Orion V-Block Violet-Suppression Filter
Some Background V-Block on Double Stars Saturn: The Short and the Long of It In Sum: The V-Block is Good but Could do Better!
Some Background
"The V-Block works amazingly well with achromatic refractors of any aperture or focal length. Its sophisticated coatings selectively block wavelengths in the violet and blue (<450nm) end of the visible spectrum, while allowing up to 95% transmission of the other wavelengths." - http://www.telescope.com/.
Sometime in 2003, I started hearing reports about "minus-violet" filters being manufactured by Sirius Optics of Redmond Washington USA. The idea of suppressing this baneful color made sense to me. Now you ask "Why baneful?". And I answer "'Cause Violet is such a nice color, it deserves to be blended with it's buddies red and green - not hanging out all by itself as it does in the Pup and to a lesser degree in Ms Vicki...
Meanwhile, in November I began testing a Lumicon Deep-sky filter using Argo, Ms Vicki, & the Pup. As it turned out I discovered some very interesting things that you Dear Reader are free to follow for yourself at Lumicon Deep Sky Light Pollution Filter . But why not also abstract the relevant insight (and I quote):
"Cause a couple things got flagged while using Ms Vicki and the Pup for Lumi tests. Now if you recall both Ms Vicki and the Pup are achromats. That means of course that these two refractors are only designed to bring the colors red and green to a common point of focus. Both scopes have trouble handling the color violet since refractors "bend" light and each refractive element bends light differently depending on the frequency (wavelength) of the light involved. Violet of course, is a short wavelength - which means the light cone is steeper than green and much steeper than red (the longest visible wavelength to human occular perception).
So where am I heading with this? Why during daylight testing of both achromats I noticed that the sharpness of both scopes improved with the Lumi in place. Get my stellar drift??? So I picked up a minus violet "V-Block" filter from Orion..."
Now today (New Year's Eve 2004) is almost a month after that first experience with violet suppression and of course a few details remain unsaid from that original insight.
After picking up the V-Block from Orion I naturally did a daylight comparo between the Lumi and the Ori. Turning the F5 Pup achromat on a distant hillside, I noticed that the Lumi did a superior job of eliminating "purple fringe". What I saw in particular was that both filters worked well in suppressing on-axis violet fringe, but the Lumi eliminated off-axis violet as well. All this said to me that the Lumi had superior rejection band capabilities in the blue-violet region. And in comparing data sheets provided by the distributors my observation was confirmed: The V-Block suppresses roughly 80% of 400-440nms wavelength light while Lumi eliminates more than 95%.
Now you might ask: "Is this enough of a difference to prefer one filter over the other?" And to answer that question I'd say: "Not necessarily - especially when you consider that you lose all those yellow-greens with the Lumi as well". Probing further and more pointedly you might ask: "Wouldn't it be desirable for a violet suppression filter to eliminate all fringing?" And I'd say: "Well to be frank, based on the off-axis image quality comparison between the two filters I'd give a provisional "Ah, well, Sure!"
But all this doesn't mean the V-Block is not a thumbs-up acquisition. It simply means that you may find a superior alternative - but right now I couldn't tell you what that alternative is cause I've only used the V-Block...
V-Block on Double Stars
The weather these parts is typical. For a couple weeks before Christmas the Sun sinks down toward the southern horizon - but at least you can see it do so. Then Christmas arrives and we get clouds and precipitation (sometimes snow :>) which lingers for a week or two, cuts you some slack and then hangs out some more. This goes on until March when you may - or may not - get a chance to do a bit of a Messier Celebration.
("Come on jeff - get to the point, will ya?")
Oh yah the point! Well you might think that I'm about to say that there will be little forthcoming from this source concerning the V-Block filter for awhile. But that ain't necessarily so. Cause you see I made some hay while the Sun was still shining. And all that hay is stored away on AstroTalk waiting to be fed to the horses... So hear, let me log in and grab some reports...
Here we go:
"Friday I picked up a V-Block filter from Orion. Saturday night the sky remained unexpectedly clear so I grabbed the Pup and made a few observations using the filter. One thing I noticed right away is how well the V-block helped suppress the diffraction rings. In every case all stars showed a significantly fainter first diffraction ring. Obviously ring-suppression bodes well - at least in Double Star work.
The sky this evening, of course, had to be pretty stable for me to run these tests. Even with the Lyre well-outside the skies middle third I was able to turn up four tight little airy disks at 210x. Meanwhile - partially due to the poorer transparency to the northwest (say 3.5 ULM) - none of the stars showed a diffraction ring. This despite the fact that the 10.5 magnitude come could just be held direct..."
I also observed Delta Cygni on that same occasion (December 14, 2004):
"- using the ring suppressing V-Block on the Pup at 150x brought out another fixed brightning to the southwest of the bright primary. More observations are needed but Cygnus is sinking fast..."
And then there was Pi Aquilae:
On a whim I turned the Pup - at some 210x - on Pi this evening. Sky was quite stable - despite Aquila having long exited the skies middle third. Now if you've been following this thread in its entire I have not as yet - 'till this evening - been able to decisively see 6.8 magnitude Pi-B. And yes - using the filter I was quite surprised to actually detect the secondary. So it would appear that ring-suppression is an important component of resoving faintish secondaries inside the first diffraction rings of primaries.
Now to be sure this was not an easy optical feat. At some 210x and through 4.0 ULM skies a 6.8 magnitude secondary at Dawes limit presents a very small airy disk in an 80mm scope.
I also had some good views of Saturn but - let's hold off on that until tomorrow.
Saturn: The Short and the Long of It
I've got a lot cooking right now. So I kind of set this project aside. Thankfully though I got an email from Otto Piechowski as follows:
Jeff,
Two questions about the use of minus violet filter with the ST80. Does the use improve the view of the cassini division? In this small of an aperture, does the loss of light caused by the filter negate the value of the removal of color fringing and scatter?
Otto
Skinny Answer number one: "Yep." Skinny Answer 2: "Nope."
The long answers to both:
On Sunday December 12, 2004 I had a look at Saturn. Planet had not quite exited the skies middle third - pretty much on a line from Castor to Pollux but toward the south and slightly west. Stability was good - certainly 7/10. I got a decent view of Cassini at 150x. SEB apparent. Polar regions showed their usual darkening. Caught the planet's shadow on the ring system.
But there was this other thing - a certain faint purplishness about the view - especially between ring and disk. Didn't I still have that V-Block filter installed on the diagonal? Yep, d**nation! That's why I bought the thing - to get rid of that false color mucking up the view.
So right from this very first view of Saturn you can see that the V-Block from Orion doesn't quite x-out the Violet...
But does it really do anything for the view? Hard to say - but it didn't hurt! Check this out...
One week later I recorded the following observation on AstroTalk:
Had a superb view of Saturn this am through the 80mm Pup. With V-Block filter in place on the 12.3mm ED plus the 3x configured Ultrascopic and 1.8x Shorty barlow (that's a LOT of glass) for some 175x - .45mm exit pupil) and though 8/10 stability skies:
- Cassini looked about as sharp as a serviceable - but not recently sharpened - lead pencil.
- I could follow the division to within about 25 degrees of the central meridian of the ring plane anterior to the planet and about 30 degrees posterior to the same.
- The shadow of the anterior portion of the ring could be seen against the globe of the planet.
- The planets shadow could be seen delineated against the leading posterior portion of the ring (to terrestrial west).
- The Encke Minima (not division) just outside "Cor's white stripe" encircling Cassini in Ring B could be detected leading the planet - but not trailing.
- There were hints of Ring C - the dusky ring - visible - but nothing confirmable at 80mm aperture...
- I thought I could just discern the limb of the planet hanging out north of a now visibly less expansive ring presentation.
- The SEB was well delineated along both the northern (more equatorial) and southern (more polar) frontiers.
- Variations in the texture of the sub-equatorial region could be made out - especially toward the polar hood.
- A large ovacular brightening could be seen terrestrially west of the planets pole.
Finally I also noticed that as soon as Saturn got say - some 40 degrees away from the zenith the view began to degrade. One might speculate however that someone's house heater kicked in to account for it BUT I also viewee Jupe which was no more than 40 degrees above the horizon at the time (~5:00am) and all I could make out were the two main belts...
On Monday, December 20, 2004 I turned 102mm achromat "Ms Vicki" on Saturn as well. Here are my notes from AstroTalk:
Again 8/10 seeing stability skies (some wavering and jitters but otherwise very nice!).
The view of Saturn through Vicki at 150x (1.8x barlow and 12.3mm ED plus V-Block) was superior in aesthetics to the Pups of the previous morning. Everything described above but just a tad sharper. One difference however that huge white oval region lay pretty much on the CM rather than toward the leading (western) limb. Another difference - geez. Vicki is so close to resolving detail north of the SEB - pretty sure I caught a minor belt about halfway up from the SEB to the pole plus other "on the edge" detail.
Although I observed at only 150x I'm pretty sure I could have run the power up to 50-60 x per inch and the image would not have degraded significantly - so yah nice stability.
Saturn's color was very acceptable through Vicki - less of the 'ol yellar seen through the Pup the previous mornin'.
So there you go Otto - the short and the long of it...
In Sum: The V-Block is Good but Could do Better!
Really that's it. I recommend looking for something that knocks out all the Violet - especially if you have a fast achromat tweaked for good high-power performance. The V-Block's seems to be fine in slower achro's - like Ms. Vicki (F10 and 102mms aperture). Keep in mind however, that as aperture goes up slow becomes fast quickly. So if you have an F8.3 120mm you will probably want to get something stronger.
PS: If I get a good look at Jupiter in the future I may add some more to this page. But Jupe's pretty much skating the skies middle-third this apparition - so it will be hard to really make a solid comparison from past views through the achros...