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watcher
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Reged: 11/21/07

Loc: St. Louis, MO
How would you get a 27% CO in an SCT?
      #5785737 - 04/08/13 11:31 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.


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ed_turco
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Reged: 08/29/09

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Re: How would you get a 27% CO in an SCT? new [Re: watcher]
      #5785890 - 04/08/13 12:39 PM

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.


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Mike I. Jones
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Re: How would you get a 27% CO in an SCT? new [Re: ed_turco]
      #5785949 - 04/08/13 12:59 PM

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


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watcher
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Reged: 11/21/07

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Re: How would you get a 27% CO in an SCT? new [Re: Mike I. Jones]
      #5786105 - 04/08/13 01:59 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.


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don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: How would you get a 27% CO in an SCT? new [Re: watcher]
      #5786153 - 04/08/13 02:30 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:



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

Edited by don clement (04/08/13 04:25 PM)


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rik ter horst
sage


Reged: 11/01/10

Loc: Ewer, the Netherlands
Re: How would you get a 27% CO in an SCT? new [Re: don clement]
      #5786232 - 04/08/13 03:36 PM Attachment (57 downloads)

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

Edited by rik ter horst (04/08/13 03:45 PM)


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GlennLeDrew
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Re: How would you get a 27% CO in an SCT? new [Re: rik ter horst]
      #5786622 - 04/08/13 06:46 PM

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

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gpelf
sage


Reged: 12/28/11

Loc: Kentucky
Re: How would you get a 27% CO in an SCT? new [Re: GlennLeDrew]
      #5786657 - 04/08/13 07:11 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


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GlennLeDrew
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Re: How would you get a 27% CO in an SCT? new [Re: gpelf]
      #5787107 - 04/08/13 10:55 PM

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

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watcher
Carpal Tunnel
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Reged: 11/21/07

Loc: St. Louis, MO
Re: How would you get a 27% CO in an SCT? new [Re: rik ter horst]
      #5787158 - 04/08/13 11:34 PM

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

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rik ter horst
sage


Reged: 11/01/10

Loc: Ewer, the Netherlands
Re: How would you get a 27% CO in an SCT? new [Re: watcher]
      #5787236 - 04/09/13 01:26 AM Attachment (38 downloads)

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.


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don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: How would you get a 27% CO in an SCT? new [Re: rik ter horst]
      #5787308 - 04/09/13 02:53 AM

Did you use the vacuum method for the corrector?

Don Clement


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rik ter horst
sage


Reged: 11/01/10

Loc: Ewer, the Netherlands
Re: How would you get a 27% CO in an SCT? new [Re: don clement]
      #5787794 - 04/09/13 10:45 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.


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don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: How would you get a 27% CO in an SCT? [Re: rik ter horst]
      #5787883 - 04/09/13 11:46 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


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Ajohn
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Reged: 12/03/07

Re: How would you get a 27% CO in an SCT? new [Re: don clement]
      #5803970 - 04/17/13 06:00 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 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
-


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: How would you get a 27% CO in an SCT? new [Re: Ajohn]
      #5804238 - 04/17/13 10:05 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.

Edited by Asbytec (04/17/13 10:59 PM)


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GlennLeDrew
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Re: How would you get a 27% CO in an SCT? new [Re: Asbytec]
      #5804568 - 04/17/13 01:38 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.


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Chuck Hards
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Re: How would you get a 27% CO in an SCT? new [Re: don clement]
      #5804592 - 04/17/13 01:50 PM

Quote:

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.


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: How would you get a 27% CO in an SCT? new [Re: GlennLeDrew]
      #5804652 - 04/17/13 02:21 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.


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rik ter horst
sage


Reged: 11/01/10

Loc: Ewer, the Netherlands
Re: How would you get a 27% CO in an SCT? [Re: Asbytec]
      #5806949 - 04/18/13 03:34 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


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