Jump to content


Photo

How would you get a 27% CO in an SCT?

  • Please log in to reply
25 replies to this topic

#1 watcher

watcher

    Vanguard

  • *****
  • Posts: 2120
  • Joined: 21 Nov 2007
  • Loc: St. Louis, MO

Posted 08 April 2013 - 10:31 AM

What focal ratio would you need to make an 8" SCT to get a small (under 30%) central obstruction? Would it be similar to a MAK, around F/15? Just curious.

Thanks.

#2 ed_turco

ed_turco

    Apollo

  • *****
  • Posts: 1391
  • Joined: 29 Aug 2009
  • Loc: Lincoln, RI

Posted 08 April 2013 - 11:39 AM

You could design an SCT with a longer focal ratio, (slower taper in the light cone) like f/3, or increase the amplification factor in the secondary, which leads to a smaller diameter, (call it 5x). And you'd end up with an f/15, just like the Mak you mentioned.

No way around it; you cannot retrofit a current SCT to do these things; my alternative is an awful lot of hard work.

#3 Mike I. Jones

Mike I. Jones

    Gemini

  • *****
  • Posts: 3259
  • Joined: 02 Jul 2006
  • Loc: Fort Worth TX

Posted 08 April 2013 - 11:59 AM

And, the CO is actually driven by the light baffling, which is in turn driven by the Cass parameters, and largest unvignetted field diameter desired. The CO for a properly baffled Cass will always be larger than that due to the secondary mirror itself. It truly has to be a system-level design, not just the Cass optics by themselves.
Mike

#4 watcher

watcher

    Vanguard

  • *****
  • Posts: 2120
  • Joined: 21 Nov 2007
  • Loc: St. Louis, MO

Posted 08 April 2013 - 12:59 PM

I knew it wouldn't be any kind of retrofit. I was just curious as to if it could be done at all, and why there are no "planetary" SCTs on the market, considering the way MAKs are coveted by many. I would think that a SCT made to those specs would be cheaper to mass produce on a commercial scale than the MAKS.


Thanks.

#5 don clement

don clement

    Vendor (Clement Focuser)

  • -----
  • Vendors
  • Posts: 813
  • Joined: 02 Feb 2011
  • Loc: Running Springs, California

Posted 08 April 2013 - 01:30 PM

Max Bray came close in making his F10 Mak-Cass by incorporating a third curve for the secondary right onto the corrector as shown here:

Posted Image

BTW Max Bray’s Mak Casses are excellent planetary scopes especially when paired with Clave Plossl eyepiece.

#6 rik ter horst

rik ter horst

    Mariner 2

  • -----
  • Posts: 228
  • Joined: 01 Nov 2010
  • Loc: Zuidwolde, the Netherlands

Posted 08 April 2013 - 02:36 PM

Almost done.... My latest 250 mm F/15 Schmidt-Cassegrain with 25% linear obstruction. Tube and back plate are made of carbon, primary mirror (F/2.5) of zerodur. Need to finish the optics in the coming months.

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.
Cheers, Rik

Attached Files



#7 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 11033
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 08 April 2013 - 05:46 PM

I love the *clean* lines, Rik! At some point, a picture looking down the front end would be gratifying...

#8 gpelf

gpelf

    Ranger 4

  • -----
  • Posts: 315
  • Joined: 28 Dec 2011
  • Loc: Kentucky

Posted 08 April 2013 - 06:11 PM

A question for those in the "know". Would you be able to detect a difference from 27 to 30% ? I have heard statements going both directions.

Greg

#9 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 11033
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 08 April 2013 - 09:55 PM

I highly doubt one could at all detect the very slight additional diffraction a 30% obstruction introduces compared to one of 27%.

#10 watcher

watcher

    Vanguard

  • *****
  • Posts: 2120
  • Joined: 21 Nov 2007
  • Loc: St. Louis, MO

Posted 08 April 2013 - 10:34 PM

Outstanding! Exactly the instrument I had in mind. Wish I had your talent. I'd never buy a scope. :bow:

#11 rik ter horst

rik ter horst

    Mariner 2

  • -----
  • Posts: 228
  • Joined: 01 Nov 2010
  • Loc: Zuidwolde, the Netherlands

Posted 09 April 2013 - 12:26 AM

Thanks Joe and Glenn!
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.

Attached Files



#12 don clement

don clement

    Vendor (Clement Focuser)

  • -----
  • Vendors
  • Posts: 813
  • Joined: 02 Feb 2011
  • Loc: Running Springs, California

Posted 09 April 2013 - 01:53 AM

Did you use the vacuum method for the corrector?

Don Clement

#13 rik ter horst

rik ter horst

    Mariner 2

  • -----
  • Posts: 228
  • Joined: 01 Nov 2010
  • Loc: Zuidwolde, the Netherlands

Posted 09 April 2013 - 09:45 AM

Yes Don, I use a vacuum method. Great invention of Mr. Schmidt!
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.

#14 don clement

don clement

    Vendor (Clement Focuser)

  • -----
  • Vendors
  • Posts: 813
  • Joined: 02 Feb 2011
  • Loc: Running Springs, California

Posted 09 April 2013 - 10:46 AM

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. However one of the best planetary view I have ever seen was a 10” Cave Newt many years ago down in Long Beach.

Don Clement

#15 Ajohn

Ajohn

    Mariner 2

  • *****
  • Posts: 283
  • Joined: 03 Dec 2007

Posted 17 April 2013 - 05:00 AM

Thanks for the deepsky comment Rik. I reasoned that this sort of F ratio could be used for deep sky and have come up with a pure cassegrain design that is essentially perfect across a 0.6 total degree field. It's F3 to F14.5 and uses a wynnish corrector using 3 bought lenses and one that has to be made. Flat field too :D Can I make it - don't know but I am going to try. First hurdle is an F3 parab. 2nd the 2dry and so on.

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.

John
-

#16 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 8438
  • Joined: 08 Aug 2007
  • Loc: La Union, PI

Posted 17 April 2013 - 09:05 AM

Hey, the front end of my MAK looks like Rik's. There is a certain beauty to it...no baffle (or at least a very small one not easily seen.) :)

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.

#17 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 11033
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 17 April 2013 - 12:38 PM

Norme,
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.

#18 Chuck Hards

Chuck Hards

    Fly Me to the Moon

  • *****
  • Moderators
  • Posts: 6122
  • Joined: 03 May 2010
  • Loc: The Great Basin

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.

#19 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 8438
  • Joined: 08 Aug 2007
  • Loc: La Union, PI

Posted 17 April 2013 - 01:21 PM

Glenn, yes I can see the cresent very near the edge of the optical back. I did not see that prior, however. Yes, I was concerned about that very problem.

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.

#20 rik ter horst

rik ter horst

    Mariner 2

  • -----
  • Posts: 228
  • Joined: 01 Nov 2010
  • Loc: Zuidwolde, the Netherlands

Posted 18 April 2013 - 02:34 PM

"Hey, the front end of my MAK looks like Rik's. There is a certain beauty to it...no baffle (or at least a very small one not easily seen.)  "

Hi Norme,

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.
Cheers, Rik

#21 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 8438
  • Joined: 08 Aug 2007
  • Loc: La Union, PI

Posted 18 April 2013 - 05:11 PM

Thank you, Rik. Surely you are correct. The OP was how to get a smaller CO with SCT, and I tossed out that idea (not recommending it because there are risks) but wondering about it. In my case, I cant see any visual (planetary, lunar, and visual inspection) difference with the secondary removed.

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.

#22 rik ter horst

rik ter horst

    Mariner 2

  • -----
  • Posts: 228
  • Joined: 01 Nov 2010
  • Loc: Zuidwolde, the Netherlands

Posted 19 April 2013 - 01:43 AM

In order to check a baffle system I do the following: Put an eyepiece in the focuser that gives you lowest magnification and largest field (2 inch 60mm Plosll for example) and point your scope to the (day-) sky. 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. Now move your head sidewards untill the pupil disappears. Untill its entirely gone you shouldn't see any light arc around the pupil. If you dó see an arc, the baffles are not properly dimensioned and you can be sure this will affect contrast.
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!

#23 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 8438
  • Joined: 08 Aug 2007
  • Loc: La Union, PI

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.

#24 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 11033
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 19 April 2013 - 01:47 PM

Rik,
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.

#25 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 8438
  • Joined: 08 Aug 2007
  • Loc: La Union, PI

Posted 19 April 2013 - 07:25 PM

Glenn, that's interesting, too. So, you can actually look into the scopes mechanical parts using a magnifying glass?






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics