How would you get a 27% CO in an SCT?
Posted 08 April 2013 - 10:31 AM
Posted 08 April 2013 - 11:39 AM
No way around it; you cannot retrofit a current SCT to do these things; my alternative is an awful lot of hard work.
Posted 08 April 2013 - 11:59 AM
Posted 08 April 2013 - 12:59 PM
Posted 08 April 2013 - 01:30 PM
BTW Max Bray’s Mak Casses are excellent planetary scopes especially when paired with Clave Plossl eyepiece.
Posted 08 April 2013 - 02:36 PM
Optical system is similar to the earlier 250 mm SCT's I have made during the nineties. Its an entirely different design than that of a C8 which is more multi-purpose. My scope was designed primarily for planetary observations, although practice learned that it can be used for Deepsky evenly well: with a 60mm plosll you still get 63x power and a field of 0.8 degrees.
Posted 08 April 2013 - 05:46 PM
Posted 08 April 2013 - 06:11 PM
Posted 08 April 2013 - 09:55 PM
Posted 08 April 2013 - 10:34 PM
Posted 09 April 2013 - 12:26 AM
Here's a front view of one of the earlier versions of this 250 mm F/15 system. With such a small obstruction your unvignetted field is only a couple of mm. In practice you'll not be bothered by it however.
Posted 09 April 2013 - 01:53 AM
Posted 09 April 2013 - 09:45 AM
I once made a smaller Schmidt-corrector by hand (so local polishing), but this took so much time that I decided to go for vacuum.
Posted 09 April 2013 - 10:46 AM
Posted 17 April 2013 - 05:00 AM
On the SCT conversions I wonder. Would it be possible to use the existing corrector plate and figure a 2ndry to compensate for the change in magnification? I'm wondering what to do with a C9 1/4 with a broken corrector plate. It wouldn't be so bad if actual dimensions and radii etc were available.
Posted 17 April 2013 - 09:05 AM
I managed to reduce my CO by about 9% simply by removing the seconday baffle. However, as Glenn might remember, this was due to vignetting at the secondary baffle reducing effective aperture. I am not sure SCTs suffer from vignetting at the secondary baffle or any reduced effective aperture, so the additional gain with increased aperture - in addition to reduced CO diameter - might not be that dramatic.
When I found my scope was operating at a reduced aperture due to vigneting on the secondary baffle, I carefully removed it and began observing. Basically, the CO percentage improved from about 37% (52mm baffle opening/140mm Effective aperture) to 28% (42mm Secondary mirror dia/150mm full aperture.) Again, the boost in effective to full aperture helped to get this much reduction.
Upon observing, really there seemed to be an immediate impact visually on the some brightest stars (Arcturus, Alderberan and Capella.) The fifth diffraction ring was gone, it went form faintly seen to not seen. The fourth ring did appear more difficult to see in good seeing. Diffraction effects on bright lunar features were subjectively, if not objectively, reduced. Yes, I believe they are improved.
It took a good long while observing Mars and Jupiter to notice a difference. Nothing jumped out as being improvement, nothing was immediately recognizable as an improvement. However, over time I did notice my sketches were showing a bit more detail.
Star testing, however, was pretty terrible. Scary. I nearly put the secndary baffle back. Where as prior to removing the baffle, both inside and outside were very clean, high contrast, and not really the same (SA) but pretty much "textbook." This was no longer the case with the baffle missing. Outside of focus showed some pretty good residual HSA. Subsequent testing, however, I believe the system to be balanced for HSA nad this sudden appearance of blurry outside focus (only) patterns was quite normal. Some definition in the rings can be seen out to about 10 to 12 waves defocus depending on seeing. I did Ronchi test for a turned edge and could not detect one, so I believe the blurring is, indeed, residual HSA. That would be consistent with star testing, as well, and take comfort in Roland's comments on the subject of "textbook" patterns.
Again, inside focus is unchanged, clean, and nearly textbook for some SA. I can routnely do 50 - 60x/inch on Mars and doubles without before some degradation. In focus patterns still remain relatively clean. Outside, they are just ugly (and probably not a problem for SCT.) To this day, observing the moon and even the dark side of the moon, I can find no additional light in the FOV. I guess what that means is, the ugly patterns outside focus (possibly not seen in an SCT, anyway, if HSA) and removing the secndary baffle did no harm.
So, in my experience it's /possible/, I guess, to operate a CAT without the secondary baffle and not experience any ill effects of immage degradation or stray light. YMMV, of course. That might be one way to gain a few percentage points on CO percentage. As Glenn said, it might not be noticeably different at small percentages, but at near 10% it can be.
Posted 17 April 2013 - 12:38 PM
When you pull the eyepiece out and peer into the opening from as far an off-axis angle as possible, do you not see a crescent-shaped gap of direct skylight getting past the secondary and through the innermost opening of the primary baffle? The one and only purpose of the secondary baffle is to block such direct, non image forming light from reaching the focus and falling inside the FOV.
If removing the baffle does not result such a crescent of skylight being visible from the edge of the field for your widest angle eyepiece, you're lucky. Furthermore, such a case would suggest that the primary baffle (in conjunction with the secondary baffle) is either narrower than necessary or protrudes farther up into the OTA than required.
Posted 17 April 2013 - 12:50 PM
I believe one of the factors of a good planetary scope is really smooth surfaces. Maksutovs are spherical and tend to make good planetary scopes. The vacuum method certainly allows for a smooth surface in aspheric because of polishing to a sphere then the aspheric surface is made after the vacuum is released. Too bad we cannot create super smooth Newtonian aspheric surfaces by vacuum method. Don Clement
The next best thing, Don, is the flexed mirror as conceived by the late Bill Kelley and refined by Alan Adler. I don't understand why this hasn't caught-on more with the mirror-making crowd- especially with the thin blanks that are ubiquitous today. An incredibly smooth parabola is much easier to obtain.
Posted 17 April 2013 - 01:21 PM
With the diagional in place, (I'd have to verify) I dont believe I can make out the crescent. That might mean some light still reflects inside the baffle, visual back, and diagonal, but I cannot see much difference with the eyepiece in place. I did some rather crude ray tracing based on accurate measurments and found most (straight) rays just clearing the edge of the secondary 'spot' terminated at or very near the knife edge near the end of the baffle.
I also observed the dark side of the moon, reminiscent of Jon's solar filtered moon example, to evaluate stray light. I cannot see any of the moon, if memory serves, beyond the eclipsing secondary 'spot.'
When I get back and find some good observing weather, I'll re-lok at that. Or maybe just use an illuminated wall. Let me get back on that.
Posted 18 April 2013 - 02:34 PM
There is a baffle around the secondary though. And a central baffle of course, so it is entirely baffled for a 40 mm diameter field and the unvignetted field is about 10 mm. Baffles are so important in a telescope! The baffles of this scope are extremely black and this contributes to a significant boost of contrast, especially during daytime observations (but also for deepsky!). So it is not only a smaller obstruction that counts for good image quality.
Posted 18 April 2013 - 05:11 PM
Glenn's comments spurred a closer look time permitting. Can you recommend any visual tests that might show the effect of the secondary baffle being removed? The primary baffle still seems to be pretty tight.
When I first glanced at your front end, it looked as if it had no baffle. In any case, your CO does look pretty small - the same impression I get looking down my own optical tube.
Very beautiful scope, Rik. Well done.
Posted 19 April 2013 - 01:43 AM
Also, often internal reflections can easily be seen in this way, they can come from the focuser, from the diagonal, from the eyepiece, whatever. ALL these reflections should be eliminated using flockingpaper, black paint or anti-reflection thread. Its a nice job for a cloudy period ;-)
And worth the effort!
Posted 19 April 2013 - 01:53 AM
From some distance(!) you look in the eyepiece and now you should see the pupil with the obstruction. The periphery should be as black as possible.
Perfect, thank you. I've done the test using farily higher power while looking at the moon. I do not know how to interpret those results (some occasional rainbow type effect outside the pupil best I can tell.) But, the test is not how you describe, so I'll repeat it.
If memoty serves, I believe this effect was seen only or most severely with a barlow lens. I suspect a reflection somewhere with the Barlow installed and my baffle is flocked (inside.) When I get back, I'll repeat the test.
Well, I apologize if this got off topic, but this is how I reduced the CO in a comercial CAT.
Posted 19 April 2013 - 01:47 PM
For some years I've been examining the exit pupil and surroundings in just the way you recommend. For any size exit pupil, and especially the smaller ones, I use a magnifier of roughly 20-30mm f.l. This allows to examine in great detail the inside of the scope as 'seen' from the focal surface. The actual surfaces scattering light are readily discriminated. By moving the magnifier a small distance fore-aft, different depths inside are brought into focus.
Posted 19 April 2013 - 07:25 PM