Collimating using a star test Vs Cheshire...which is "right"?

[Notty]

Strange question I know! So I went through the steps to collimate my 200PDS using the very good guide by Astrobaby, and everything seems lined up nicely. I do a bit of observing last night and my Star Test shows the black dot not central in the star. So I adjust the main mirror and get it spot on. Out of interest this morning I had a look though the Cheshire and it's misaligned, by quite a lot; the black dot and circle were overlapping but the big mirror needed readjusting to get it all central again.

So which is right? Is it maybe a function of not having the star bang on in the centre of the eyepiece when I did the star test? Obviously as I tuned the main mirror it moved the star out of the centre so I had to re-slew to centre it each time, and I called it "done " when I had the black hole in the star fully centralised with the star centred in the eyepiece.

Does anyone have any ideas, because I would expect if it's collimated in the Cheshire it should be central in the Star test and vice versa.

Thanks for any tips.

[Moonshane]

star testing is very tricky in my humble opinion. you need high power and you also need to get the star exactly in the centre of field or it's a waste of time. I never bother and rely on the Cheshire result as long as the images are sharp and contrasty.

[todd8137]

I go with the above I always check my dob every outing with a Cheshire never to a star test if it's ok with the Cheshire then it's good to go ,

Pat

[Northern Soul man]

Hi Ya Notty, I was always led to believe that the star test was the main and only test that would show perfect collimation, but as Moonshane says, it can be a bit hit and miss.  I only have an SCT now, but the star test in the SCT has good reference points - so long as the atmosphere is steady, the star your using is a good way up the sky, your scope has cooled enough - all these add to the problems of collimation - I think a lot of gazers rely too much on perfect collimation - and then become a little "collimation" sensitive when they think that the views could be better.  so long as your near enough and everything looks central - just go and observe - the atmosphere will play havoc with the views on some nights, when I check collimation, some nights just aren't good enough, as you have, to a certain extent, concentric rings in the view down the draw tube, the SCT concentric rings just in and out of focus are a really good reference point - you can see right from the off - if there's even a slight adjustment needed - this will show up well in the rings, but with the Dob/newt your concentric circles become the different mirrors - trying to get the mirror clips on the main mirror in view evenly and the shadows of the edge of secondary nice and round can be a little taxing at best - but with the SCT you only have 1 mirror to adjust.

When new Astronomers come into the hobby, it becomes an absolute nightmare to try and tell them that there is no "special" scope - they are all the same to a certain extent - we can all "see" the same up there, there may be a slight difference in the quality, but all the main players in telescope making use mass produced techniques to "mass" produce their mirrors from only a handful of factories - as we know, the one may just be a "clone" of the other - there's no "magic" scope that does all - so we all have to use the equipment we have - nearly all the same when we start out - yes, when we have been doing it for a while - then if you wish - you can look into "specialist" items - but we all start with the same optics to a certain extent.

I find that the atmosphere will degrade the image FAR MORE than a slight mis - alignment in the optics will - if your anything like me, when I first started I spent more time looking down the wrong end of the scope - I had to keep telling myself - GET YOUR HEAD OUT OF THAT END OF THE TUBE AND WALK JUST INFRONT OF THE FOCUSER AND TRY LOOKING AGAIN!!!!!

Paul.

[laudropb]

I have found the exact same thing Notty, but as I am purely visual observering I just go with the Cheshire and ignore star tests

[Notty]

Thanks for all the replies everyone, unfortunately I bought the 200 pDS for imaging, and from what they tell me this scope needs the collimation nailed for it to perform. By the sounds of it without a reticule eyepiece (which for some reason seem to be priced £££ cough) I may just have to rely on the Cheshire.

[Moonshane]

seeing and cooling will affect visual observing more than collimation

physical shifting of the mirror during imaging can affect collimation as can tube flex due to the weight of all the gear hanging off the focuser.

there is no reason a cheshire cannot give you a close to perfect collimation but in my experience, star testing for collimation purposes is laced with more problems than solutions.

[Andy Milner]

I had the same issue with my 200p. I don't star test fir exactly the same reason. I find it's not far off in a star test and I just carry in and get good images anyway. Everything looks just fine, moon, Saturn, Mars, etc. I'm yet to properly try DSO's but got a good few shots of m42 when I started out in February.

Sent from my iPhone using Tapatalk

[russ.will]

In my experience, do not rely on the primary mirror donut being central. Both of my 305mm Dobs were circa 2mm off, which in an F5 or less scope is a lot.

If star testing delivers a better result and more tightly focused stars, run with it.

Russell 

[ambro]

Helo,

Star test is easier for SCT because central obstruction is concentric.
Fast Newt has big offset. If you defocus too much you will see offcenter shadow from secondary. I am using central mask bigger than SM (about 70mm for 63mm SM diagonal) to move shadow at center for star test time. With mask you can adjust main mirror correctly. Cheshire or barlowed laser will show the same results.

Examples from aberrator:

No offset

small offset

big offset

Clear Sky,

Robert

[Notty]

Robert, thanks for that. I think it might be getting a bit clearer, as the astro baby guide talked about the offset of the secondary, and I guess the 200pds counts as "fast" but can anyone confirm I should still aim to get the dot and donut matched in the Cheshire, but on star testing the black circle will be offset and this is correct?