DeepSky: Televue TV85 APO & Orion XT4.5 Dob-Newt
Some Background The Seeing The Optics The Views In Sum
Some Background
Not everyone has the bucks - or the inclination - to buy high-end observing equipment. Some folks are new to our High Art and Science and are not sure whether they are in love with the Night Sky or simply infatuated. Laying out a couple thou for everything needful - scope, mount, accessories - is tough enough on the budget. But scopes like El Marko's TV-85 cost that much just for the OTA! On the other end of the spectrum however are a number of inexpensive options. My own choice was an ST-80. El Marko's son Dylan went with the Orion XT 4.5 dob-newt - eyepieces and finderscope included - for 200 dollars american.
El Marko's TV-85 rides on an alt-az mount. The finder is the one-power, small-bore BB-gun unit provided by Televue. At 600mm's focal length the scope really needs no finder. Magnifications in the mid-20x range are achievable with the ubiquitous 25mm eyepieces most amateurs have in their kits. The same eyepiece yields 36x in the F7.9 newtonian of 114mm aperture. Both scopes are capable of "wide-field views".
The little newt comes with a 6x26mm finder. Although using a 35mm ep gets the newt down to 25x as well - scope use is assisted by the finders presence.
Although I prefer an alt-az ride to that of a dobsonian, I discovered through use of the two scopes that - for me - having a finderscope made astro-navigation easier. But despite the fact that Dylan placed the XT on a booster platform, I preferred the TV's observing position - especially given the fact that El Marko's mount includes an extensible jack stand to reposition the working height of the OTA. This jack stand idea - like the grab handles found on both the XT4.5 and Argo's OTA - is an "almost-must-have" as far as convenience is concerned.
As a rule I did my best to match magnifications between the two scopes. I brought along my own eyepieces. As mentioned, using a 35mm Ultrascopic gave the same low-power magnification on the dob as the 25mm on the TV (25x). Using a 10mm ultra gave 90x through the dob. Adding a 2x barlow to the refractor gave 80x - close enough - especially given the divergent exit pupils of the two scopes.
It was not always possible to compare simultaneous views through the two scopes. Dylan and I began observing through the XT earlier in the evening. The TV was not setup until considerably later. Of all the studies only the Veil complex overlapped - everything else was done by mental comparo. Because I have strong images of how many of the same studies appear through 150mm MCT Argo and 80mm ST-80 Achro "the Pup" it is not difficult for me to make comments on how the other two scopes would have fared as well. The one caveat here is that seeing transparency up on the Bonny Dune ridge - on a new moon night - were exceptionally transparent...
Seeing
El Marko had a late evening at work, so Dylan and I observed for more than an hour before we became a troika. We started out by assessing sky conditions. There was a stillness to the air and overhead the Great White Way was displayed in great glory. There was almost a sense of graininess to the star cloud regions in Scutum and Cygnus. Unfortunately the view south was obstructed by trees - but we were going to focus on things zenithial anyway.
Dylan and I made a quick check of Ursa Minor - all stars comprising the little dipper were easily held - even in the San Jose lightdome to the north. We then started picking out faint stars unaided in Cygnus. One 5.6 magnitude star could be held - but the night was still young.
Mustering 90x we turned the XT on the Double-Double in Lyra. At this magnification the more matched E2 pair displayed a clean separation while the more disparate E1 was a bit more of a challenge. Dylan reported distinguishing both pairs - but Dylan's visual acuity is such that he is reported to have resolved Albireo without optical aid... I dropped the magnification down to 60x (15mm Ultrascopic). Occasionally I could distinguish the E2 pair but nada on E1. Again Dylan reported seeing occasional breaks between both pairs. Based on my own experience I would say that seeing stability was 7/10 Pickering - although I would have needed to resolve an airy disk to be sure. Although the Double-Double was well overhead and seeing transparency was exceptional - I was unable to catch sight of the two 12.0/12.5 magnitude comes that lie between E1 & E2.
To check limiting telescopic magnitude we turned to M57. At 90x the 12.8 magnitude star could be held with moderate aversion. A 12.3 magnitude star in the "Chalice of the Ring" was just possible direct. Using Astro.Geekjoy's Limiting Magnitude Calculator at 90x through 7/10 stability 5.6 ULM skies the scope should hold stars direct to magnitude 12.4. The XT4.5 was good to form...
An hour later with the sky perhaps .2 magnitudes deeper, I turned the TV on the Double-Double. Using the 10mm Ultrascopic with 2x barlow (120x) four perfect airy disks with only the slightest hint of a first diffraction ring was resolved. Seeing stability - at that time and through the 85mm scope was 8/10. The two faint comes could be held with the slightest aversion. Using the 15mm/2x combo E2 was separable while E1 appeared elongate however the comes were MIA. A check of the ring region at 120x showed the 12.8 reference star with the slightest aversion - so stars to 12.5 could be held. Plugging in 5.8 ULM 8/10 stability and 120x on the 85mm APO stars to magnitude 12.6 should have been possible. Meanwhile normalizing the two scopes to 90x through 5.6 ULM skies (as though both scopes had been used at the same magnification and stability earlier) the TV85 comes up just a tad short of the XT4.5 (12.1 vs 12.4).
Optics
From the outset of using the XT I noticed four very interesting behaviors.
- The slow (F8) Newtonian has a remarkably flat field
- The background sky was quite dark and studies showed high contrast
- Stars - though not quite pinpoints - were aesthetically pleasing.
- It was difficult for me to get precise stellar focus.
Any scopes field flatness is effected by focal ratio. Because the newts aperture is relatively small - even F8 leaves the scope very portable. Of course a slow scope should have greater depth of field and therefore make it easier to set focus - but this seemed not to be the case with the XT. Turning the scope on Vega gave contradictory results. Although both afocal tori showed sufficient lumination to warrant the epithet "diffraction-limited", I noticed that outside focus there was a "spikeyness" to the image suggesting potential roughness at the mirror surface. Why this did not appear inside focus as well is a mystery to me. Its possible however, that - had a higher magnification startest been done - the mystery would have revealed itself. Possibly the whole thing was atmospheric (extra-focal star images tends to incorporate more near-atmospheric disturbance). However none of this was visible in the TV - irrespective of how low - or high - magnification was used.
Of course chromatic aberration should not be apparent in a reflector - unless poor quality eyepieces are in use. Vega's color quality was excellent - all sides of focus.
Surprisingly, excellent color correction was also seen in the TV. Despite the relatively fast optics - and lack of any flourite glass - I would have had to work my imagination overtime to ding the ED doublet in this regard. Checking Vega at 120x also revealed almost identical afocal tori - each with a single dimple of light and faintish interstitial fresnel rings. There was the slightest hint of a "cats cradle" outside the tori extrafocally - but that was it. Certainly the doublet is funneling all them photons in the 1/6 plus range of spherical aberration.
Meanwhile it was clear to me that field flatness was superior even to the slow newts - although the newts background sky was equally as contrasty-dark. Meanwhile those larger airy disks were much easier to bring to focus and were far more immune to atmospheric disturbance. The scope - despite a slight oscillation in the ride - was easily brought to focus at all magnifications 18 - 210x.
Clearly then spending 10x as much for a high quality OTA does bring some advantage optically!
The Views:
I am always amazed when I turn a small scope on the Double-Double and resolve all four stars into those beautiful "pearls of luminous irredescence". This unfortunately was not the case with the XT4.5 - perhaps had we doubled the magnification this may have been possible - but there would always be the issue of the scopes difficulty in achieving perfect focus. But the TV85 did not fail to amaze. In fact all four stars were so well presented that I simply couldn't make out a diffraction ring among the four of them. This is never the case in the three scopes I normally use. HD 102 Achro "Vicki" always shows a single diffraction ring around each of the three 4th and 5th magnitude components. 80mm "the Pup" does so as well but that ring is not evenly illuminated - due to the difficulties inherent in collimating an inexpensive F5 achromat perfectly. 150mm Argo shows rings around all four components. When tube currents are present (early after setup) the rings can take on hellish shapes. Once thermal stability is achieved ring-luminosity is well-balanced but often overbright - due to Argo's 35% central obstruction.
Another view I always look for with the Double-Double is the presence of the two small comes that lie between the main pairs. These stars - magnitude 12.0 and 12.5 - are often "glared out" by the brighter pairings. Through 150mm Argo they can be held direct with difficulty at 120x - but only through decent (5.0 plus) skies. Neither of these stars were revealed by the XT at 90x. However they both could be held in the TV85 with slight aversion at 120x through excellent (~6.0) skies. Another optical game I often play with the Double-Double is to see how low a magnification can be used to resolve both pairs. Interestingly, the superb optics of the three and change TV85 played this game as well as the four and change newtonian - a tribute to faint first diffraction rings...
It was also difficult to really see any difference between the views of M57, M56 & M13 between the two scopes of the evening. The ring was cleanly delineated at all frontiers through both scopes - meanwhile the half-dozen faint stars making up the "Challice" were also in evidence. Under the excellent dark sky conditions both scopes revealed the "scarab" esque qualities of the Great Hercules Cluster. To the 4.5's credit a few stellar flickers in the core on eye movement were seen. In many ways the two instruments gave views similar to what 150mm Argo reveals under half plus magnitude less transparent skies... Dylan and I were both able to resolve a dozen or so stars associated with 8.2 mag globular cluster M56. This, however was only possible on moderate aversion - whereas Argo could hold as many using direct contemplation under similar conditions.
We turned the 4.5 on Albireo as well. Dylan saw gold and pale purple - I saw gold and aqua-blue. But Dylan had been using the 10mm plossl provided with the scope. Switching to the 10mm Ultrascopic, Dylan too saw the aqua of the 5th magnitude "secondary".
Where the 4.5 had a clear advantage over the 85 was when we turned both scopes at 35x on the Veil Complex - especially the western Veil commonly called "the Witches Broom". Even without a filter the thinner northern extension could be held with moderate aversion. Adding an ultrablock filter also revealed the fan-shaped region to the south. Through the 85 it was difficult to see the northern broom-handle without extreme aversion. It would seem that when it comes to extended nebulosity there is a clear advantage to aperture.
We also turned the 4.5 on the Crescent Nebula (NGC6888) along the spine of the swan. At best - under conditions where 150mm Argo would definitively detect nebulosity - the 4.5 came up short. And so it goes - optical fervour is undone by aperture fever when it comes to faint and extended studies...
In Sum:
Irrespective of magnification the two scopes are comparable in magnitudinal reach and contrast. The shorter focus APO gives about 1/3rd wider fields for the same eyepiece set while its superior optics allow magnification to be pushed much higher than the 1/3rd greater aperture newtonian. Because of optical superiority the Double-Double gave a more pleasant view through the APO while views of most non-stellar studies (the Veil Complex) were visibly - but not significantly brighter through the 114mm newtonian. Larger planetary nebula (such as M57) are a wash. I would venture that smaller, brighter planetaries probably give finer views through the optically-perfect APO.
Certainly the fast APO gives a wider, flatter field and superior stellar aesthetics. Views of high contrast condensed studies (Double stars, the planets, the Moon, probably small planetary nebula) are enhanced whenever superb optics are turned their way. An excess of photons misdirected in their path often confuse the eye and reduce definition. The smaller scope is also advantaged by seeing - large airy disks go less disturbed by vagaries of the air than smaller ones. Looking through a smaller column of air introduces less RMS distortion than larger columns.
The TV85 is a whole scale of magnitude (richter - not stellar) more expensive than the dob-newt. To spend this kind of money wisely means to be sure the instrument purchased is of the highest possible quality and will be put to the kind of use compelling it's acquisition. For 2K it is possible to purchase a lower-quality 16 inch dob-newt - with all the accessories. The same money could outfit an observer with a eight inch Schmiddt Cassegrain or 6 inch Maksutov-Cassegrain - again with all things needful. For someone young and new to observing - such as Dylan - 200 dollars may be as hard to come by as 2K for a well-heeled adult who knows what he or she is about astronomically...
Not all of the TV85's greater cost has been sunk into the optics. OTA construction materials and assembly quality are obviously superior to the inexpensive newt. The TV85 is a fine scope and is easily admired for both its appearance and construction. This is an instrument to be ceremoniously handed down from one generation to the next of amateur astronomers. This, of course, is not the case with the dob-newt. It is after all a starter scope - one that is purchased and immediately "handed down" to the wide-eyed child in one's life to begin - rather than crown - a career of numerous nights under the beguilements of the Night Sky.