Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Anybody optimizing their SCT optics?

  • Please log in to reply
81 replies to this topic

#51 wfj

wfj

    Surveyor 1

  • *****
  • Posts: 1667
  • Joined: 10 Jan 2008
  • Loc: California, Santa Cruz County

Posted 02 June 2018 - 06:21 PM

Norme's remarks actually address Rolo's issue. He isn't suggesting a rush to rash action, but a methodical process as an informed experimenter.

 

He might be seeing some improvement in his planetary sketches, which are quite good considering the limited aperture he's using. But he's careful to not "over claim" without conclusive results, as a careful observer should.

 

The baffles are there to prevent "sky flooding" that Rolo alludes to on his ETX experiment. I can attest to this myself having picked up unbaffled ATM cassegrain "attempts" that were ill considered.

 

They are also present to reduce contrast loss for high ambient light conditions like middle of the day terrestrial viewing.

 

But also they are present to the degree of "manufacturability" of a scope, like on the f/11 C90, who's truncated primary baffle is a cost compromise. It's "flat face" gets illuminated by bright targets, and the backscatter reflects off the secondary reducing contrast, as well as other effects. I suspect that the "good enough" short baffle meant that assembly of the top/bottom halves wouldn't have to risk collision with the baffle tube, as a manufacturing loss issue.

 

The Questar thin, long, conical approach is an optimization (this thread is about optimization, not about selling benefits of optimization). If you take any cassegrain and examine how the baffles work (and don't work) on bright objects, some of them work quite differently than others. Sometimes this may be transferable, other times not. (I've been experimenting with 3D printed alternative baffles too.)

 

And like Rolo/Norme's two sides of the same "is it useful or nonsense" concern, an objective, quantitative measure is necessary. You'd like to be able to quantify scattering, diffraction, and correction changes to assess various optimization (I'll add a fourth: High order Spherical Abberation - HSA, or above the first five Siedel aberrations (google "The Five Seidel Aberrations" for more)). This is harder than it sounds to reproducibly measure (some designs, like the C9.25's slower primary, lead to less HSA).

 

However, manufacturers put varying degrees of work into optimizing designs for cost or performance, as well as consistent quality control/assurance. Same is true for ATM and DIY improvements.

 

As to why manufacturers wouldn't add an improvement ... Norme's "need" is very narrow - one scope, a narrow set of equipment he uses, at limited certain BFL's, on a limited set of targets, where he's never going to ask for a manufacturer return. The manufacturer sells to a much broader range of these with its large customer base, with a significant number of returns even in the most perfect cases. The two don't compare - once a manufacturer "fixes" on a design instance to be manufactured, that's it for the most part.

 

Should others rush out and cut off baffles? No, because the manufacturer addressed their need already. Should experimenters like Norme "learn" the bounds of a scope carefully? Sure - as long as they are honest with themselves about objective criteria to measure improvement/loss. And have a reversible "plan of action", because most experiments fail.

 

(By the way, Rolo has some of the best images I've ever seen from common SCTs - that speaks for itself as to the effectiveness of a manufacturers choice in design.) Something to measure against any "optimizations" - you'd need to exceed that quality. Yet also, one can increase contrast with sampling and scale expansion with a sensor, while with the eye it's a different game as well.

 

add:

If we could accurately measure as mentioned above, good scientific method would likely be to apply the improvement to 5 or 7 prior "un-improved" scopes, then take  5-7 already optimized scopes and remove the improvement to resemble the "un-improved" ones. From this we'd look for the mode/mean of results and present error bars on them to conclude effect, possibly with images/sketches to communicate said effect in a useful way.

 

Even manufacturers don't often do this.


Edited by wfj, 02 June 2018 - 06:33 PM.

  • Asbytec likes this

#52 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 14807
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 02 June 2018 - 07:37 PM

wfj, you said a lot above. Great comment. Where to start...

 

There is probably little need to optimize a commercial scope, surely they are fabricated with some amount of thought going into the design. How much or to what end is anyone's guess. And that's the thing about optimization, we need some sort of objective measure to determine how effective it is and whether it's worth it. I just did not know how to do that, measure the peak intensity of the central disc, for example. Maybe metaguide, but my scope is undriven and no laptop to perform the test in the field. The best I have is before and after mental images. 

 

My Jupiter observations seemed more productive, but that could well have been written off to gaining more experience observing the planet at a slightly higher magnification. I mean, it was not like the mod slapped you in the face with a noticeable improvement. Or with major problems. The best I can say is maybe a tiny bit less diffraction artifact on the lunar limb. It appeared not quite as pronounced. Another was the 5th diffraction ring visible on very bright stars in steady seeing /may/ have been somewhat reduced. Not sure where that tiny bit of light would have gone, maybe into the first ring or maybe into the central disc. But, good luck measuring any small change in apparent brightness of either. Besides, that 5th ring, if improved, probably had such a small impact on contrast transfer that any visual difference probably won't be seen.

 

What may be observable, but not really significant IMO, is the 7% reduction in the obstruction from 37% at a reduced effective aperture to ~30% at full aperture. This gain may have been offset, to some extent, by a slight increase in primary spherical aberration (from the properly sized primary mirror) along with any correction offered by the meniscus. Gaining the additional 10mm aperture was also not earth shattering. But, it is pleasing to know the scope operates as good as it ever did without the troublesome thought it was being vignetted for whatever design reason the manufacturer decided was important. I suspect they decided to make the meniscus invisible from the visual back with a tight baffle (they succeeded) or to "please the star test crowd." Someone once said the baffles on this commercial MCT were not as tight as an Intes design. I disagree, they were very effective. Too tight, more than was needed, apparently. You absolutely couldn't see the meniscus from anywhere at the visual back. 

 

I did a lot of testing (and observing). I watched the moon's light and how it behaved on both the secondary and primary baffle at different positions in the FOV. You could actually see the moon illuminate the outside of the primary and secondary baffle as it slewed across the FOV and get a feel for how much the field was vignetted with a large bright object. Ran a ruler down the front of the meniscus to see where it protruded into the light path, did the collimated laser "flashlight" test. Took the scope apart and measured everything, including the primary diameter at 162mm, the primary baffle opening at 26mm (going from memory.) Everything, including the distance between the primary and secondary baffle openings and their diameters along with the diameter of the silvered spot. I did ray traces of light rays from every angle, and even tried to figure out a way to cut the baffle back a little without removing it. The whole nine yards before deciding to *try* the modification. 

 

I agree, though, gosh, such a mod is not for the faint of heart. I was very faint hearted about it, but determined to explore the vignetting issue. I left the mod in such a condition for two years that it could be reversed. During those two years I never ran across a reason to reverse it. So, I hoped the scope would perform, as marginal as it may be, with a slightly smaller, more optimized obstruction. My guess is it does. When one crunches the numbers on the effect of the obstruction, I am hoping the final image on the focal plane is consistent with a middling RMS and modest obstruction bringing the theoretical "numbers" to the level of diffraction limited performance. So, with or without much visual difference either way, I left the mod in place.

 

Images of the obstruction shadow on either side of focus show a ratio near 80%, which according to Suiter is about 1/6 PVW of pure primary spherical aberration (not including the correction of the meniscus). The shadow breakout, prior to the mod and with a slightly larger obstruction, was between 1.5 and 2 to 1, also consistent with good correction with star test images not unlike Suiter's images of pure LSA with a 33% obstruction. The star test can be confusing because it looked nearly perfect prior to the mod and scarey afterward. I almost reinstalled the baffle right then, right after my heart sunk. But, decided to stay the course for a while and see what is up with that. After a long while, nothing bugged me either way.

 

In the meantime, the Orion 150 was off the shelf. I hope they addressed the design issues to reduce vignetting and offer a good scope at full aperture. I plan not to have any seller's remorse with this scope after 7 years of faithful service. So, the mod stays. I still have the baffle, though. I run across it every now and then. :)


  • wfj likes this

#53 Edd Weninger

Edd Weninger

    Apollo

  • -----
  • Posts: 1058
  • Joined: 16 Feb 2014

Posted 02 June 2018 - 10:47 PM

The Questar 3.5 has a baffle spot slightly larger than the mirrored secondary spot on the meniscus.  This is because additional baffling is needed during daytime use.   A few people have removed the baffle spot and reported a slight improvement under night time conditions.  You might be seeing something similar.  A ray trace in OSLO would show this.

 

Most owners have not done this because, like me, we use Questars in daylight viewing nature.  


  • Asbytec and wfj like this

#54 yellobeard

yellobeard

    Mariner 2

  • *****
  • Posts: 208
  • Joined: 11 Sep 2013
  • Loc: Netherlands

Posted 03 June 2018 - 02:55 AM

I think that 'the need for optical modification' of a SCT also is pressured by the 'afraidness' of owners and the more complex optical system.
In most cases, one must have some higher optical manufacturing skills, to be able to think of mods that really add a lot to the image quality. Lets face it: How many on this world make their own optics for their home build newtonians, and how many do the same for SCT's? I think that there is a big chance that the Netherlands is the no1 country as it comes to the amount of people that are capable of successfully make good SCT optics, and, correct my if I'm wrong, that number of people stops at 3.
Therefore, most SCT 'lovers' do not know much about the possebilities to (with a lot of effort!) make a SCT that performes much better than the commercial models.

Much better as in:
-All optics repolished for higher optical quality and low surface roughness.
-Additional flocking and baffling to fully preserve contrast: Many point at the size of the central obstruction as the 'bad guy' as it comes to fine contrast, but they mostly forget the huge importance of flocking and baffling!
-Scope sufficiently insulated to bring both optics and internal air at ease, preventing deformations in the optics by cooling down too fast, and preventing internal tube currents
-Ultra thin secundary including very low weight of secundary mount, to make them easily follow tempetature changes (not causing heat plumes!), my F/15 secundary (16" SCT) including mount measures 150 grammes.
-Parabolizing (undercorrected) of the primary mirror: The bigger SCT's suffer from spherochromatic aberrations due to the power of the schmidt corrector plate, with the primary some 50-70 % parabolic, spherochromatic aberrations are way less.

I am fully aware, that with all things I just mentioned, you probably feel forced to make a SCT totally from sctratch.
But I experienced, that great improvements already can made to commercial SCT's, by making new (parabolic) secundaries and new schmidt corrector plates, which have noticably less surface roughness (very important for highest possible contrast)
But then again, how many people can make perfect schmidt corrector plates?...
  • wfj likes this

#55 BKSo

BKSo

    Mariner 2

  • -----
  • Posts: 231
  • Joined: 08 Dec 2015

Posted 03 June 2018 - 04:42 AM

We know the spherical aberration of an SCT changes with focus plane position. So anyone parfocalize to the position with minimal SA?



#56 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 76049
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 03 June 2018 - 05:06 AM

I think that there is a big chance that the Netherlands is the no1 country as it comes to the amount of people that are capable of successfully make good SCT optics, and, correct my if I'm wrong, that number of people stops at 3.

 

There's you, there's Rik and then whose the third person??  And Rik, as I understand it, is a professional optician so I am not sure he counts.. :)

 

When conversing with Rik, I commented that ATMing is popular in regions where observing opportunities are few and far between.  From what I gather, that probably describes the Netherlands.. 

 

Jon


  • gnowellsct likes this

#57 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 14807
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 03 June 2018 - 06:15 AM

But then again, how many people can make perfect schmidt corrector plates?...


Occasionally we run across an ATMer in the ATM and Classic forums who are refurbishing corrector plates. Wonder how much of a market would exist for such a skill.

#58 yellobeard

yellobeard

    Mariner 2

  • *****
  • Posts: 208
  • Joined: 11 Sep 2013
  • Loc: Netherlands

Posted 03 June 2018 - 08:21 AM

There's you, there's Rik and then whose the third person?? And Rik, as I understand it, is a professional optician so I am not sure he counts.. :)

When conversing with Rik, I commented that ATMing is popular in regions where observing opportunities are few and far between. From what I gather, that probably describes the Netherlands..

Jon


Ok, but then I don't count too, as I'm a professional optician too..
The 3rd person would be Hans Dekker, who is a retired professional optician..

So, turns out to be that there are no skilled amateur optical specialists who can make schmidt corrector plates from scratch.
  • Jon Isaacs, macdonjh and gnowellsct like this

#59 yellobeard

yellobeard

    Mariner 2

  • *****
  • Posts: 208
  • Joined: 11 Sep 2013
  • Loc: Netherlands

Posted 03 June 2018 - 08:29 AM

Occasionally we run across an ATMer in the ATM and Classic forums who are refurbishing corrector plates. Wonder how much of a market would exist for such a skill.


I'm very interested in reading about those people who refurbish plates, do you perhaps have links?
Only things I found on the web is an explanation of how you can manufacture schmidt corrector plates, using the vacuum method, but they never go into detail on the small things u need to know before you can ever have a schmidt plate of acceptable quality.

#60 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 14807
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 03 June 2018 - 08:44 AM

I'm very interested in reading about those people who refurbish plates, do you perhaps have links?
Only things I found on the web is an explanation of how you can manufacture schmidt corrector plates, using the vacuum method, but they never go into detail on the small things u need to know before you can ever have a schmidt plate of acceptable quality.

Let me see if I can find one. As I recall one guy was refurbishing an old Dynamax 8 and, I believe and unless I'm mistaken, was working the corrector. It was in the Classic Scope forum a while back.

Here: https://www.cloudyni...midt-corrector/

Edited by Asbytec, 03 June 2018 - 09:10 AM.

  • Jon Isaacs likes this

#61 wfj

wfj

    Surveyor 1

  • *****
  • Posts: 1667
  • Joined: 10 Jan 2008
  • Loc: California, Santa Cruz County

Posted 03 June 2018 - 12:24 PM

Lets face it: How many on this world make their own optics for their home build newtonians, and how many do the same for SCT's?

I did (with my dad) when I was twelve (Icon is pic of scope with that mirror, and teaching my son who inherited that scope - he's 25 now, and was looking at Jupiter just last night). Did another by myself the following year, made a 5" corrector plate (vaccuum method) at 16, and a 2.5" MCT at 18 by repurposing microscope illuminator Magin mirrors. (I don't post in ATM/DIY threads due to horrible experiences about a decade ago, and have learned to avoid posting on optics testing anywhere.) I quietly work on experiments, mostly using OSLO to check out abstract mathematic designs I rarely fabricate.

 

I understand very much that many take optics as kind of a "blood sport", which drives some away as well. No sense of humor?


  • davidc135 likes this

#62 yellobeard

yellobeard

    Mariner 2

  • *****
  • Posts: 208
  • Joined: 11 Sep 2013
  • Loc: Netherlands

Posted 03 June 2018 - 01:11 PM

Hi wfj, very nice to hear that you also made SCT's and MCT"s.
I also sometimes have quite severe fights on forums, but I feel positive energy from lots of forum members who want to learn from others, and they make me to continue writing posts about my projects, and answer questions they have.

So you are one of the very few who successfully make SCT's.. Be proud of it!
  • Jon Isaacs, wfj and davidc135 like this

#63 davidc135

davidc135

    Viking 1

  • -----
  • Posts: 621
  • Joined: 28 May 2014
  • Loc: Wales, UK

Posted 03 June 2018 - 02:13 PM

I think so few have succeeded in making corrector plates because so few have tried. You need enough enthusiasm and persistence to be up to tackling a Newtonian. With a plan and a bit of research there's a good chance of achieving a creditable 1/4 wave standard or better depending on aptitude and experience.   David


  • Jon Isaacs and wfj like this

#64 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 76049
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 03 June 2018 - 02:34 PM

I think so few have succeeded in making corrector plates because so few have tried. You need enough enthusiasm and persistence to be up to tackling a Newtonian. With a plan and a bit of research there's a good chance of achieving a creditable 1/4 wave standard or better depending on aptitude and experience.   David

 

waytogo.gif

 

https://www.cloudyni...dt-camera-r2897

 

An interesting read , an article by David on a 7 inch F/3 Schmidt camera .

 

Jon


  • davidc135 likes this

#65 YAOG

YAOG

    Viking 1

  • *****
  • topic starter
  • Posts: 992
  • Joined: 12 Oct 2015
  • Loc: SoCal of course!

Posted 03 June 2018 - 05:02 PM

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

Hi Clark,

 

I have never seen any glare in a C6 when observing in the dark and I have observed with C6s that were not flocked and one that was flocked and even the DIY owner said it was a waste of time and money. The owner tried but could not see any improvement of any kind and certainly not on the order you mention. This is pretty much what I have experienced with the flocked SCTs I've seen. I mean I get that people like the idea of flocking but so far having seen no discernible improvement which in practical use has not appeared makes this more about style than function at least IMO anyway. I have seen some people claim otherwise but not generally when used in the dark. Of course a lot of these SCTs have been around a long time and at some point some budget models may not have had effective AR coatings or any coatings for that matter. This could explain why some people have claimed to have seen flocking making a difference whereas the majority have not. 

 

Chip 



#66 Auburn80

Auburn80

    Viking 1

  • *****
  • Posts: 652
  • Joined: 01 Apr 2013

Posted 04 June 2018 - 10:16 AM

I've had several SCTs over the years and had views through a number of others. None of those suffered from poor stray light control . . .until I got my C6 last year. Others have had the same problem in the last few years with the C6 and Celestron has no cure. Luck of the draw.

Clark

#67 Mitrovarr

Mitrovarr

    Vanguard

  • -----
  • Posts: 2405
  • Joined: 12 Sep 2004
  • Loc: Boise, Idaho

Posted 06 June 2018 - 12:11 PM

I think some of the benefit people get from whole-tube flocking of SCTs is because the flocking paper acts as insulation, particularly if there's a small air-gap.
  • Jon Isaacs likes this

#68 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 14807
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 06 June 2018 - 07:09 PM

I think some of the benefit people get from whole-tube flocking of SCTs is because the flocking paper acts as insulation, particularly if there's a small air-gap.

On flocking, I tend to agree there is no discernable difference at the eyepiece in my experience when viewing at night. However, when you look up the baffle from the visual back at a bright object, the primary baffle interior is lit up pretty good. So, some flocking won't hurt, it deadens the interior of the primary baffle and does make a noticeable difference in that sense. Others have benefitted from flocking the primary baffle reducing artifacts. It's a nice safeguard to have in place primarily for brighter objects. 

 

I flocked the interior of the tube, even though we should not have to. It's pretty tough for light to bounce around the interior of the tube and make its way down the primary baffle. But, I left it installed, anyway, as a feel good measure. In our modest ambient temperatures, I have not noticed any improvement with the insulation effect. But it may be present. 

 

One important consideration when flocking is to identify bright sources of internal reflections by pointing the scope out a brightly lit window and identifying (especially) reflections close to the exit pupil of a low power eyepiece. Some reflections will be found in the visual back. You can identify and deaded (all of) those reflections. It seems to help with bright objects, but deep sky there is just not enough light, direct or otherwise, to cause any problems. Unless you're observing with a streetlight nearly on axis. 

 

YMMV


Edited by Asbytec, 06 June 2018 - 07:10 PM.


#69 YAOG

YAOG

    Viking 1

  • *****
  • topic starter
  • Posts: 992
  • Joined: 12 Oct 2015
  • Loc: SoCal of course!

Posted 06 June 2018 - 07:46 PM

From the replies in the thread it seems like not many people bother to check and correct their SCTs for mechanical and optical alignment before secondary collimation and just hope the parts are all lined up. A lot of the SCTs out there are getting pretty old and based on what I have seen of the materials used by Meade and Celestron to mechanically retain the glass bits in the optical tubes many if not most scopes older than 10-12 years would probably benefit from replacement with better materials and adjustment to restore the correct relationships between the primary, corector and secondary relative to the baffle tube and to confirm that the primary is pointed at the center of the secondary. Performing this work did wonders for my dumpster grade Meade LX200 10" f/6.3 OTA but I had to research how SCT optics work, build some fixtures and about $400 of Howie Glatter laser gear and spend a lot of time scratching my head to find a solution. But now I have an optically outstanding M10" f/6.3 SCT OTA. 

 

Chip


  • davidc135 likes this

#70 Mitrovarr

Mitrovarr

    Vanguard

  • -----
  • Posts: 2405
  • Joined: 12 Sep 2004
  • Loc: Boise, Idaho

Posted 07 June 2018 - 10:18 AM

I wish optimizing would help my Meade 10". It suffers from distinctly different intrafocal/extrafocal images and image softness at high power. Sadly, I think that means it's undercorrected, which I dont think you can optimize your way out of.

Fortunately, it's not dreadful or anything, and is a great deep sky scope and a tolerable planetary one. I do wish it live up to its planetary potential, particularly since I'm starting planetary AP now. I actually had better results with my C6 (which has optics that are at least good and might be better).

#71 YAOG

YAOG

    Viking 1

  • *****
  • topic starter
  • Posts: 992
  • Joined: 12 Oct 2015
  • Loc: SoCal of course!

Posted 07 June 2018 - 02:51 PM

I wish optimizing would help my Meade 10". It suffers from distinctly different intrafocal/extrafocal images and image softness at high power. Sadly, I think that means it's undercorrected, which I dont think you can optimize your way out of.

Fortunately, it's not dreadful or anything, and is a great deep sky scope and a tolerable planetary one. I do wish it live up to its planetary potential, particularly since I'm starting planetary AP now. I actually had better results with my C6 (which has optics that are at least good and might be better).

Hi Mitrovarr,

 

What have you done to assess your OTA's optical alignment? My Meade LX200 Classic 10" f/6.3 SCT OTA at one point during my adventure into optimizing the OTA displayed every possible optical error including what I originally thought was poorly corrected optics. Once fully adjusted this M10" f/6.3 OTA now displays a nearly perfect star test with smooth balanced and centered rings both in and outside focus. I don't know if you have star tested a Chinese made Celestron scope but starting with the Tri-Lobe CPC and later OTAs almost all will star test virtually perfect. My 2002 production Meade 10" f/6.3 optics look just like the Chinese Celestron star tests. It may be that your SCT's optical softness may not be the glass parts but optical alignment. 

 

I've heard a lot of Meade and Celestron SCTs owners complain about having soft high mag images at the eyepiece and have popped in one of my own eyepieces and found them to be okay for the conditions. I think people don't realize how much every part of the optical chain influences the image seen at the eyepiece. People often don't consider atmospheric and light pollution conditions at their location which hurts sharpness and more importantly, contrast, and that image quality is always relative to the seeing conditions. The larger the mirror or objective the greater the influence the air has over a scope at the eyepiece. With a good M10" f/10 that is well adjusted and collimated you'll need excellent seeing conditions and a pretty dark sky to take advantage of the even part of the resolution potential of a mirror this size. In my experience a bigger scope is only better if you have a perfect scope and the skies to support it and don't forget the diagonal and eyepieces, they are huge! 

 

Our club's property is typically Bortle 4 sky and generally above average seeing and transparency. My bespoke Parallax/AT130mm apo utilizing all Ohara mating glasses and an FPL-53 center element will generally put up better views of solar system objects than my optically excellent M10 f/6.3 or Celestron NexStar 11 SCT OTAs. But obviously it's not resolution that's the source of these better views but this unique refractor's higher contrast that allows deeper more detailed observation with a smaller 130mm apo. But when the air is perfect the big SCTs for fleeting moments display vastly superior views to the 130mm apo just to tease me. 

 

Chip Louie


  • gnowellsct likes this

#72 Mitrovarr

Mitrovarr

    Vanguard

  • -----
  • Posts: 2405
  • Joined: 12 Sep 2004
  • Loc: Boise, Idaho

Posted 07 June 2018 - 04:10 PM

I haven't done anything but collimating it. I don't have all of the advanced calipers, etc. needed to really check the whole optical train, although if you have any suggestions I'm happy to hear them.

I don't think it's just seeing or failure to cool down. First of all, the secondary shadow is very different in size inside and outside of focus, which is a pretty bad sign. Secondly, I generally know how to recognize both bad seeing and tube currents. I could be wrong, but it doesn't look like it. Finally, I also have a 10 and 12" dob and I haven't come away with the impression they have bad optics - I think both are actually quite good. So I have seen better large telescopes too.

Oh, and my C6 is a later model Chinese made scope. It does have very good optics.

Edited by Mitrovarr, 07 June 2018 - 04:11 PM.


#73 wky46

wky46

    Vanguard

  • *****
  • Posts: 2438
  • Joined: 12 Dec 2005
  • Loc: West central, Ky.

Posted 07 June 2018 - 04:21 PM

Like other posters, all I ever did was clean the outside of the corrector every few yrs. and kept it collimated using Bobs Knobs

 

But this time I took the plunge and just finished cleaning both the corrector and primary. Cotton balls, Dawn, warm tap water and a good rinse with distilled water. Left it alone after the distilled water rinse. Lined up the corrector with the factory applied marks. Absolutely clear and beautiful. No streaks or cloudy. 

 

Mold was forming on inside edge of lens and looked like spider webs and other light crud on the primary.

 

Ants made a home in there. I’ve always kept it outside but covered well

The scope is a good 25 yrs old

 

Can’t wait to get it collimated and out again under the stars.

 

*what the heck, gonna be hazy tonight but still gonna get it tweaked tonight

feels like I’ve got a new scope! Glass sure looks new, anyway


Edited by wky46, 07 June 2018 - 04:50 PM.


#74 Peter Besenbruch

Peter Besenbruch

    Fly Me to the Moon

  • *****
  • Posts: 6883
  • Joined: 21 Aug 2014
  • Loc: Oahu

Posted 07 June 2018 - 05:28 PM

I've done a little optimizing, but not to an SCT. Still, some of the things I have done apply to them, too.

 

I insulated the tube. No more cool down.

 

I bought an Astrozap dew shield, and added a layer of insulation to that, too. The insulation keeps it rounder. wink.gif

 

I use a good quality diagonal when viewing the moon and planets at high power. My latest acquisition is a Baader diagonal with a fine focuser. I'm finding it helpful.

 

Finally, I got a Baader Clicklock visual back. It does zip for the optics, but it makes diagonal rotation easy when using an equatorial mount.


Edited by Peter Besenbruch, 07 June 2018 - 05:29 PM.


#75 YAOG

YAOG

    Viking 1

  • *****
  • topic starter
  • Posts: 992
  • Joined: 12 Oct 2015
  • Loc: SoCal of course!

Posted 07 June 2018 - 06:24 PM

I haven't done anything but collimating it. I don't have all of the advanced calipers, etc. needed to really check the whole optical train, although if you have any suggestions I'm happy to hear them.

I don't think it's just seeing or failure to cool down. First of all, the secondary shadow is very different in size inside and outside of focus, which is a pretty bad sign. Secondly, I generally know how to recognize both bad seeing and tube currents. I could be wrong, but it doesn't look like it. Finally, I also have a 10 and 12" dob and I haven't come away with the impression they have bad optics - I think both are actually quite good. So I have seen better large telescopes too.

Oh, and my C6 is a later model Chinese made scope. It does have very good optics.

Hi Mitrovarr,

 

I didn't use calipers for anything but I did use some Howie Glatter laser collimation tools which are pretty handy to have. I might write up my process for others to try or at least go through my thought process and outline it.

 

I had this thought few months ago that you just reminded me about but I have not verified it. This may have some influence on your inside vs outside focus star test results and explain why I don't see this issue which with the extra large secondary of my 10" f/6.3 optics I would think could exaggerate the issue. I use the big port on my 10" Meade with a 2.5" Moonlite focuser. The difference in mirror position is small but I wondered if moving the mirror position might have an influence in the brightness of the rings inside vs outside focus. I also noticed a difference in diffraction ring brightness inside and out when the secondary baffle was not well centered but to do this alignment I used a circular projection from the HG laser collimator. It is difficult to precisely optically center the secondary holder without some reference point. This could have an effect on your star test but I have played with my scope after it was well adjusted because it's a PITA to put it back. 

 

Side by side comparisons are more reliable than memory, even then there is some judgement involved. What people think they remember is rarely what they describe in the moment even with good notes which is why eyewitnesses testimony is so unreliable. 

 

Chip


  • gnowellsct likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics