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

Improving Optics For Older C8 SCT Telescope

  • Please log in to reply
14 replies to this topic

#1 JRG8806

JRG8806

    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 10 Jan 2020

Posted 11 January 2020 - 10:11 AM

Hello Cloudy Nights!

 

This is my first post on this forum and before I ask my question, I just want to say how incredible this community truly is.  I have been reading forum posts on this website for the past 6 months and am blown away by the support of all who are registered here.  

 

I was given a 1980's Celestron C8 SCT telescope earlier this year.  It was not in the best condition so over the past few months I have been rehabing this scope.  My friend is big into astronomy and helprf me clean the optics, collimate the scope and make sure all is working.  I finally have the scope in working order, all parts working as they should be. 

 

My only concern is the optics of this scope.  I do understand that my telescope was manufactured during the darker times of Celestron, in which they did not manufacture the highest quality telescopes.  So I am guessing right off the bat, my optics will not be great.  However, I feel like when I am observing, especially lunar, I see some hazy smudges on the image.  The scope does focus pretty well, it just seems like I am looking at the moon with some blurry patches on the image.  They are not terrible, but enough for me to think about how to fix.

 

I have read that if your primary mirror isn't completely dirty, to leave it alone.  I read that even cleaning the primary, if it is quite dirty, will only lead to slightly improved optics.  Under the light in my living room, the mirror looks very clean; however, when I shine the flashlight on my iPhone into the telescope, I can see blue smudges and streaks from when my friend cleaned my telescope.  Something else to note, is my corrector plate is missing the (2) cork shims that were installed by Celestron to properly align the corrector plate.  I have read this may or may not make a big difference in the image.

 

My question is, with the above information, is it worth removing the primary mirror for a proper cleaning?  My first telescope was a reflector telescope in which I easily center-spotted and cleaned this mirror with no issue re-installing.  I have read SCT telescopes are much hard to put back together.  On this scope, I originally tried to clean the primary with it installed and found it very difficult to orient my hands inside the telescope.  

 

Thank you very much for your help.  Your responses are much appreciated as I begin my journey into astronomy!

 

 

 



#2 D_talley

D_talley

    Gemini

  • *****
  • Posts: 3,431
  • Joined: 07 Jul 2005
  • Loc: Albuquerque, New Mexico

Posted 11 January 2020 - 10:27 AM

Welcome to CloudyNights.  I have a C8 from the 80's that has some of the best optics of any scope I own.  Not all scopes from this period are bad.   As for your scope, one rule is to never shine a bright light into a SCT because you will see things you should not see, making you think it is bad.   There are other things you need to look at first before touching the primary mirror.

 

1. Are you sure it collimated? 

2. Was the secondary mirror cleaned when your friend cleaned the scope. 

3. The blurry patches you saw, did they move around like bad seeing or are they in the same place in your view.  

4. Do they change when you change eyepieces?

5. Have you used the scope in the daytime, looking at a far object?  How is that view?

 

Get more time behind the eyepiece and determine the issues you are seeing. 


  • Joe1950 likes this

#3 Jeff Lee

Jeff Lee

    Surveyor 1

  • *****
  • Posts: 1,838
  • Joined: 17 Sep 2006

Posted 11 January 2020 - 10:44 AM

The shims could be very important as is the orientation of the corrector. If there is a local club with some more expertise you might contract them (Or join:)). What does the star test look like? Can you post an image for the experts to see of an out of focus (both in and out) of a star? When all is well I'd recommend a FR6.3, seems to make my year 2000 C8 a better scope.



#4 sg6

sg6

    Fly Me to the Moon

  • *****
  • Posts: 6,866
  • Joined: 14 Feb 2010
  • Loc: Norfolk, UK.

Posted 11 January 2020 - 10:54 AM

No reason why the optics should be bad, an awful lot is down to chance and after chance a lot of imaginary reasons appear.

 

Best is find something to look at, M42 might just fit otherwise find something smaller.

 

Will say I have never had a good view through an SCT, and the same question has appeared here. Suppose it may not be the scope/design maye just the rather simple fact that Synta/Celestron mass produce them and people seem to think they will be delicately and accurately assembled. Truth - mass produced and it will be numbers out that count not high quality.

 

Looking down the wrong end of a scope usually leads to cries of dispair. Seems a poor idea to do so. Reflectors of any sort seem the "worst" option to look down.

 

If you supplied a location then someone may suggest a club. However expect the same answers - Leave it and look through it, then decide.



#5 markb

markb

    Messenger

  • *****
  • Posts: 490
  • Joined: 16 Jun 2006
  • Loc: Long Island; in transition to Arizona

Posted 11 January 2020 - 11:03 AM

Short answer, almost everything fails a flashlight 'test', and yours sounds more than clean enough. What you were told about cleaning is correct. I have a usable Meade with a bubbled, pitted and tarnished secondary (a failed Meade experiment with silver mirrors) that still gives fairly sharp images. It should be a great scope once recoated. Notably, the Ronchi test indicates the basic figure is very good.

 

Many scopes of the time were still good to excellent, my brother's C8 of the period was superb. So do not condemn it based only on age.

 

A terrible impact on inherent scope sharpness is disassembly of the scope followed by reassembly without using the factory alignment marks. Great scopes easily turn into soft scopes. My GPS11 was botched by a cleaning and badly mishandled Bob's Knobs install and was awful, very soft. After restoring all the factor alignments it is excellent. It may yet be improved further, but only a bit. Rotatiion alignment was not done to match figure abberations but primarily to use baffle angle and secondary -puck-housing tolerances to get the optical and mechanical axes aligned/coincident.

 

So check, and restore, your factory rotational alignment first!

 

Search the forums for rotational alignment or etched numbers to find lots of detailed posts including my own, personal, experiences. Your friend may not be the best source on this if he has already lost the factory alignment. So many folks have no clue the marks exist.

 

On corrector centering, read below. Simple spacers are likely enough with rotational alignment being the key in that  vintage scope,

 

Celestrons at the time would have an etched few numbers right at the edge of the collector plate on the front surface, that should read 'normally', not reversed, as installed in the scope. The secondary has a sharpie mark on the glass edge, not housing, for alignment. The primary might be marked, but is probably immaterial as the primary is glued to the focuser arm and is set in place (unless also disassembled).

 

Hopefully the secondary mark is visible, but might require loosening or removing it from the plastic housing, if so, Mark the housing to match making later parts easier.

 

The secondary 'stripe' and the etched numbers on the face of the corrector are aligned to the 3 o'clock position of the tube viewed from the front, corrector, end. This is the tube side opposite the focuser location.

 

Do not massively overtighten the secondary housing if removed, and the corrector retainer screws are to be snug, not tight, to avoid distorting the corrector.

 

Corrector centering is a tricky point, but you will keep it simple. Part or all of the rotational alignment is to utilize tolerances in the secondary puck/glass/plastic housing to make the mechanical center match the optical center axis, so on scopes with cork spacers they were usually used to keep the corrector centered in the cell. So if you simply center the corrector with equal thickness cork you should be fine. Without better tools, and without a badly optomechanically aligned scope (comatic images even when collimated, among other clues) I'd stop there. The goal is to match the secondary location to the primary, as constrained by the baffle tube, and the factory spin alignment should have accomplished that.

 

Nearly perfect thickness cork sheeting is a breeze to find, just get cork gasket in sheets at an auto parts store.

 

Once you check the factory rotational alignment and collimate it (again, search on CN, and do a focused star collimation, not just the centered shadow starter step).

 

Then see how it does. 

 

If you don't have good stable atmospheric conditions (I almost never do, at home), you can use a Hubble brand artificial star. You can also further test it with a Ronchi test film (a Ronchi eyepiece at $60 is ideal, Gerd Neumann tester, I think Optcorp.com sells them in the US) or even a dpac test, but the latter can easily lead to tears on good scopes with minor issues. I like the Ronchi to give me a decent idea of the optics, particularly when used with a Suiter extra focal 'test'.


Edited by markb, 11 January 2020 - 11:15 AM.

  • Magnus Ahrling likes this

#6 Gary Z

Gary Z

    Surveyor 1

  • *****
  • Posts: 1,532
  • Joined: 26 Jan 2012
  • Loc: New Mexico

Posted 11 January 2020 - 11:05 AM

As your friend is far more familiar with this scope, you might pose this question to him.  If you're seeing smudges when you look at the moon, it may be time to have a more skilled sct owner take a look. But your friend may have at some point taken the scope apart and could guide you should you both agree it needs to happen again.  Let him weigh in on the smudges to see what he thinks.  He'll either agree with you or tell you what others have said....to maybe leave well enough alone.  

 

Gary



#7 JRG8806

JRG8806

    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 10 Jan 2020

Posted 11 January 2020 - 05:33 PM

Thank you all for your comments.

 

I think my first step will be a thorough day time test of the scope at various magnifications with various eye pieces.  When I was aligning my Telrad and finderscope, I did this during the day.  Where I live, I can only see about 1/4 mile.  So I will try to find a spot that allows for greater distances.  The optics did look pretty good at a 1/4 mile with a 30mm eye piece.  I will also try and take a picture of the image with my iPhone, through the eyepiece (this can be challenging but I have done it before with some success).  The scope does look collimated with an out of focus star.  

 

Thank you for the tips about not looking into the front of the scope with a light.  Based upon what I see when my eyes look through it, the primary mirror does seem very clean.  As for the secondary, my friend did not clean the secondary mirror because he said it was in good shape.  

 

I will report back once I have done a thorough day time test.  The weather does not look great for the next week, so it may be some time, but I am looking forward to this day.  If needed, I will look into reviewing the corrector plate centering, will check the condition of the secondary mirror and review the process of factory aligning the corrector.    

 

I am joining a local club as well.

 

Thank you again.  



#8 rmollise

rmollise

    ISS

  • *****
  • Posts: 22,917
  • Joined: 06 Jul 2007

Posted 11 January 2020 - 07:33 PM

Hello Cloudy Nights!

 

This is my first post on this forum and before I ask my question, I just want to say how incredible this community truly is.  I have been reading forum posts on this website for the past 6 months and am blown away by the support of all who are registered here.  

 

I was given a 1980's Celestron C8 SCT telescope earlier this year.  It was not in the best condition so over the past few months I have been rehabing this scope.  My friend is big into astronomy and helprf me clean the optics, collimate the scope and make sure all is working.  I finally have the scope in working order, all parts working as they should be. 

 

My only concern is the optics of this scope.  I do understand that my telescope was manufactured during the darker times of Celestron, in which they did not manufacture the highest quality telescopes.  So I am guessing right off the bat, my optics will not be great.  However, I feel like when I am observing, especially lunar, I see some hazy smudges on the image.  The scope does focus pretty well, it just seems like I am looking at the moon with some blurry patches on the image.  They are not terrible, but enough for me to think about how to fix.

 

I have read that if your primary mirror isn't completely dirty, to leave it alone.  I read that even cleaning the primary, if it is quite dirty, will only lead to slightly improved optics.  Under the light in my living room, the mirror looks very clean; however, when I shine the flashlight on my iPhone into the telescope, I can see blue smudges and streaks from when my friend cleaned my telescope.  Something else to note, is my corrector plate is missing the (2) cork shims that were installed by Celestron to properly align the corrector plate.  I have read this may or may not make a big difference in the image.

 

My question is, with the above information, is it worth removing the primary mirror for a proper cleaning?  My first telescope was a reflector telescope in which I easily center-spotted and cleaned this mirror with no issue re-installing.  I have read SCT telescopes are much hard to put back together.  On this scope, I originally tried to clean the primary with it installed and found it very difficult to orient my hands inside the telescope.  

 

Thank you very much for your help.  Your responses are much appreciated as I begin my journey into astronomy!

 

What will help you more than anything is precise collimation. Removing the primary of the telescoped is NOT a job for a tyro, and you would likely do more harm to the optics than good.



#9 JRG8806

JRG8806

    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 10 Jan 2020

Posted 13 January 2020 - 02:38 PM

Hello,

 

I just wanted to give a quick update about what I discovered during my optics test.  I took the scope out during the day and went through my eye pieces.  I did not encounter what I encountered originally.  My optics for the most part were "smudge" free and everything focused properly. Saying that, I did feel like there was a slight glowing from the objects within the field of view.  It was nothing major but I just felt like the contrast was soft and there was just something slightly off about the image.  This could be a collimation issue as I have not collimated the scope since my friend returned it to me after he cleaned the mirrors.  I have used 6 or 7 times since then.  

 

I have attached a picture of the moon I took by using my iPhone and capturing some images though the eye piece.  I know this is not the preferred astrophotography method but it did capture some of the haziness I am describing.  I am curious what people think?  

 

Thank you all for the help.  

 

 

- Jason 

Attached Thumbnails

  • Moon JG.jpg

  • eros312 likes this

#10 speedster

speedster

    Vostok 1

  • *****
  • Posts: 189
  • Joined: 13 Aug 2018
  • Loc: Abilene, Texas

Posted 13 January 2020 - 04:25 PM

One corrector and primary, things can be really messed up before you see the effect.  Take a look at this:

 

https://www.cloudyni...orrector-plate/


  • davidc135 likes this

#11 Don W

Don W

    demi-god and Chairman of the Finance Committee

  • *****
  • Administrators
  • Posts: 23,746
  • Joined: 19 May 2003
  • Loc: Wisconsin, USA

Posted 13 January 2020 - 06:18 PM

Pretty good image. Terminator shows a lot of detail.



#12 eros312

eros312

    Viking 1

  • *****
  • Posts: 531
  • Joined: 14 Feb 2009
  • Loc: Tampa, FL

Posted 13 January 2020 - 08:52 PM

Looks good.  The haziness you see is probably just the seeing, or maybe thermal effects. 



#13 JRG8806

JRG8806

    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 10 Jan 2020

Posted 14 January 2020 - 01:04 PM

Thank you all for the comments, Speedster great, great information.  Thank you!  



#14 Magnetic Field

Magnetic Field

    Viking 1

  • -----
  • Posts: 854
  • Joined: 28 Aug 2017
  • Loc: UK

Posted 14 January 2020 - 03:44 PM

Thank you all for the comments, Speedster great, great information.  Thank you!  

Do you also see the blue false colour around the moon while observing through the eyepiece?

 

What eyepiece did you use?

 

You can also check collimation (under good seeing) by pointing the scope at high magnification (>200x) to a star (a red coloured star often helps) in focus. Can you see the diffraction rings around the star?



#15 Mike Spooner

Mike Spooner

    Vendor (mirrors)

  • *****
  • Vendors
  • Posts: 693
  • Joined: 06 Aug 2010

Posted 14 January 2020 - 04:52 PM

Yes, which eyepieces are used, do they all show smudges and are they clean? I have an old Nagler with worn coatings and it flares when a planet or star crosses the field of view.

 

 Mike Spooner




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