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Anybody optimizing their SCT optics?

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#26 rolo

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Posted 23 May 2018 - 07:05 PM

Maybe it's juts me but IMO flocking offers no detectable advantage to the user, mostly it is good for the person selling the flocking material. I have looked through more than one SCT flocked with every imaginable material and I have never seen any difference. If anybody has seen an objective test with a measurable increase in contrast for astronomical use please share it with us. A photo of tube glare in daylight is not proof of anything useful for visual or photographic astronomical use. Why do people do this?

 

Chip 

Correct, but I like using it as spotting scope and the difference is very obvious in the daytime. At night with a few street lights around  a dew shield will be sufficient. 


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#27 Gil V

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Posted 25 May 2018 - 08:12 PM

Every telescope I get my hands on is getting at a minimum a complete tear down of the OTA.

I’m not a typical user, though. I’m no optician, but I can most surely center and align every part of a Newtonian or SCT.

Maks and refractors, not so much. I might try anyway, though.

#28 YAOG

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Posted 26 May 2018 - 10:58 AM

I suspect that is the issue with most SCT's.   The corrector plate just was not made that well.  I should check some of the ones I have with my mono light source and flat and see what they look like.    The only SCT that I have had that do reasonably well in DPAC are the C9.25's.   DaveG has been reworking the corrector of the DX8 he has for a while now.   That is what I call a labor of love.   I think what he is doing by reworking the corrector will also require reworking the secondary.  All things of magic beyond my skills right now.  

 

I suspect that is the issue with most SCT's.   The corrector plate just was not made that well.  I should check some of the ones I have with my mono light source and flat and see what they look like.    The only SCT that I have had that do reasonably well in DPAC are the C9.25's.   DaveG has been reworking the corrector of the DX8 he has for a while now.   That is what I call a labor of love.   I think what he is doing by reworking the corrector will also require reworking the secondary.  All things of magic beyond my skills right now.  

Hi Starman, 

 

I'm confused, what is DPAC? 

 

Chip



#29 macdonjh

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Posted 26 May 2018 - 12:10 PM

New corrector (much higher quality), new secundairy mirror (aspheric thus eliminating coma), flocking OTA interior and baffle tube, isolating OTA and fans behind main mirror to circulate hot air from behind the main mirror - all those steps greatly improved my Celestron C14 to a much higher level. Not cheap, but improvements are worth all the money spend.

Where ever did you get an after-market corrector for a C14?  

 

DPAC = double pass auto collimator

 

I had a C11 for several years a while ago.  When I had mine the EDGE scopes weren't out so it was all the rage to modify your SCT to take care of some of the things the EDGE design has addressed.  I did not change my optics in any way, proper collimation was enough for me to be satisfied with the quality of the view.  I didn't touch the primary mirror, either.  What ever mirror flop there was didn't bother me enough to do anything about it.  I did do the following:

  • ​Install internal dew heater, behind the corrector.  Great modification, made set-up and tear down quicker and worked like a champ.
  • ​Installed flocking on the OTA and secondary baffle.  That had no effect, and the flocking eventually came unglued from the secondary baffle and I had to remove it.  No effect (I will admit I never used my C11 as a spotting scope during daylight), but it sure looked cool.  Made the inside of the scope REALLY black.
  • Installed two 60mm fans in the rear cell.  Good modification that helped cool the scope prior to observing.  Both fans blew in; air exited through the primary baffle after blowing around the primary and up through the front of the scope.  If I needed air circulation while observing, I turned on one fan and the air escaped through the other fan.
  • Added Crayford focuser.  A good addition, I never liked the standard SCT focuser.

There was a guy, several years ago, on either Astromart or the Yahoo C14/C11/C9.25/C8 group, that was working on fully disassembling his Celestron scope (I think C14).  His goal was to center and square the primary with the mechanical axis, center and properly rotate the corrector, center and collimate the secondary.  His writing style was pretty abrasive so he didn't get much help from the community, and most everyone following the thread believed he was fixing problems that didn't exist.
 


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#30 roadi

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Posted 27 May 2018 - 04:35 AM

No issues with vignetting chief. Its probably the biggest contrats killer in daytime spotting scope mode which is the reason for. Its easily removed if causes heartburnlol.gif . Not much of a difference at night unless there's some lights around like where I live. Makes no difference from a dark site

Beautiful scope! They did it right with the paint job! cool.gif  



#31 Asbytec

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Posted 27 May 2018 - 05:25 AM

I removed my secondary baffle (it was vignetting to smaller effective aperture) then flocked the primary baffle and the visual back to remove as much chance of stray light as possible. Removing the secondary baffle did no harm when viewing the moon. 

 

https://www.cloudyni...ary-baffle-mod/



#32 Jeffmar

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Posted 28 May 2018 - 03:18 PM

I tried the flocking thing with my old c11 and ended up chipping the primary a little bit on the edge. I am pretty sure that neither the flocking nor the chipping made a visible difference in the scope. I finally learned to leave my scopes alone when they are still working well. Now days I only take my SCT's apart if the mirrors get really dusty but that is a rare occurrence. Like so many people have said before, collimation make the biggest difference. 


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#33 AxelB

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Posted 28 May 2018 - 04:52 PM

No mention yet of active cooling?
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#34 Jeffmar

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Posted 30 May 2018 - 05:33 PM

No mention yet of active cooling?

Oh yeah! That can make a big difference!



#35 YAOG

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Posted 30 May 2018 - 06:08 PM

No mention yet of active cooling?

AxelB,

 

I was thinking about internally adding a low speed 100mm fan to breakup thermal plumes that I can sometimes see. Not sure it will be effective though.

 

Chip  



#36 Astrojedi

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Posted 30 May 2018 - 09:55 PM

In my opinion there is not much to optimize in newer SCTs except collimation (at least for the ones I have from Celestron). Some folks will replace the focuser for imaging which also I have found to be optional.

I do cringe at some of the mods I see on these forums. I am unlikely to ever buy an optimized SCT on CN or AM.

#37 yellobeard

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Posted 31 May 2018 - 08:50 AM

Where ever did you get an after-market corrector for a C14?

Uhhh.. From me.. Including the aspherical secundary. I already modified many SCT's, from C5's to C14's and 2 Meade 14"
The main issue is the surface roughness on the corrector plates, you can find a reasonable one, but its a long search.

Edited by yellobeard, 31 May 2018 - 08:51 AM.

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#38 macdonjh

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Posted 31 May 2018 - 09:53 AM

No mention yet of active cooling?

Post #29, third bullet point...



#39 macdonjh

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Posted 31 May 2018 - 09:55 AM

Uhhh.. From me.. Including the aspherical secundary. I already modified many SCT's, from C5's to C14's and 2 Meade 14"
The main issue is the surface roughness on the corrector plates, you can find a reasonable one, but its a long search.

That is impressive.  I've heard about ATM making Schmidt correctors, but never read any posts about how to do it without all the equipment Celestron and Meade have.  What do you do, in general no "trade secrets", to polish the correctors you work on.  Any risk of changing the figure?



#40 Penarin

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Posted 31 May 2018 - 09:57 AM

Probably not what you are talking about, but I did some "optimizing" on my 6" SCT-

 

AstroZap flexible dew shield

Compression Ring 1.25" visual back by Blue Fireball

TV 1.25" dielectric diagonal

 

Also went with a desiccant endcap and a bulb blower to keep the front plate clean.

 

Very happy with the views through this scope.  I haven't yet worked up the nerve to crack the scope open for flocking and such...  


Edited by Penarin, 31 May 2018 - 09:57 AM.


#41 Auburn80

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Posted 31 May 2018 - 10:17 AM

Maybe it's juts me but IMO flocking offers no detectable advantage to the user, mostly it is good for the person selling the flocking material. I have looked through more than one SCT flocked with every imaginable material and I have never seen any difference. If anybody has seen an objective test with a measurable increase in contrast for astronomical use please share it with us. A photo of tube glare in daylight is not proof of anything useful for visual or photographic astronomical use. Why do people do this?

Chip


Chip, you must not have seen the glare in a C6. Go slightly off axis on Jupiter and it looks like a diamond ring formed with the planet being the diamond. Improving the primary baffle tube with paint or flocking usually helps.

Clark
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#42 Mitrovarr

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Posted 31 May 2018 - 12:03 PM

that sounds like the cost of almost what a C14 costs.


It sounds much more expensive than a C14 to me.

#43 davidc135

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Posted 31 May 2018 - 12:56 PM

That is impressive.  I've heard about ATM making Schmidt correctors, but never read any posts about how to do it without all the equipment Celestron and Meade have.  What do you do, in general no "trade secrets", to polish the correctors you work on.  Any risk of changing the figure?

A fairly recent thread in the ATM section describes the making of a 4'' corrector for a B&L 4000 to 1/4 wave accuracy. (Not 1/3 as stated in the thread). The method depends on using a powered turntable and seems a reasonable approach for ATMs up to 8'' or so aperture. A 7'' F/3 camera has been made in the past and I'm in the early stages of a 6'' corrector at the moment.

Dave Grolski has made a start on an 8'' corrector and it would be very interesting to hear more from Yellobeard (plenty of trade secrets, please).

 

David



#44 yellobeard

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Posted 31 May 2018 - 02:15 PM

What do you do, in general no "trade secrets", to polish the correctors you work on. Any risk of changing the figure?

I make complete new schmidt corrector plates, with hand picked glass without inhomogenities in refractive index. With the risk of getting the figure right!

My most challenging project was making a second new corrector plate for my 16 SCT. The first corrector is 8mm thick, and has slight impurity of refractive index, although very usable, I wanted to try to make one, only 5.7mm thin, with no impurities. It was a "no cure no harm" project, as I still enjoyed the 8mm plate. But the new thin plate turned out to be noticably better.

Next will be a 24" corrector plate for my 24" RC, which, yet to be build, has a flexible primay (as well as my 16"), and so can be converted to a SCT.

My wish to home make schmidt corrector plates already started back in 1997, when I already was in love with the SCT setup, but not happy with the general quality of the commercial aviable optics, even if I would have had the money to buy one..
In 1997, I completed my first 5" totally home build SCT.

Edited by yellobeard, 31 May 2018 - 02:36 PM.


#45 treadmarks

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Posted 31 May 2018 - 02:53 PM

In my opinion there is not much to optimize in newer SCTs except collimation (at least for the ones I have from Celestron). Some folks will replace the focuser for imaging which also I have found to be optional.

I do cringe at some of the mods I see on these forums. I am unlikely to ever buy an optimized SCT on CN or AM.

I think fans or insulation could count as reasonable optimizations too. Maybe also getting a reducer/corrector because that does improve the optics off-axis. As far as rough or misaligned correctors and such, I know of a practical method that amateurs of any skill level can handle. The technical term for this method is Return Merchandise Authorization grin.gif



#46 Jon Isaacs

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Posted 31 May 2018 - 03:03 PM

It sounds much more expensive than a C14 to me.

I make complete new schmidt corrector plates, with hand picked glass without inhomogenities in refractive index. With the risk of getting the figure right!

 

Yelowbeard:

 

How much does one of your C-14 corrector plates cost?  How much does an aspheric secondary cost ?

 

Jon



#47 rolo

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Posted 31 May 2018 - 03:14 PM

I removed my secondary baffle (it was vignetting to smaller effective aperture) then flocked the primary baffle and the visual back to remove as much chance of stray light as possible. Removing the secondary baffle did no harm when viewing the moon. 

 

https://www.cloudyni...ary-baffle-mod/

So the optical engineers put a baffle for nothing...Interesting, I tried that with an ETX125 and it was a horrific  loss of contrast from stray light.


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#48 Asbytec

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Posted 31 May 2018 - 07:37 PM

So the optical engineers put a baffle for nothing...Interesting, I tried that with an ETX125 and it was a horrific  loss of contrast from stray light.

Stray light and glare was my greatest fear. I looked at it from every angle: meniscus end, visual back, and even through the eyepiece looking at the moon on axis and off. I probed it with a laser pointer and looked up the visual back with and without the diagonal. While testing, I observed faint albedo on the dark side of the moon and just about everything I could think of including albedo across Plato's floor and saw no ill effects.

 

The secondary mirror alone is large enough to keep light rays grazing the edge of the silvered spot from reaching the focal plane directly. All rays terminate somewhere near the end of the baffle. I reduced any reflections (indirect light) by flocking and adding some baffles to eliminate any internal reflections near the exit pupil by looking at the exit pupil through a brightly lit daytime window. Yea, no problems, thankfully. I was prepared to reinstall the baffle if performance was affected, but after two years of observing I finally removed the foam attachment ring. 

 

It's not a project for the faint of heart. The only problem, which scared the bejeezus out of me at first, was allowing the marginal rays to contribute to the image. The start test was noticeably more under corrected as opposed to the /nearly perfect/ tests before. But, again, after 2 years of observing Jupiter and having a great time doing it, I decided there was no visible effect on the image. There may have been some slight improvement in the reduced diffraction effects from the larger obstruction. I am sure there is, but it's hard to observe any improvement. But, it's at full aperture over a small field of a high power eyepiece and plenty large to encompass Jupiter many times over. 

 

I am not sure what the purpose of the baffle is or was. The primary baffle is plenty tight by itself. In fact, the baffles in the older Orion were too tight. It may have been either poorly designed or purposefully designed to vignette increasing the obstruction and reduce the marginal rays. I thought about trimming back the primary baffle, but decided against it as the mod is irreversible. 

 

Best Small.jpg


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#49 rolo

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Posted 01 June 2018 - 06:17 AM

You should contact the manufacturer and tell them they don't know what they're doing. Tell them and all their scopes should have the baffle removed  so they can perform like a Questar. Also, since you really didn't notice any considerable improvement it wasn't worth the risk. Leave it alone I say. 


Edited by rolo, 01 June 2018 - 06:21 AM.

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#50 Asbytec

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Posted 01 June 2018 - 09:03 AM

Synta apparently remodeled the 150, I got the older version on closeout before they disappeared from the shelves for a few years. Complete with a solid Sky View pro mount. I observe with my hand on the legs without any vibration. Dampening time is incredibly short and mirror shift is very minor.

Yea, it was a difficult project to undertake as I liked the scope so much. Maybe so much I wanted to know more about how it worked and why it was vignetted to 140mm aperture. Yea, marginal, if any, improvement was noted. But at least it's now at full aperture with a smaller obstruction, for what difference 7% co reduction can make.

It was good as is and could have left well enough alone. I hope the new ones are better. It sounds like they are better suited for more back focus.

Edited by Asbytec, 01 June 2018 - 09:06 AM.

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