Sky Training the Pup
(See also Otto Piechowski's ST-80 Tune Up on Astro.Geekjoy)
As my few readers may have noticed, I have stopped adding daily entries to this particular website. Basically, I've moved on to a new publication format: "AstroTalk". On AstroTalk I can post stuff. Dialogue with friends and fellow observers. Make all kinds of "off the wall" comments. Record specific observations. Discuss equipment. And most importantly, develop new ideas, and have them checked and verified - or refuted - by others in a well-spirited and brainstorming environment.
Every once in a while, however, I intend to pull together learnings and experiences to be shared with the general public. This document "Sky-Training the Pup" is an example of such. It is, in a way, analogous to a discussion that documents my experiences with 150mm MCT Argo - a series of experiences which taught me a great deal about Maksutov Cassegrain scopes in particular, and about practical optics, in general.
This discussion, however, does not take the same "narrative form" seen in argo.html. What you read here is a series of posts originating in a special section of AstroTalk where I now add notes relating to issues astronomical and telescopic. In this series, and unlike other topics posted on AstroTalk, only one reply originated from someone other than myself. Fellow starsplitter and scopist Cor Berrevoets added a comment verifying the fact that air spaced doublets are, in fact, impacted chromatically by small changes in gap between elements.
So without further preamble, you, the general observing public are cordially invited to follow my efforts to "Sky Train the Pup"...
Hi Folks,
This new board is where I plan to post notes to myself concerning various astronomical projects. This first thread has to do with activities intended to "fine tune" the performance of my observing tool associate: "the Pup" - an 80mm ShortTube 80 Achromat mounted on an Orion EQ-1 mount.
The Pup is already "the tool of choice" for very wide field studies (greater than 1 degrees). However another goal is to bring the Pup online as an "aid de camp" for 150mm Argo to generate baseline data in support of present and future astronomical projects.
Feel free to follow along and post questions and insights...
Clear and Steady,
jeff
AstroTalk Thread Started on: 09/18/01 at 16:04:01The Pups collimation is subtley out of alignment. At high magnifications (120X+) diffraction rings obviously and consistently brighter to one side of airy disks than the other. This is the subtlest sort of "coma" and must be corrected before the Pup can provide reliable "on the edge" 80mm aperture data associated with lunar/planetary and double star observations.
The immediate performance goal for the Pup is to get to the point where Delta Cygni's disparate 6.4 magnitude 2.4 arc-second separate companion can be definitively and repeatably seen in the region of the 2.8 primary's glare and first diffraction ring.
Already the Pup gives Dawes limit resolution for matched pairs - so it is clear that the scope can achieve good results for disparates as well.
But this will take work and requires good to excellent collimation of all components in the optical path.
Last nights step was to shim the object glass to spread the first diffraction ring as centrally around Polaris' airy disk as possible.
This phase is now more or less complete - but may have to be revisited once visual back section alignment is looked after.
That alignment is phase II and is now in progress.
AstroTalk Reply #1 on: 09/18/01 at 16:11:52
This afternoon took advantage of the fortuitously placed cell phone tower on a hillside several kilometers from my home. The goal was to ensure that the top point of the tower would center in the 120x field of view as I rotated the mirror diagonal around some 200 degrees of motion.
The first step in this process is to make sure that all mechanicals - focuser, adapter, barlow, diagonal and eyepiece fit snugly together and that there is no visible "droop" or "wimpyness" along the line of this assemblage.
I had accomplished the above task several weeks ago so it was simply a matter of making sure that everything was "snug".
The next step was to remove the diagonal and center on the tower reference in the refractor "straight thru" viewing mode. With the image centered at 120X I then aligned the crosshairs on the finderscope to coincide. Thus with diagonal removed and not a contributing variable, there was some kind of reference to check the diagonal mirror placement against.
As it turned out, installing the diagonal resulted in about 8 arc-minutes of delta between finder centering in the straight thru mode and in the position of the cross hairs on diagonal centering.
To fix this I had to ultimately redo the position of the cardboard shim in the diagonal and rotate it 90 degrees anti-clockwise.
To finalize shim position and its thickness had to rotate the diagonal through 180 plus degrees and determine what kind of offset existed in each of three positions.
By the time I was finished the finderscope properly acquired the target within plus or minus 3 arc-minutes instead of the original +/1 8.
One final benefit: The finder crosshairs now accurately center irrespective of the orientation of the diagonal which will make high power double star study acquisition MUCH easier than it has been in the past.
Noticed one other issue. The finder support mechanism is a wee bit unreliable so some adjustment of crosshair position may be needed each evening out...
The next phase is startesting...
AstroTalk Reply #2 on: 09/18/01 at 17:09:53
Suffice to say, there are those who know far more about startesting optics than I. Typically, startesting is best done before buying a used scope OR during a trial period after taking delivery. But outside a few experts in the field most of us are content if our star images don't show obvious signs of coma, astigmatism or excess chromatic aberration. And generally I've been pretty satisfied with the Pup over the last six months or so.
Once you buy a scope and have it for a while there is little you can do about the quality of the main optical components. Things like the object glass, or the primary and secondary mirrors CAN be replaced - but it is unlikely that most amateurs will burden themselves with the expense and downtime required. Usually poorer scopes end up on the after market. Some of these may have fine optics but simply require a thorough going over to make sure everything is properly setup. Others have faulty components and the newer owner consoles themselves with the idea that they got the scope at half the price etc.
But sooner or later most amateurs sensibilities and sensitivities wake up and they realize that they are not satisfied with the quality of their scopes optical performance.
That's where I am now with the Pup.
As it stands now I've done about all I can to bring the Pup's components to a point of alignment and cooperation. The object glass is squared. The visual back is secure. And the mirror diagonal is properly shimmed. What do I hope to see the next night out while viewing Polaris as a test star?
1. Can I see a perfectly "round" airy disk at high powers?
2. Can I move the star through focus and see the outfocus image collapse cleanly into an airy disk and out again without comatic or astigmatic aberrations?
3. Is the inside and outside extrafocal "star globe" visually round and in the center of the field of view both sides of focus?
4. Does the airy disk center inside the extrafocal images as focus is traversed?
5. Is the airy disk centered perfectly in the first diffraction ring arc?
6. Under perfect seeing, does the diffraction ring extend all the way around the airy disk in a complete circle?
7. If less than perfect seeing, does the ring arc swing around the airy disk or is it always stuck in the same position?
8. Are there any "brightenings" on the diffraction ring that more or less stay put? (Zonal errors in the system.)
And finally (but not really that important if all the above factors play out as hoped)
9. Do inside and outside diffraction rings map against one another reasonably well?
Well tonight (perhaps) I should have a few answers based on this afternoons adjustments...
AstroTalk Reply Reply #3 on: 09/18/01 at 17:27:59
Well folks,
Here's the verdict on this evenings trials:
Quote:
1. Can I see a perfectly "round" airy disk at high powers?
Yes, this has been the case since first getting the Pup. A scope that shows an airy disk can at least perform to 7/10 seeing potential. And the Pup is able to do this...
Quote:
2. Can I move the star through focus and see the outfocus image collapse cleanly into an airy disk and out again without comatic or astigmatic aberrations?
There is, and remains some very slight coma on focus traversal visible in the Pup at 160x.
Quote:
3. Is the inside and outside extrafocal "star globe" visually round and in the center of the field of view both sides of focus?
As above there is some slight "ellipticity" to the Pups a-focal images. In the past there was no point where afocal globes were "round" anywhere in the field of view. The afternoon's work improved this and I could find places in the field of view (not at center) where the image was visibly "round".
Quote:
4. Does the airy disk center inside the extrafocal images as focus is traversed?
This is and remains the main problem with the Pup. The "starpoint" is visibly offset.
Quote:
5. Is the airy disk centered perfectly in the first diffraction ring arc?
No the diffraction ring is neither complete nor equally spaced vis a vis the airy disk.
Quote:
6. Under perfect seeing, does the diffraction ring extend all the way around the airy disk in a complete circle?
This evenings tests revealed that the first diffraction ring was complete but not concentric.
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7. If less than perfect seeing, does the ring arc swing around the airy disk or is it always stuck in the same position?
There were times when atmospherics caused the arc to swing all the way around the airy disk. This suggests that good to very good results based on adjustment of components is a possibility.
Quote:
8. Are there any "brightenings" on the diffraction ring that more or less stay put? (Zonal errors in the system.)
Happily this is not an obvious issue with the Pup. But it will require excellent collimation to make such an issue salient.
Quote:
9. Do inside and outside diffraction rings map against one another reasonably well?
the Pup's inside focus star image is visibly superior to the outside focus. I'd say the inside focus is 1/6th wave corrected and the outside 1/4.
This evening, I ran through the above series of tests on Polaris at 160x and got the above results. I then began a series of adjustments to reshim the diagonal and the object glass. At a certain point I was getting very good but not ultimately acceptable results. At that point I removed some shimming in the lens cell and all hell broke loose. Simply terrible coma. In tracking this down I noticed that the lens now wobbled side to side. By removing this last shim there was nothing that could laterally support the main lens when screwing in the retaining collar.
Unfortunately, by the time a re-installed the needed lateral shims the marine layer moved in and I lost the sky.
So next night out I will get a chance to check my latest shimming and evaluate the Pup's perfromance based on the criteria established above.
Oh yes, with the Pup "almost right" was able to consistently make out a brightening southwest of Delta Cygnus on the first diffraction ring. Unfortunately, at its best, that ring was not equidistant from Delta's airy disk. This is what prompted my to remove the lateral shimming since it was that shimming that elongated the ring out to the south of the airy.
RULES FOR SHIMMING AN OBJECT GLASS
Rule One: "Add material (beneath the object glass) to draw diffraction rings in that particular direction."
Rule Two: As more material reinforces a particular point, add thin layers of flanking material to prevent the lens from "teetering".
Rule 3: "Know when to quit."
Despite not getting it perfect this evening, the method of shimming the object glass results in definite improvements in alignment. In fact, was able to remove shimming from the diagonal originally thought necessary.
As long as I don't break anything, its just a matter of time and perfected technique before the Pup will meet the requirments identified above...
AstroTalk Reply Reply #4 on: 09/18/01 at 23:07:46
Well Folks,
Given the fact that I will not have another chance to "star test" the Pup before a road trip, had no choice but to re-baseline the optics using the daylight "cell phone tower" method today.
As mentioned elsewhere, this method basically involves adjusting the Pups optic-train components in such a way that a distant tower remains at center in the telescopic field of view when the scope is setup with and without barlow lens.
To do this, the scope is configured in the refractor "straight through" configuration. Use of the barlow effectively gives two points of reference - one closer to and the other further away from - the object glass. If the scope is able to center the target based on a single finderscope crosshair setup in both positions, then it may be assumed that the convergent image cone of the lens is directed isometrically down the length of the tube. If the scope can not be so aligned, then the convergent cone "angles" and centering the tower in the field of view of the eyepiece will result in the crosshairs of the finderscope pointing to different positions near the tower.
Once straight through adjustments are made, the diagonal is installed and cardboard is used to shim the diagonal mirror to keep the tower on target as the diagonal is rotated.
You definitely don't want to do these steps out of order since straight through alignment is essential to diagonal shimming.
So, what's about to happen is that I plan to take the Pup on a trip to meet with friends at a nice dark sky vacation spot. While there. I'll do some deepsky and double star observing (maybe even a gas giant or two).
If the cell tower method proves out, the Pup should approximate the alignment goals defined above. (I've printed them out and will write a more or less detailed response to each question.)
Meanwhile I'll have no tools with me so I'm stuck observing using the Pup as is.
In a way I'm putting my observing where my mouth is...
Let you all know how things turn out...
AstroTalk Reply Reply #5 on: 09/19/01 at 15:27:11
Hi All,
And Oh Yah forgot to mention,
80mm ST80's (such as the Pup) are air spaced doublets. I know, because while doing all that shimming and stuff, the two elements separated and I had to reseat them.
It's true! When the elements separate like that there is a lot of "air" between them...
Suppose this will cause me any problems on my trip?
Crossing my fingers,
jeff
AstroTalk Reply Reply #6 on: 09/19/01 at 18:16:44
Jeff,
In a double the actual spacing between the elements can be used to control the amount of spherical/chromatic aberrations to a certain extent ! Unluckily the producers of these scopes do not share the optical design features with the users, else you could find out with software what the optimal spacing is.
It is like the spark-plug settings of a car, you can have them working or have them optimized. Optimized the car can perform better, the big difference ..... a car can be optimized in a garage rather simple .... try to find yourself a workshop that does optical-spacing in telescopes.
Cor
AstroTalk Reply Reply #7 on: 09/19/01 at 23:20:02
Hi Cor,
I do not currently have time to post fully on the latest efforts to "Sky Train" the Pup. Suffice to say, I am now in a position where the need to develop a method to properly collimate the first element of the Pup's doublet vis a vis the second is essential. Star testing has revealed a 90 degree comatic shift between the in- and out-focus images. This astigmatism, plus a visibly increased amount of chromatic aberration, tells me the Pup now requires a great deal of tweaking.
Of course, I love doing this sort of thing BUT the fact is that there are so many degrees of freedom in the relationship between the two elements that I will probably end up damaging one of the elements before a satisfactory outcome is achieved!
If I can not bring the Pup back to its previous level of performance I will probably use it seldomly in the future. If I can not get rid of the original "skewing" in the diffraction pattern the Pup will be of little use to me in collecting data on doubles and (possibly) limiting magnitudes.
Hopefully I will get a full trip report written up and posted on astro.geekjoy - but it won't be pretty...
Wish us luck,
jeff
AstroTalk Reply Reply #8 on: 09/23/01 at 16:04:53
Hi All,
As you may know, I've been away since Thursday in the early am. Left around 4:00am in the morning and some 10 hours later (2pm) arrived at Crater Lake National Park in south-central Oregon. Though the trip was not primarily focused on observing, that first evening (Thursday, September 20) was spent observing a variety of deepsky studies and double stars through the 80mm Pup.
But there was no joy in Muddville, Mighty Casey had struck out. The optics on the Pup were a mess (as mentioned in the previous post). The culpret was not collimation, no it was that separated doublet mentioned in an even earlier post (and addressed by Cor).
But to be complete, and as promised, here are the answers to the series of questions which evaluate to the "success" or "failure" of the Pup Sky Training project:
Quote:
1. Can I see a perfectly "round" airy disk at high powers?
Referenced Polaris which showed a "roundish" airy disk at 160X. BUT perfect symmetry seemed to be lacking...
Quote:
2. Can I move the star through focus and see the outfocus image collapse cleanly into an airy disk and out again without comatic or astigmatic aberrations?
Astigmatic image traversal was the order of the night! Ellipticity was seen to migrate at 90 degree angles. Quote:
3. Is the inside and outside extrafocal "star globe" visually round and in the center of the field of view both sides of focus?
Notes say that the outfocus major axis of the elliptical "star globe" was oriented 120 degrees clockwise from the finderscope (as viewed from the eyepiece end of the scope). I recollect the major axis as being approximately 50% greater in length than the minor axis both sides of focus. There was no locale in the field of view where the "star globe" was just that - a globe...
Quote:
4. Does the airy disk center inside the extrafocal images as focus is traversed?
The "star point" was one foci of the outfocus ellipse and held a position opposite the 120 degree angle ellipticity documented above. That position was maintained as inside focus transitioned.
Quote:
5. Is the airy disk centered perfectly in the first diffraction ring arc?
Not even close.
Quote:
6. Under perfect seeing, does the diffraction ring extend all the way around the airy disk in a complete circle?
On bright stars (Vega, Arcturus, Antares) three diffraction ring arcs were seen adjacent to the star point described above. (These were "arcs" of about 120 degrees that did not continue to the opposing side.)
Quote:
7. If less than perfect seeing, does the ring arc swing around the airy disk or is it always stuck in the same position?
There was no way to evaluate seeing based on star images. However a split of both pairs of the Double Double was cleanly accomplished (at 133X). And Pi Aquilae could be seen elongated (at 160x). The diffraction ring position remained constant and distracting to the eye in all cases.
Quote:
8. Are there any "brightenings" on the diffraction ring that more or less stay put? (Zonal errors in the system.)
There can be no answer to this question until diffraction rings are continuous, complete and concentric.
Quote:
9. Do inside and outside diffraction rings map against one another reasonably well?
Diffraction rings better organized inside focus than out...
AstroTalk Reply Reply #9 on: 09/23/01 at 18:39:12
Hi All,
Made some good progress with the Pup this evening. Started out by separating and cleaning the elements with optical fluid - just in case particulate contibuted to irregular spacing between elements.
Before disassembly marked the lens cell to ensure it was re-assembled in the same way it came apart (white out and black marker pen worked fine).
While cleaning, noticed that there are three small thin strips of "tape" used to space the elements. So at least I've identified the gapping "mechanism". After cleaning, made sure the proper side of the first (thinner, dual convex) element was reinstalled with original side face in. Also seated the element as "flat" as possible within the concave section of the back element.
All of this was for not. No improvement...
So decided to see if by tilting the lens cell at different angles I could significantly reduce the elongation of the a-focal "globes". Saw enough improvement to suggest that "round" globes were a possibility.
Then started knife edge testing to see which way the elongation oriented. Based on this, dissassembled everything and re-positioned the primary element to change the "tilt" slightly.
This approach showed huge potential. Even the slightest tilt of that fully convex element gives visible results. So by knock off time (around 9:00pm) and with a whole bunch of "business card spacers" shoved up under the lens cell, started getting decent - if not perfect - afocal globes and almost circular diffraction rings.
Much better than the Sick Puppy taken to Crater Lake.
So things are on the mend. But there will probably be several more cycles of test, dissassembly, primary lens re-positioning etc. before the original goals of this project are met.
Only question is whether or not the Pup will survive the training...
I plan to quit all this as soon I get an airy disk centered within a complete first diffraction ring.
Really, I will!
jeff
AstroTalk Reply Reply #10 on: 09/23/01 at 21:35:34
Well Folks,
Right now the Pup's as healthy as it's ever been. Spent the entire evening expeimenting with the spacing between the elements in the doublet. Used the thinnest possible material (optical cleaning paper) to raise one or more of the three contact points which separate them. Each time I raised one point, elongation of the afocal image distended in that direction. There was no position where adding these thin strips made everything (diffraction rings) concentric to the star. Finally, just gave up. Cleaned the outermost element in warm soapy water (Dr. Bonners) and carefully reassembled.
Based on what I've seen, it IS likely that careful spacing of doublet elements will "perfect" diffraction ring concentricity and possibly ring surface brightness distribution. However, the amount of travel required to effect the change is extremely small - especially in such a short focal length instrument.
So in effect, despite adequate optical shaping of the doublet pair (1/4 wave plus), it is an extremely time consuming and meticulous process to get the two elements perfectly spaced and planar to one another. For that reason, the Pup will remain a 7/10 seeing stability scope (since the requirement for 8/10 seeing is that the first diffraction ring must completely circumscribe the airy disk of the star in an equidistant fashion).
Despite this, the plan remains to "sky train" the Pup. But for the time being, this has become a "back burner" operation awaiting some new idea about how to effect the very small changes in gap needed to perfect the physical relationship between the two elements in the doublet.
One final thought: When you purchase a fine refractor - say an AP, Televue, or Takahashi, along with those exotic materials (low dispersion or flourite glass) you also pay for someone to take the time to "perfect" the relationship amongst the elements and mount them in an optical tube in such a way that collimation is easy and stable. This is not the case with inexpensive scopes...
To get the most out of scopes like a ShortTube 80 (150 dollars!) you must personally play the role of the optical technician. You must make all the required adjustments and test the results. Along with those exotic materials and superb mechanicals, that technician's time is a significant part of the 2,000 plus dollars payed for that scope.
Personally I am very pleased to have taken the time to dial in both Argo and the Pup. It really is a joyful thing to do! The only difference between Argo and the Pup comes down to the fact that Intes of Russia built in all the mechanical adjustments needed to accomplish the task. Synta of China however, (manufacturer of the Orion ShortTube 80) provided absolutely nothing to assist in aligning optical elements and mechanical components. I'd certainly have payed twice as much for the Pup had such adjustments been added to the design. To be sure Synta makes some decent optics and a fair focuser - all that really is needed to make the ShortTube 80 a "research grade achromat" is the ability for the user to integrate all of its parts into a well-functioning whole.
But that is not the case and I'm left at an impasse...
Clear and Steady,
jeff
AstroTalk Reply Reply #11 on: 09/25/01 at 23:16:48
Hi All,
Reflection on what i've learned so far in sky training the Pup led me to a fundamental insight into the scope's problem. Noticed in making last evening's adjustments that inside focus diffraction rings could be rendered concentric to the central star point but the outside focus could not. Reasoning that the inside element of the doublet pair had more control over the "green" color of the achromat and that the outside element controlled the "red", put 2 + 2 together and realized that it was the outside element that needed to be properly aligned - not the inside element.
Now the problem was to determine how to implement a very small shift in the outside element position. Surely I could not accomplish this by hand! A fast achromat is just far to sensitive to the gap and planarity between elements.
However there was no reason why I couldn't use more subtle methods...
And that method was right before me even fixed onto the scope. All that needed to be done wasto induce somekind of precision pressure against the outside element bwhile spinning the object glass retaining ring in place...
So cutting a small square of business cardboard, placed it on the precise spot needing depression on the outside element. Then cranked down the retaining ring.
Miraculously, diffraction rings round Vega became far more concentric on the the outside of focus. The only problem was that a shield-like "pinch" on the star globe was induced by the pressure. So backing off a little on the retaining ring, crossed my fingers and hoped that the shift in the outside elements position would remain.
And it did - at least throughout the evening while I enjoyed a superb view of Selene. In fact, hints of several brightenings in crater Plato and a fine view of the Straight Wall were seen. Tonights view was the best ever seen of Selene through the 80mm Pup and seeing stability was not in the least spectacular (6+/10).
Now, the hope is, that on the next really excellent night the Pup will reveal that close, dimmish companion of Delta Cygni as more that a simple "brightening" on the first diffraction ring. Should this prove feasible AND the Pup remains stable and healthy there is no doubt the scope will make a fine counterpart to 150mm Argo as work continues on limiting magnitudes, optimal scope apertures and disparate double resolution.
Wish us luck,
jeff
AstroTalk Reply Reply #12 on: 09/26/01 at 22:19:06
Hi All,
Adjustments made last evening to the Pups first element seem to have remained stable. However, poor seeing stability this evening prevented me from completely verifying that the Pup is capable of getting the most out of 8/10 seeing skies (tonight's were about 5/10 overhead).
The immediate performance goal for the Pup is to definitively resolve Delta Cygni by showing the 6.4 magnitude secondary as a star on or at the primary's first diffraction ring (to the southwest). This really won't be possible for the Pup until seeing stability = 8 / 10. Such nights occur around here perhaps twice a week - and I may have already had mine before making that latest set of adjustments!
One interesting phenomenon did manifest itself while attempting to get a decent view of Mars (under 3/10 seeing). While watching the boiling cauldron of the planet drift across the 240x field of view (10mm Kellner with 2X shorty and 3X Ultrascopic barlow combination) suddenly a huge red ring flashed around the planet! Had the idea this may have been an optical and not an atmospheric phenomenon - so I shifted the planet back and sure enough the red ring showed again just as the planet approximated the same field of view position.
My suspicions are now that everything is so well collimated that some form of organized light scatter manifested itself. The position where this occured was about halfway across the field and 1/3rd the way from field center to the 12 o'clock position. At the time the diagonal and scope were rotated about 90 degrees from vertical (basically lying on its side at about the same height as the equatorial mounts counterweights and pointed up about 25 degrees above the horizon...)
Interesting eh?
jeff
AstroTalk Reply Reply #13 on: 09/27/01 at 21:32:20
Hi All,
Really starting to close in on this thing now. The outer element of the doublet seems to be remaining quite stable. Outside focus "globe of light" is round and the star point remains in the center. Several fresnel rings are possible - both inside and outside focus. Organization of light in the rings is not as good as Argos. Part of the problem is the chromaticism which makes distinguishing the rings difficult.
Haven't made any more adjustments to doublet elements since "pinching" down at the one point needed to square up the first element to the second. However there are still some subtle problems with mechanical alignment (collimation). By knock off time this evening the star point inside focus was pretty well centered. But this will need to be rechecked.
Under 7/10 seeing thought I caught Delta's secondary several times. Need a solid 8/10 night to nail it down one way or the other.
I'm very confident that if I can get a full and equidistant primary diffraction ring this should be achievable.
Clear and Steady,
jeff
AstroTalk Reply Reply #14 on: 09/28/01 at 22:58:03
Yowser!
Got up early this morning for a peek at the Gas Giants through the Pup at 80 and 43mm.
Both planets looked better in the Pup at 80mm's than ever before - and who knows what the seeing was really like?
You may not believe this but with the Pup stopped down to 43mm the view of Jupiter was like a mini-Argo! Edge was absolutely sharp, snap to focus incredible. Sure the planet was darker and I lost resolving power but the aesthetics were astounding.
Can only hope that the 80mm view was seeing impacted. Otherwise, I may be stopping the 80mm down permanently to something between 50 and 75 mm for double star testing. Of course the stop will be removable so I can go to rich field deepsky mode.
All of this testing does say something of real interest.
1. Buy an inexpensive large aperture achromat (120mm ougth to do.)
2. Planarize the air spaced doublet.
3. Stop it down until you get satisfactory chromaticism and excellent wavefront optics. (Conceivable you could end up with a very fine 90mm lunar planetary scope that can be easily converted to 120mm for deepsky.)
Unfortunately you can't do this sort of thing with an MCT, SCT or Newt. The central obstruction would kill you.
And an expensive APO does not need this kind of treatment.
But a cheap achromat - yowser!
jeff
AstroTalk Reply Reply #15 on: 09/29/01 at 14:16:52
Hi Everyone,
About to take a bold new step with the Pup this evening...
Went down to the hardware store and bout a black polypropeline end cap. The cap is intended for 3 inch ADS drainpipe use.
Cut a 67mm hole in the center with an exacto blade. Cleaned the cap thoroughly (had a real tough time removing the price tag!).
After it dried fit it into the 80mm Pup's dewcap. (Easy enough to install and remove plus the dust cap fits on the end fine as well.)
Now I have a 67mm (2.5 inch plus) F6 achromatic refractor!
Should be able to show stars down to magnitude 12.0 at 120x on an 8/10 sseing night. Should be able to resolve matched doubles down to 1.7 arc seconds. Should be able to show delta Cygni's 6.4 magnitude 2.4 arc second separated companion. Should be able to give views comparable to what Argo would show as a 2.5 incher. Should have a beutiful flat wide field with only the vaguest traces of color around the Moon and Gas Giants. Should enable me to baseline limiting magnitude, sky conditions, and disparate double stars in conjunction with 150mm Argo. Should be able to cleanly focus Gas Giants. Should show 4 belts on Jupiter and a conspicuous Cassini Division brodering saturn's Ring A...
Lot's of shoulds. I'll keep you posted.
Carpe Noctem,
jeff
AstroTalk Reply Reply #16 on: 09/29/01 at 18:25:09
Hi All,
Last night had a chance to try out the 67mm F6 version of the Pup on a number of double stars plus the Moon. Of all the checks made, the most suggestive was that of star testing the scope on Vega and inspecting the Fresnel patterns both sides of focus...
As you probably know, if you throw your scope out of focus on a bright star (such as Vega for small apertures or more conveniently Polaris for larger apertures) you may see a series of bright circular lines interspersed with darker regions. These lines - both bright and dark represent places in the a-focal image where light from a star "interferes" with itself. Where such light re-inforces you get brightenings, where they de-inforce you see darkenings.
Generally its the brightenings you pay especial attention to. Their shape (round like your mirror or lens) contrast, intensity, and organization provide excellent clues as to the alignment of your optical system, and how well the various elements are shaped, surfaced, refined, coated etc...
In general, a perfect test is only possible whan a single star lies in the field and is placed against a pitch black sky of absolute stillness. Under such conditions the only light entering the scope is that of the star and that light remains undisturbed by the effects of a churning, refractory atmosphere.
But this condition can not be met outside the laboratory. So the contrast between bright and dark linesis hard to see. In addition light scatter due to the glass through which the light passes, roughness of the surfaces off which it may bounce, results in the need to use very high magnifications and look very carefully.
Past star tests of the Pup (at 80mm) have been inconclusive. Collimation errors, chromatic aberration, and the very fast F5 system complicated the matter. At best, the Pup was just "diffraction limited" in optical quality. The inside focus pattern appeared better defined than the outside. On double stars the scope managed to pretty much resolve matched pairs at Dawes limit - when sufficient magnification could be mustered...
Despite matched pair resolution, views of the Gas Giants were less than satisfactory and this performance issue reflected itself in disparate double star resolution issues (Delta Cygni anyone?)...
Meanwhile star tests of the Pup aperture stopped to 43mm were FAR better. The kind of pattern seen - well balanced intra/extra focally, along with superb definition and organization were frankly better than 1/6th wave rated Argo and and that of a 102mm FS102 Takahashi APO I'd observed through.
But where was the line? How much aperture stopping was necessary to get Argo's level of results per aperture?
I guessed F6 - and I was right. At 67mm the Pup's fresnels are comparable to Argo inside focus and better than Argo outside focus. The 67mm version simply said is outstanding for a cheap achromat. Sure the green of inside and purple-red of outside is still there - but much diminished. You have to look pretty hard to notice chromatic aberration while viewing the Moon for instance. And Vega appeared blue white with just a hint of a red-violet aura on averted vision.
So now I have a 67mm Pup which is capable of giving performance similar to Argo's at that same aperture (and without the central obstruction).
But there are still problems. I noticed that eyepiece's now vary in there suitability. The Kellners don't give the same results during star testing as the Plossls or the Ultrascopics. And for another thing there still is no method to adjust and perfect the scopes collimation.
AstroTalk Reply Reply #17 on: 09/30/01 at 15:04:04
What remain's really needful now is to design some kind of precision mechanism that can be used to:
1. Adjust the spacing and planarity of the doublet pair. (Still not quite happy with some minor splaying seen in the extra-focal image...)
2. Adjust the bulk orientation of the object glass lens cell. (Still not able to quite get the inside focus star globe perfectly round in the very cebnter of the field of view.)
3. Support the ultrascopic barlow and fosuser extension used to enable the 400mm scope to achieve 200X plus for tight double stars and lunar-planetary work.
4. Create an easily adjusted variable-sized aperture stop which can be used to tune the aperture for different separations of double stars. Or to quickly morph the scope over for wide field deepsky use...
Meanwhile the EQ-1 mount that came with the scope is handicapped when viewing studies on the zenith overhead and to the deep south. Various mechanicals contact one another at certain points and the slow motion controls must be removed regularly to slew to stars overhead.
So there remains much in the way of work to do to perfect the usability and performance of the Pup.
Upgrading the OTA as described above, plus developing a variable aperture stop (iris), and upgrading the mount will take a great deal of time and resources. At this point finances are also a factor.
But it should be fun to at least "paper planning and designing" the mechanisms involved and thinking about what a "Super-Pup APO Eater" would look like as well as how it would perform and finally how it could be used to precisely measure double star separations using variable aperture resolution and matchematical modeling...
Anyone interested in exploring the possibilities?
Carpe Noctem,
jeff
AstroTalk Reply Reply #18 on: 09/30/01 at 15:16:57
After a very successful night of double star questing the previous evening, woke to Selene shining through the west-facing window. Rubbed the sleepy dust from my eyes and noticed the sky merging from dark to light. Knew it was still dark enough to catch the Gas Giants. Jumped out of bed. Through on some jeans and flannel shirt. Headed for the study. Within five minutes had the 43mm aperture stopped version of the 80mm Pup turned on Saturn at 120X.
No trouble getting a nice edge on the ring and the planets limb. In fact, at F9.3, depth of field was so good that a not inconsiderable amount of focus travel gave the same precise and equally gratifying image quality.
Cassini Division at 43mm was more present than I'd ever seen it before - even at 80mm's. Think about this, a scope with an aperture less than many a finder, was showing the Cassini Division between Saturn's Ring A and B!
With the 67mm aperture stop in place (F6) depth of focus was lost - but it was possible to precisely snap to. Cassini now looked like someone had taken a black pencil and traced a line between Ring A and B. Contrast improved with the additional aperture. So did resolution. No, Cassini's didn't look like someone had scribed it into the ring system with an exacto blade (as it appears in 150mm MCT Argo) - more like a slightly used pencil line. Still very impressive.
But the question now was, would the image quality hold at the full 80mm (F5)?
And the answer was yes - but with slight qualifications. "Snap to" focus was not quite achievable. However Cassini remained undiminished and the planet's SEB might have been a hair darker than at 67mm. So I took several moments to mentally outline some of what I was seeing at 120x:
- Almost, but not quite, sharp limb focus.
- Cassini Division like a pencil line more than 70% the way toward the polar regions from the eastern and western ansae.
- Ring A outside Ring B (implied by seeing Cassini Division).
- Thin line of contrast demarcating the planet's body and anterior Ring B.
- Yellowish Equatorial Zone
- Brownish South Equatorial Belt.
- Fade to Blue Gray in the South Polar Region.
- Clean shading of terminator to planets east (especially southeast).
Things not seen:
- Crepe Ring.
- Ring B transition to Crepe Ring.
- Shadow of planet on posterior Ring at southwest (leading edge).
- Encke Minima
- Blue gray mottling to the SPR.
- Irregularties in the SEB edge.
But Saturn is, in general, more forgiving of optical rectitude than the other Gas Giant - Jupiter!
Previous to tuning the Pup the best I ever saw of the planet was the two main equatorial belts and the North Temperate Belt. Now at both 67 and 80mm's I was seeing:
- The two main belts (with hints of the rift in the South Equatorial Belt).
- The North North Temperate Belt (NNTB).
- A much diminished North Temperate Belt (what's going on here!) during moments of greatest steadyness.
- The South Temperate Belt (easier than NTB, more difficult than NNTB).
Interesting fact was that I did not get this level of definitive detail while first viewing the planet. Along the way, took a look at a setting Selene and found the focus was a hair off (at 80mms). It was after precisely re-adjusting that the more subtle belts began to emerge on Jupiter.
There are a host of micro-level features associated with Jupiter that were not visible through the 80mm Pup. For instance the SEB rift was suggested - not obvious. Could not make out any barges, ovals or textures on the planet as occasionally seen through 150mm Argo. But the key here is that the planet achieved focus and presented enough detail to be interesting.
All the blueprinting done with the scope over the last several weeks has really payed off. The 80mm achromat is now giving the kind of views I would expect of any "optically correct" unobstructed scope costing 5 to 10 times as much. The keys to this performance improvement basically come down to:
1. Perfecting the collimation - within the limits imposed on the scope due to a lack of the kind of micrometer adjustements available on "high end" refractors.
2. Planarizing the two elements of the achromatic doublet in such a way as to get the star point to center both inside and outside of focus within reasonably "round" outfocus "globes of light".
Neither of these two adjustements has been accomplished to my ultimate satisfaction - and I'm not sure that the limited available mechanisms to accomplish these tasks will ever make "perfection" possible. But the proof that such adjustments can lead to superior results lay right there before my eyes...
Carpe Noctem,
jeff
PS: And oh yes, Selene - 2 brightenings visible in Plato at 120x. But no, still have yet to resolve Delta Cygni but suspect that this will be achievable once we lose the Moon and contrast with the night sky improves...
AstroTalk Reply Reply #18 on: 09/30/01 at 15:16:57
Hi All,
Just thought I'd wrap this one up. As radio commentator of old Paul Harvey would say, "The Rest of the Story".
Last night (Thursday, October 11, 2001) put the "finishing touches" on the Pup's collimation. Used Vega as a test star. In so doing, found that I needed to stuff two wads of business card cardboard up under the lip of the lens carriage to give it "it's best tilt". Also had to insert a fine washer as a spacer under the lip of the focuser where it meets the OTA. All this was done with the scope in the diagonaless "look through" viewing mode. (Now that's what I call "getting down" - the eyepiece was maybe two feet above the ground.)
With these changes the "starpoint" now took to the center of both the inside and outside diffraction rings. All diffraction rings were concentric in the middle of the 160X field of view. (However they are visibly elongated well away from that point just as they do in 150mm MCT Argo.)
The next step was to install the diagonal and verify similar presentation with it in place. UGH! Traversal of focus was asta-comatic!
Terrible!
Rechecked the straight-through presentation - whew - fine.
Then started adding thin washers and cardboard shims to the "dissectable" diagonal.
Another hour later - VOILA! Straight through and diagonal modes performed identically. More important, the d**n able tail of comatic diffraction rings around Vega were gone! And traversal of focus was sweet - causing this gorgeous, blue white first magnitude star to expand and collapse through focus concentrically.
Color too was minimal - averted vision needed to see the telltale maroon of perfect focus.
Turned the scope on Delta Cygni. Nice round concentric first diffraction ring (though a bit "walky"). More importantly, under 5.5 ULTM skies and the slightest aversion - a tiny little point of light to the southwest very near the one and only diffraction ring.
Yes, ladies and gentlemen of our High Art and Science all the hard work has payed off. The formerly box-stock 80mm achromatic Pup now gives near (if not) cutting edge research-grade image quality!
At 80mm and 13X, could easily take in the Entire Veil Complex (OIII filter, of course). Pinpoint stars over 2/3rds the field. (This is an F5 scope after all!). Coma only obvious near the very edges of the 35mm Ultrascopic eyepiece field stop. (In fact, the Pup may very well outperform 150mm MCT Argo using in this regard through the same eyepiece.)
There do remain a few concerns about the scope - primarily whether or not the alignment will "hold" during transport and use. I will probably replace the cardboard with thin washers at the lens carriage. Eventually more elegant methods may be employed to support collimation adjustments. But the bottom line is that the optics are capable - they just need to be properly supported by the mechanics.
Tune Your Scope, Tune Yourself,
jeff
AstroTalk Reply #20 on: 10/12/01 at 13:11:51
Although I have no further plans to add to the AstroTalk thread in the future, check back here in a few months and I hope to make a few comments about how the work has gone forward with the Pup. I'd also be happy to hear from others (via email) concerning their own efforts to "blueprint" their achromats...