Collimating Maksutov-Cassegrain

[luigis]

Hi all,

I own a Skymax 127 Mak-Cass. In some photos I´m noticing the stars are not 100% perfectly round, this can be either a guiding problem or misscolimation. I don´t know which one it is so I would like to verify if the scope is perfectly collimated.

I know Maks hold collimation very well but mine travels a lot so it may be an issue.

I wonder which would be the best way to test collimation, cheshire? artificial star? Just a collimator cap?

Defocusing a star is one way but I would need to have the star perfectly centered in the eyepiece for that so I´m not sure...

Which way woud you recommend me to test collimation and collimate the scope if needed?

Thanks!

Luis

[JamesF]

I stripped my 127 Mak down and reassembled it so needed to collimate it afterwards. I used a cap (actually the lid from a 5L plastic carton with a small hole drilled in it) on the visual back to make sure everything is properly centred (mine wasn't at first, though how I'm really not sure) and then did most of the rest by eye. I sat the scope on a desk and moved so that my eye was at the same level as the centre of the scope, about a metre and a half away from the front. You should see concentric rings disappearing to infinity. If they drift off to one side you need to adjust the collimation to bring them back into line. When it was so close that I couldn't see anything wrong I took it outside, put it on the mount and did a final collimation against a defocussed star.

I ended up with this, which is as good as I think I can get against a real star:

At some point I'll have a go with an artificial star, but opinion seems to be that you need to get the star a fair distance away for it to work (I've heard talk of 30 to 40 metres).

James

[A McEwan]

I used an artificial star set about 40 yards away from the objective of my 180mm f15. That has got it "close" but I still need to fine-tune it using a real star to get it spot-on.

Ant

[nightfisher]

On the skymax, are the 3 little screws just lock screws, and the 3 larger ones for making the adjustments

[JamesF]

They're push/pull as far as I recall. Three go through threaded holes in a backplate on the end of the tube to pull that side of the mirror "in" and three push against it to move the mirror "out".

James

[luigis]

Thanks for the replies. I'm still not sure if the problem is collimation or guiding, 1500mm is pushing the guiding somehow and my seeing is always very bad. I guess I will have to test everything and go crazy like a maniac. In other words another normal day...

Any recomendation for artificial stars?

[JamesF]

Any recomendation for artificial stars?

They're fairly easy to make yourself, but I have a link for one that's availble from the US for not very much. Other people have said it's not bad to the best of my recollection. I'm not at home for a couple of days, but when I get back I'll post the details if someone else hasn't already done so.

James

[JamesF]

This is the one I was thinking of:

http://www.hubbleoptics.com/artificial-stars.html

James

[Duke2k]

Hi al. I have the 150 mak and I am struggling to collimate mine. Could you please explain how you use a collimating cap? James F said that he made a cap from a lid and drilled a hole in it. Do you look through the hole or from the front of the scope? I think a lot of the problem is that the supplied diagonal doesnt fit centrally in the back as the grub screws push it off centre.

[Duke2k]

Hi all. sorry I forgot to mention does it matter what focal length eyepiece I use?

[NGC 1502]

I collimated my 90mm Mak (bought second hand) on an out of focus star, kept in the centre of the field of view.

Worked a treat and has not needed further attention since doing that over 4 years ago.

Best done at high power, 200x plus. That scope splits tight double stars very nicely indeed.

If using a collimation cap you look through the hole in the cap, but as already said, didn't do it that way.

Regards, Ed.

[JamesF]

The ideal way is probably to use an artificial star, but you probably need to be able to place it perhaps fifteen to twenty metres from the scope.

When I did mine I started off by finding a plastic cap that would fit over the visual back. I drilled a small hole in the centre of that (probably about 1.5mm diameter) and used it to check everything looked central from the back of the OTA. I didn't use the diagonal at all during this process.

As I said above, placing one's eye at the same height as the centre of the OTA a metre or two away will show if the alignment is badly out because the reflected images you see will drift off in one direction or another if alignment is out. If you have things right then they should be concentric. With care I reckon you can actually get quite close by repeating the process of checking this way and adjusting the collimation to reduce the "drift" until you can't actually tell that there is any.

James

[JamesF]

Little update on this since I found the bits whilst having a tidy up of my astro kit yesterday...

Here's the cap I made for checking alignment:

James

[JamesF]

And here's what I've replaced it with. I bought this aluminium 1.25" cap from Astroboot a while back:

It turns out that if you remove the plastic bit with the logo in the middle there's a hole all the way through:

As that's rather neater than my plastic one, I shall be using it from now on.

James

[djhartist]

Couple of years late to this thread, but wanted to say thanks for info.

My Skymax 90 mak seemed to be a bit out of collimation lately so upon checking using techniques above (view through front + rear view with cap) i realised it was, and that the observed high magnification top heavy diffraction (?) patterns around stars echoed the misalignment.

With a bit of caution and trial and error, and regularly using the front / rear view techniques, I carefully adjusted the six allen screws on the back. This has improved the concentric alignments and apparent collimation. Daylight tests seem better and sharp at x140 - yet to do a star test (too cloudy) but am thinking of trying to set up an artificial star to test.

Front view technique: http://www.robincasady.com/Astro/collimation/

I have since found this pdf from Orion for the Starmax and Apex 90, 102 + 127, which are the same as the Skymax. I would probably recommend using this approach (combined with the front view method), as it gives a logical explanation of how to use the push / pull screws, which is probably better than the trial and error technique!

http://www.company7.com/library/orion/Inst_makcasscollim.pdf

[damnut]

Great find & thanks for posting.