Jump to content

Banner.jpg.b89429c566825f6ab32bcafbada449c9.jpg

Star Collimation


uminded

Recommended Posts

I used Astro Baby's collimators guide to tune up my scope and I took it outside (Last night was the first night not 100% overcast since Oct 8th  :cry:), took it outside to do a star collimation.

Problem is I do not get an "airy disk" with any difference of focus... Its all just one blur or a pinpoint of light. I can not possibly be that far out as I observed two dark bands on Jupiter last night. The only pattern I could get it if I backed the focuser tube 99% out I could see the secondary and primary mirror alignment swimming in the view. I then focused back down and past and if I see any pattern at all its the secondary shape when slightly out of focus.

Am I doing it wrong? I tried 50mm 20mm and 8mm eyepieces on my F6 scope.

Link to comment
Share on other sites

You will need as you suggest a higher magnification. You will also need a long enough cool down period to be sure that tube currents are not present, and you will also need good atmospheric conditions. Getting all these factors to come together takes luck and patience.

Link to comment
Share on other sites

I don't bother with star tests. That said you need to make a fraction of a turn only either side of focus.

That surprised me. I always thought that the Star test was the absolute test to the theory and practical set-up of a properly collimated telescope under perfect conditions, ie fully cooled to ambient, perfect seeing conditions ect, but alas,  those features are not always obtainable here under UK skies?

I just thought the test itself was like the MOT of a  Newtonian set-up...passed.

After all, if the  final out of focus image  ( Airy Disk ) is not fully concentric, any previous efforts to set-up are in vain and the set-up can be improved, obtaining the best image from your system?

Link to comment
Share on other sites

It also helps if the star you choose to use to collimate on is high in the sky - say 70 degrees up to directly overhead. This will ensure that there is the least atmospheric disturbance possible. Lower viewing angles will result in a thicker layer of atmosphere to be passed through by the image.

Link to comment
Share on other sites

Try collimating on a star at the zenith with a non tracking mount :-)

Also the star has to be exactly central. I don't have a reticule eyepiece either.

I have concluded it is not worth it if the views meet expectations; I would sooner be observing than collimating.

Link to comment
Share on other sites

With a planetary scope, that last bit of collimation is the difference between an ok view and eking out the finest detail ;)

For my size scope, with a tracking mount, and at a high power (for me x300 to x400), defocus the star (mag 2-3) until it fills the view - you will know when it is central ;) Adjust the focus inwards until you see outer rings with a tiny dot in the centre. Adjust collimation until the rings are concentric.

Find a mag 4-5 star, use at least x400 and focus the image. You should have an Airy disc and a diffraction ring(s). Fine tune collimation until the diffraction ring is even - it should snap in and out of focus evenly.

When that is done you are ready for the kind of planetary detail which eludes most people :)

Link to comment
Share on other sites

Well I don't think I get clear enough skies to even attempt 400x magnification... But I did manage to see some fading detail on Jupiter on a 6/10 viewing night in my backyard so my collimation is good enough for the 1hr backyard sessions for now. Once my barlow shows up and I go on a country drive I will see what difference it makes.

For now I just finished a green laser finder and a prototype remote motorized crayford focuser. Will post a little write up once I prove it to work.

Thanks for the help!

Link to comment
Share on other sites

Getting an airey disk can be a pain - you need a brightish star, good conditions with stable air, well cooled scope and a lot of magnification.  you need to defocus by a hair to see the [removed word] and it wont be as big as you may be thinking.

I use one of these http://www.hubbleoptics.com/artificial-stars.html as an indoor artifial star - with the star set up I can use the full length of the lounge and hallway to back the scope off enough to get the distance.  Its a brilliant little bit of kit for the money.

Mel

Link to comment
Share on other sites

I've tried this in the past and not been able to do it. Its always been too difficult to get a stable image which I put down to the poor atmospheric conditions which we're often plagued with in the uk.

Try a mag of about x200 or more and see how you get on.

Link to comment
Share on other sites

Why are folks defocussing ? The airy disk is only visible at focus on e you have done all the misalignment adjustments on a brighter quite defocused star.

While I really like the Hubble, it's of limited use on my bigger scopes due to the horizontal position while collimating.

What I find works is to get unfocused bright star diffraction patterns all concentric while poinnting at a bright star at say 120 az, 60alt (I'm using a German equatorial) to save my neck, get the star centered through both sides of defocus, then on a mag 4 star, focus a s best as I can and use the airy disk to guide me to collimation. The fainter star makes all the difference.

I also find that once collimated, the star may no longer be central to the out of focus patterns, but travel through as the focus shifts. This Probably indicates residual mirror misalignment ...

Go back to a bright star afterwards and I can normally see a much better defined airy spot than before, even if the seeing is wobbling it all over the place!

Mike

Link to comment
Share on other sites

That surprised me. I always thought that the Star test was the absolute test to the theory and practical set-up of a properly collimated telescope under perfect conditions, ie fully cooled to ambient, perfect seeing conditions ect, but alas,  those features are not always obtainable here under UK skies?

The last bit here is why I don't find it a helpful method.

With the unpredictable UK weather, if I had to rely on it in order to collimate my 'scope I'd be lost! :)

Link to comment
Share on other sites

That surprised me. I always thought that the Star test was the absolute test to the theory and practical set-up of a properly collimated telescope under perfect conditions, ie fully cooled to ambient, perfect seeing conditions ect, but alas,  those features are not always obtainable here under UK skies?

I just thought the test itself was like the MOT of a  Newtonian set-up...passed.

After all, if the  final out of focus image  ( Airy Disk ) is not fully concentric, any previous efforts to set-up are in vain and the set-up can be improved, obtaining the best image from your system?

Even when given perfect sky conditions, quality collimation tools and quality scope components will produce better collimation results:

1- You can't use star collimation to make secondary mirror adjustments

2- For primary mirror adjustment, star collimation will tell you when the central star enters the "coma-free" zone. It will not tell you with good accuracy if the central star is at the center of the "coma-free" zone.

For those who make the claim that star collimation is the most precise method for collimation, I suggest the following experiment:

1- Start with a perfectly collimated scope (including the secondary mirror). Leave a marker on one of the primary collimation knobs.

2- Ask someone to decollimate the scope by giving few good twists to the primary knob with with marker.

3- Your task is to recollimate by using only the same knob using only a star. When done, ask the other person to make a note of how close where you able to bring the marker to its original position. You will not be told, yet.

4- Repeat the above few times by asking the other person to twist in random directions and random number of twists. After each recollimation, the other person will take notes of the placement of that marker.

5- Finally, you will be give the results.

You can do the above with a collimation cap indoors.

Precision implies repeatability. See which method is more repeatable -- hence more precise.

Note: I did state quality collimation tools which includes a precisely placed centeral spot + quality scope components such as a good focuser. 

Jason

Link to comment
Share on other sites

jason D.....I hope I have not mislead you or anyone else in my comments.  I would'nt carry out a 'Star Collimation' myself, although it could be done, but very time consuming?

The fact is  I have misread the title of this thread, and maybe uminded has misread Astro Baby's article in which she recommends a 'Star Test' which  is carried out after you have collimated your telescope using , as you mention quality tools.

Its the Star Test that verifies my collimation is good. Thanks for making me aware.
Link to comment
Share on other sites

jason D.....I hope I have not mislead you or anyone else in my comments.  I would'nt carry out a 'Star Collimation' myself, although it could be done, but very time consuming?
The fact is  I have misread the title of this thread, and maybe uminded has misread Astro Baby's article in which she recommends a 'Star Test' which  is carried out after you have collimated your telescope using , as you mention quality tools.
Its the Star Test that verifies my collimation is good. Thanks for making me aware.

Hi Charic,

No worries...

Jason

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.