Jump to content

Collimation question


Recommended Posts

Hi all

I'm trying to collimate my Skywatcher 130PM for the first time (don't worry, this isn't a 'how do I collimate my scope' question as such - I know there are plenty of guides out there already).

As I can't afford a laser collimater I did what most guides suggest and found a standard size 35mm film canister. Drilled a 2mm hole in the top of it but my problem is, the canister is fractionally too wide for my focuser, so just sits on top of it, rather than in it.

So my only option was to tape it into place, which I've done. But my question is, is this accurate enough to use for collimating, as there's nothing to ensure the film canister is dead centre and aligned exactly parallel with the focuser tube.

thanks

Link to comment
Share on other sites

I have never used this method but remember seeing details about making sure the pin hole in the cap is centered. So this is your first doubt - is it exactly centered? Then comes the problem you describe. If the canister wall is not distorted (out of round) , centering it on the top edge of the focuser tube should work (?).

To elliminate these potential problems that would give you false results, why not just use a bright star near the zenith on a relitively calm night (a "good seeing" sky). You simply rack the focuser in or out to see the difraction rings (light and dark halos of light and dark). If the rings are concentric around the shadow of the secondary mirror that should appear at the center of the image, you will be 90%+ there for a good colimnation. Usually just adjusting the primary cell adjusters is enough to get you more than 90% there. Any further adjustments would be not redily noticeable.

So that's what my opinion is. Lets see if anyone has any comments or additional help on the subject. Good luck and let us know what you ended up doing.

Link to comment
Share on other sites

Hi guys

Thanks for the replies :)

Hi Dannyboy. If the film cannister is a slack fit in the focuser, all you need to do is wrap some tape around it to give a snug fit. Experiment to find the right amount of tape, and this will work fine.

Regards, Ed.

Ed - unfortunately I have the opposite problem to this - the canister is fractionally too wide to fit in the focuser, so as Mr Q says, I've just had to stick it on top instead.

I have never used this method but remember seeing details about making sure the pin hole in the cap is centered. So this is your first doubt - is it exactly centered?

Hard to say exactly, but I've looked several times and it's as near as dammit :)

To elliminate these potential problems that would give you false results, why not just use a bright star near the zenith on a relitively calm night (a "good seeing" sky). You simply rack the focuser in or out to see the difraction rings (light and dark halos of light and dark). If the rings are concentric around the shadow of the secondary mirror that should appear at the center of the image, you will be 90%+ there for a good colimnation.

Oh ok, that's interesting - I've read numerous guides but so far all involve a laser collimator or film canister. So doing it using a bright star, do you just look through the open focuser tube instead?

Link to comment
Share on other sites

Hello Dannyboy. I presume you have found that your scope actually needs collimating. The idea of using a film cannister with a hole in it is to make sure your eye is reasonably centered in the focus tube, without it you will find this difficult. This process will ensure that the collimation is at least close. Ultimately you will need to use a star test to make sure, for this you will need the eyepiece in the focuser. If possible use Polaris as your target, it will stay still in the field of view, using other stars which move whilst you are fine tuning will drive you bonkers. Get the classic out of focus collimated image at low power and then refine it by gradually upping the magnification. You will need a night of good seeing for best results. Good luck. :)

Link to comment
Share on other sites

leave a note out for "santa"

you never know lol :)

Heh, it's worth a try, although Santa's already bringing me some PC games, and a Philips SPC webcam so I can try my hand at some astro imaging (hence the need to finally collimate my scope after 2 years of ownership :) )

Link to comment
Share on other sites

Hello Dannyboy. I presume you have found that your scope actually needs collimating. The idea of using a film cannister with a hole in it is to make sure your eye is reasonably centered in the focus tube, without it you will find this difficult. This process will ensure that the collimation is at least close. Ultimately you will need to use a star test to make sure, for this you will need the eyepiece in the focuser. If possible use Polaris as your target, it will stay still in the field of view, using other stars which move whilst you are fine tuning will drive you bonkers. Get the classic out of focus collimated image at low power and then refine it by gradually upping the magnification. You will need a night of good seeing for best results. Good luck. :)

Hi Peter

I'm pretty sure it needs some collimation as I've had it 2 years and never done this, I suppose due to a combination of ignorance and fear of making it worse. Also when looking through the hole I made in the film canister, I don't see a nice circular image of the mirror, so I think that confirms it does need doing.

Thanks for the tip too - polaris it is then. Just waiting for a clear night now :)

Link to comment
Share on other sites

you say it is too wide - did you trim off the thin lip around the opening of the cannister?

Hi Chris

I managed to miss your comment in amongst the others - soz. :)

I'm pretty sure the ones I got don't have a little lip around the top, but I'll double-check tonight.

cheers

Dan

Link to comment
Share on other sites

Buy one of the posh dust covers from Astro engineering - £3.49 inc postage. Turned aluminium and they have an indentation dead centre that you can drill out to make the perfect collimation cap... for about the price of a pint!

Wow, nice - I just purchased a black one - and i haven't even got a scope yet - nice shiny things... :-)

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.