Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.



SCT 9.25 Collimation lines through Airy discs

Recommended Posts

I was wonder is someone with the same set-up as me can tell me if this effect is normal for a Celestron 9.25 SCT. I have not had the telescope long so I am still getting used to setting up. I check the collimation before starting each session and have noticed that 2 dark lines are apparent when I have centred the star within its Airy disc. They are like the hands of a clock and quite well defined. Any thought as to what is causing this. I was wondering if the Bob's Knobs are too tight and are causing distortion of the secondary but I cannot find any examples on the internet of this effect to back this idea up. I can't find any reference anywhere to this. Any ideas of what is causing this and how to cure it would be very much appreciated.

Share this post

Link to post
Share on other sites

I think this is your diagonal causing this.

Do you get it if you just insert an eyepiece?

Share this post

Link to post
Share on other sites

Sounds to me like a heat plume rising from the internal baffle tube , I get this on the Mak180Pro until things have reached equilibrium inside the tube.

An image down the page here of similar effect ... http://www.sergepetiot.com/?p=360

And a discussion here ... http://www.cloudynights.com/ubbarchive/showflat.php/Cat/0/Number/615474/Main/612225

Edited by Steve Ward
  • Like 2

Share this post

Link to post
Share on other sites

Whatever is causing the issue, it won't be the 'Bob's knobs' as they are isolated from the secondary mirror itself.

You could try rotating the eyepiece in the focuser, see if the dark lines you mention turn as well, if so, problem with that EP.

But from what you are saying, the most likely cause is mentioned in post # 3, wait an hour and see if full cooldown cures it.  You can still use the scope while waiting, especially for low power faint fuzzies, where critical definition is least needed, but planetary or double star views are compromised before the scope is fully cooled.

Setting the scope up as soon as you can before observing will help, with these light evenings most of us are waiting around for it to get dark.

If you can store the scope in a cool room or building, that helps too, less temperature difference from outdoors.

Hope you sort it, Ed.

Edited by NGC 1502

Share this post

Link to post
Share on other sites

Many thanks guys for your time. You have reassured me that it is a cooling down matter and not a problem with the optics. I am just getting used to the scope as I had an archromat before getting this, so I didn't have any of this to cntend with. I think I am not leaving the scope long enough before checking the collimation, although I would say that I had left it more than an hour last night as I set up while it was still light. Having said that the temperature did drop considerably last night so I guess the scope was always trying to catch up. I am convinced from what has been said that it is a heat plume causing the effect, not the eyepiece. What I observed was a close match on the first link article photo at the end. Many thanks for the helpful advice.

Share this post

Link to post
Share on other sites

Also, perform collimation without a diagonal, this will eliminate another source of possible misalignment.

Share this post

Link to post
Share on other sites

True, but if, as most do, you use a diagonal, if this alters the collimation you need to have an adjustable diagonal or collimate with the diagonal in place.  :smiley:

Share this post

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Recently Browsing   0 members

    No registered users viewing this page.

  • Similar Content

    • By PlanetGazer
      Good Afternoon
      I have been trying to get my SW 250P (f/4.7) to the best possible collimation as I couldn't manage to focus on any of the planets so far. I just collimated the scope to ready it up for tonight's session. Your feedback is much appreciated!

    • By Kronos831
      Hey guys,im currently on a school trip and brought my celestron firstscope with, tonight we ll  be trying some dsos and maybe saturn and jupiter,the telescope itself is not collimated and dirty (on the mirrors)So I wanted to ask.in the scope, there is no primary adjustment screws,only the ones to take it off .Does that mean that its set , allowing me to take it off wash it and back on without having to recollimate the primary?
    • By Demonperformer
      To put this in perspective, I bought my first SCT nearly ten years ago. And in all that time, there has been one word that has scared me witless (no, that's not a spelling mistake ) ... collimation!!!!
      There are so many horror strories around of people scratching corrector plates and simply messing it up completely and making a scope unusable, that I have always avoided it like the plague ... once even preferring to sell the scope I had and buy another one to avoid having to do it [don't you just love the logic of that one?particularly when there is no guarantee they will arrive collimated!]. I've always belonged to the "I don't care about winning, I just don't want to lose" brigade.
      Six months ago, I bought an RC6 and recently picked-up a cheshire eyepiece. No corrector plate ("But don't drop the allen key into the tube," the websites warn!). If I'm ever going to do this, now is the time. A quick look earlier this week showed the dot well out of position, so yesterday ... I procrastinated ... o come on, you didn't think it was going to be that easy, did you? But there are clear skies predicted for tonight, so this morning I went outside, set up my NEQ6, plonked the RC6 onto it, pointed it slightly downwards and went and made a cup of coffee. Returning to the scope I started to have a play.
      I kid you not. In less time than it had taken me to set up (excluding the coffee), I had the black dot slap-bang in the middle of my white circle. The allen bolts were tight, yes, but not "wheel-nut" tight; not "you need three feet of metal bar wrapped around the allen key to lean on in order to loosen it" tight. And yes, my first move was in the wrong direction ... so what??? It was so monumentally, stupidly easy ... why all the horror stories on the internet?
      Am I pleased I set it up outside where there was plenty of space, rather than struggling in the lounge? Yes. Am I pleased I got a cheshire so I could do it in daylight? Yes. Were the ten years of worry that I had endured dreading this day a waste of energy? You bet!
      So my message is to anyone who looks at the word "collimation" and immediately becomes a rabbit in headlights ... Action Cures Fear ... seriously, just do it ... if I can do it, ANYONE can.
    • By Kronos831
      Guys , i am so frustrated .I can't collimate my newt. I m here with my chesire eyepiece trying to adjust the secondary but when i turn the  centre screw, the mirror moves a lot and then shifts back to where it was. And turning the little screws arouns it ddnt so much either. I don't have much time before my session (2 hrs) pls help
    • By AntoineDemangeat
      Hello everyone, 
      As a farely new member and astronomer, I've decided to seek help for collimation. 
      The task seems pretty daunting at first but I think I got the basics down. 
      Unfortunately I cannot find any answers for the questions I have, so here it is: Is it 'normal' to see the focuser's end? 
      As you can see in the attached picture (poor quality, let me know if I should do another) we can see a rectangle at the left, the focuser.
      I don't think I should be seeing this and I don't know what causes the problem since the secondary 'seems' to be aligned and the doughnut is dead center. 
      I'm using a XT10 and the focuser is all the way in. 
      Thanks for your precious time, 

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.