Collimation. Cheshire vs Star test

[Carl]

Just done some collimation tests on my GSO RC8.
In the house i used the Cheshire and got the secondary smack bang in the centre but when i did a star test looking at concentric rings when out of focus my collimatilon was way off.
So after collimating using the star test and getting even concentric rings i then looked thru the scope with my Cheshire eye peice and noticed that my secondary was way off centre.

I did a test image and my star was sharp but when i imaged at a high iso speed the star was round and sharp but had an oval flair to one side.

I'm told the only way to accurately collimate is doing a star test, fair enough. has anyone else experienced such a huge discrepency between the two?

Regards
Carl Rainer

[bmitchell82]

cheshire will do nothing for you

Auto collimator will! as it will align your secondary to the optical center of your focuser. As the primary is a floating primary, you then utilize a star test to do it right. by focusing the star and then taking a snap of it in each coner, you then chase the flair back towards the center.

its a long arduous process but it works.!

on the other hand if it was a fixed primary.... :) it would be quite easy

[rcheshire]

He will - if you're polite!

[GeoffW1]

:rofl::rofl::rofl:what about an ice cream then ?

[RickS]

You forgot to say "please!"

[stevous67]

I have been experimenting on various methods to find a simple collmination routine. I agree that a Cheshire will do little for you, even if you give it an icecream.

Over two years I've tried many methods on a RC10" and more recently on a CDK12.5. The choice method has turned out to be a hybrid defocused star routine I come up with (haven't seen it elsewhere). Pointing to an open cluster that reasonably fills your CCD, defocus the image to reveal donuts across your field. These only need to be say 10% the height of your FOV. You'll need at even 4x4 binning a 10 second shot to see these donuts clearly (at F8 say). Having your laptop point at you whilst your at your secondary, adjust the secondary until the centre stars depict nice and centric donuts, whilst the outer most donuts elongate evenly pointing inwards. Once you achieve a nicely spread field of donuts, you will have achieved collimation.

Upon refocusing, any unbalanced flairs or reflections will be the fault of something else (eg, badly aligned primary, secondary not centre, filters not sitting perpendicular to the optical plane, etc).

Using this method takes a lot of the guessing out of which direction to tilt the secondary. Also, conditions don't have to be perfect. Lastly, I can now do it in about 15 mins, possibly you can do it faster with a camera that has USB 2.

Things to remember, when deciding which way to tilt, move defocused stars in the direction of the fat leading edge to get them concentric. Always recenter the open cluster to ensure it stays in the most helpful position.

Steve

[troypiggo]

I have some questions on your method, Steve. I'd like to try it out. I'm trying to refine by collimation and need all the help I can get.

When you mention the central stars should be perfect donuts, but the outer ones are elongated, shouldn't they all be donuts if you have a coma corrector?

When you say that upon refocusing, anything else unbalanced means there's a problem elsewhere, any tips on how to detect where to fix the problem?

[mldee]

Brendan, would you care to elaborate on the RC8 floating / fixed primary mirror?

As I posted elsewhere, I had read in another forum that the rear adjustment screws actually align the rear OTA assembly, rather than the mirror, with the secondary. That being the case, what is is the primary mirror "floating" on for support, and what are your recommendations for adjusting the rear screws?

[bmitchell82]

Okay so im not a complete expert on the whole RC system, but i have played around and helped a good friend of mine who has had both GSO and high end RC's.

As for the screws adjusting the whole rear OTA i don't think so, that would mean that the OTA would have to be disconnected from the rear cell.?

I believe that the difference between the floating and fixed is to do with the actual mirror and its mounting in particular both high end and GSO scopes use push pull - locking screws on the rear cell but unlike the high end scopes GSO is basically head in place with mirror clips where the high end is actually has a screw down fitting. Like i said this isn't gospil facts its only my understanding so if somebody else has actually pulled both types of scopes apart then please drop in.

It is this reason then why the tak collimation scope works really well on the high end scopes and the other method i mentioned coupled with the star test works well for floating systems

Brendan:thumbsup:



Brendan, would you care to elaborate on the RC8 floating / fixed primary mirror?

As I posted elsewhere, I had read in another forum that the rear adjustment screws actually align the rear OTA assembly, rather than the mirror, with the secondary. That being the case, what is is the primary mirror "floating" on for support, and what are your recommendations for adjusting the rear screws

[stevous67]

Hi Troy,

Even with the CDK, which has a forgiving flat field, when you examine stars defocused, those in the outer of the FOV (especially with the STL) are not round and balanced in shape. This is what makes this alternative easy to define visually where collimtion is, as the pattern should be that central defocused stars are nice, round and balanced, whilst those in the outer reaches of the FOV will be elongated, the fat side (if i remeber correctly) pointing inwards evenly to the centre of the FOV. I am so confident in collimating my scope now that with the full moon approaching, I'll put my CDK badly out of collimation and take some images to define the views, from uncollimated to collimated.

I'll place these on the MPAS website as a reference for an alternative approach. Brendon is correct that collimation approaches vary between types of scopes, and the GSO RC's add an extra element of trickiness to the equation, as the focuser moves with the primary mirror. I also note my Rc10 had the incorrect mirror spacing which greatly effected it's flat field. The GSO factory finally told me the correct spacing to be 493mm, but I worked it out to 495mm using Ccdware's CCDInspector, but that is another story...

I can add too that the Tak collimating scope can help you get close to collimation, but it will be only a little more effective than the Cheshire.

Just out if interest, those flairs you mentioned are mostly associated with taking images of bright stars? If so, be careful not to over concentrate on these results. Once you get collimation as best as you can, take an image of a reasonably bright deep sky object, then have a look at it. Even when I have the CDK well collimated, with CCDInspector saying zero tilt, I still get offset reflections about super bright stars, eg: Canopus.

So something is obviously causing me this issue, but I'm not going to research it, as it is not noticeable in normal imaging of DSO's.

If you like, I'll send you a link to this alternate guide once I finish it? I also recommend making a graphic guide you can refer back to that assists you, or reminds you, which collimation screws you need to move to make corrections to your secondary, including which way a defocused star needs to be moved to correct it's shape.

BR

Steve :thumbsup:

[RickS]

Steve,

Very interesting idea. I'll give it a go with my RC10. So far I'm fairly happy with the results I have been getting with the Tak Collimating scope, but if this is more accurate and easy to do then I'm in :thumbsup:

I'd love to hear the story about your secondary spacing as well!

Thanks,
Rick.

[troypiggo]

Interesting. Will give it a go if these clouds ever disappear.

[bmitchell82]

Glad to see that my knowledge of light paths and the like isn't up the duff :) Ahhh newtonians soooo much simpler! :):thumbsup:

[strongmanmike]

How does a brain get pregnant :question:

[troypiggo]

Usually happens right after a mind-f#$k. :)

[bmitchell82]

hahaha Im not.. hang on i wont say that next word but its got to do with a cartoon character :)

:D i think its more about the situation ide find myself in Mr strongman :P

As for you Troy... you just make sure you stay over there.! after dark brain pregnancies... geez