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.

sgl_imaging_challenge_banner_constellations.thumb.jpg.6034fe99df7fe590f77a776877551964.jpg

des anderson

Collimation of a SCT telescope

Recommended Posts

Hi Guys, I`ve got a 9.25 sct and I was going to get a Badder laser collimator to help with collimation, but was advised against using one!!! 

    Can anyone advise me what to use, I`ve thought of using a false star but, I `ve read somewhere that it should be positioned approx 85 yards away. Des

Share this post


Link to post
Share on other sites

I might be wrong, but I don't think a laser jobby would be at all suitable with an SCT.

With mine, I use an artificial star (tiny hole with white filler in a thick sheet of black plastic hung up down the garden) and follow the simple instructions in the Celestron handbook.  Worked a treat, did it once to check collimation, hardly ever need check it again!

Doug.

  • Like 1

Share this post


Link to post
Share on other sites

The laser collimator on a SCT will be useless. The best way is on (if not a artificial star far away) than a bright star high in the sky and center the secondary shadow on the the star out of focus at various stages at both sides of focus. 

If setting up for planetary imaging, than I do the same thing but through a CCDs.. which is probably more accurate.

 

  • Like 1

Share this post


Link to post
Share on other sites

As Doug says,but you cant beat a star test and when collimating you only need very small movements.

  • Like 1

Share this post


Link to post
Share on other sites

Laser finders for SCTs are only as good as the visual back - the slightest bit of play and the result will be miles out.  I found I soon got the hang of collimating my C9.25 using Polaris as a target.   A set of Bob’s Knobs would probably help but my SCT seems to hold up pretty well.

  • Like 2

Share this post


Link to post
Share on other sites

Yup, forgot to mention Bobs Knobs.. I got those too and they make collimation MUCH easier.

Share this post


Link to post
Share on other sites

Agree, Bob's knobs a real or artificial star and a planetary camera make it much easier, with a camera you can see the effect immediately on the screen 

  • Like 1

Share this post


Link to post
Share on other sites

For my 8" SCT

I start with an illuminated pin hole at the bottom of the garden. That hole has to be literally a pin prick, small as you can.

Then for visual you pick a star as high as is comfortable for your posture and view at several hundred times mag.

Then go through focus and look for concentric rings - plenty of examples online.

For imaging use your camera setup, DSLR's have x10 on the Liveview which is enough mag to allow fine tuning I find

Michael

  • Like 2

Share this post


Link to post
Share on other sites

You must ensure the star, real or artificial, is perfectly, perfectly centred. After adjusting a screw, centre it again.

Hotech make a SCT collimation kit, using lasers. It is pretty good too, but the process is easy enough to do on Polaris, and use a decent simple eyepiece that gives you around 300-400X for best accuracy.

If you use an artificial star, do it over grass if possible, to avoid local turbulence from ground source thermals. It goes without saying that your scope needs to be properly acclimated to the conditions before starting.

  • Like 1

Share this post


Link to post
Share on other sites

Straightforward piece here on SCT collimation:

https://starizona.com/tutorial/collimating-a-schmidt-cassegrain/

Interesting that they feel that the stock collimation screws are better than the after market ones like Bobs Knobs etc. Personally I got on quite well with the latter.

As Tim says important that the star is centered and then re-centered again after each adjustment to see the progress towards accurate collimation.

 

Share this post


Link to post
Share on other sites

Yes, Bob's Knobs for me too. Makes things much easier and holds collimation almost indefinitely. I only use a star test and, once done, only needs to be checked every now and then. I usually find when checking that no adjustment is required.

Share this post


Link to post
Share on other sites

Thank`s guys, that`s a lot of information on how to help me get me and my scope back on track. I`ve a Zwo244 camera and can use this to get an image of a star.Des

Share this post


Link to post
Share on other sites

When I had to collimate my Meade 8" SCT I waited til dusk and lined the scope with Polaris the Pole star. Once it was aligned I then switched off astro tracking and went to terrestrial, this way the scope wouldn't move from its pinpoint.

It was then a case of defocusing the star until you could check the accuracy of the collimation.

Cost: nothing

 

Regards

 

Keith

  • Like 1

Share this post


Link to post
Share on other sites

Using a camera certainly makes it easier to collimate an SCT.  As others have said,remember to exactly recentre the out of focus star before assessing how well collimated you are after each adjustment.

There is a helpful video on youtube.

'Als Collimations Aid' is a really handy tool, giving you a free floating overlay of concentric circles on your computer screen - really handy checking how well collimated you are.  You can download a copy here

http://www.iceinspace.com.au/files.html

 

Share this post


Link to post
Share on other sites

Just done mine again. All sound advice above. On mine which is the Fastar version the secondary assembly had become slightly loose which makes collimating pointless. In this case I had to remove the corrector plate, tighten the secondary assembly, replace the corrector plate and then collimate by star test. It’s not difficult to do as long as you work in a clean environment but just check your secondary assembly is not loose.

Share this post


Link to post
Share on other sites

A good way of getting an artificial star is to use a small ball bearing blue-tacked onto a black piece of card and placed a way away, or use a glint of an insulator on a pylon.

For great focus AND collimation, make a 50 pence Duncan Mask, have a look at http://alpha-lyrae.co.uk/2013/12/31/schmitt-cassegrain-collimation-made-easy-using-a-duncan-mask/

You really need your scope to be cooled down sufficiently (to make getting the "Y" nicely together) but I'll never use another method. Perfect on the first go. 

Edited by Stu Todd

Share this post


Link to post
Share on other sites

I invested in the Hotech collimator, expensive piece of kit and considering the price very poorly written documentation. I had to use that and two Youtube video's to get the set up working.

You then still need to check on a star to ensure your focal train is correct.

Bob's Knobs are a boon.

Share this post


Link to post
Share on other sites

Here is what I do...

1. Rough collimation on a centered star at x200 - aim is to get the out of focus doughnuts symmetrical.

2. Better collimation on a centered star at x200 - this time just out of focus.

2. Best collimation on a centered star at x400 plus - this time at perfect focus.

Share this post


Link to post
Share on other sites

I've got an artificial star, and yes when you use it you need to place is a good 100m or 100yards away or further!    Also, you need to place is somewhere so that your scope is pointing up, rather than horizinal or down, so as to minimise issues with mirror flop.    A real star is also a good to use.    The process is really easy, and don't be scared of it.

 

Collimation of an SCT is simply a case of lining up the secondary mirror with with other optics - as the rest are set in stone, the only thing that you need to do is alter the rotation of the secondary, which is behind the central obstruction in corrector plate.  There should be three screws holding the mirror in place.

 

Using a conveniant star - any bright star will do.   Firstly, give the scope an hour or so to settle down to ambient, So that the optics are stable.  No point collimating before this as you'll be trying to catch a moving target. (I normally set my scope up before dusk and wait for stars to appear.

 

Then use a lower power eyepiece, find a bright star and defocus the scope until you get a donut shape instead of a star.

Next adjust the screws to rotate the mirror until the donut is central.   Do not tighten the screws completely as that is too tight, it's about the rotation not doing it all up tight.

Once you have good collimation, switch to a higher power eyepiece, and do it again.

Repeat using a more and more powerful eyepiece until you get to your most powerful eyepiece that you can use with your scope.

 

Each time you put in a higher power eyepiece, the hole will shift slightly, just recenter it, and note that the changes should be smaller and smaller each time.   When you run out of pieces/cameras   you are done.

 

Having done this method several times, I've got to the point where I only ever need to make small tweaks to my setup with that, I jump straight to my imaging camera and use that.  As it's the highest power that I intend to use on my scope.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Recently Browsing   0 members

    No registered users viewing this page.

×

Important Information

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