Collimation Problem

[Tommohawk]

Hi all.

I recently upgraded from a Sky-watcher 6" to an 8". Got it for a bargain price from you know who bay - only been used for display apparently.

Initially I was very pleased - it looked in good order - unmarked and well packed. I set it up and did some photos even though conditions were far from ideal just to test it out.

One thing i noticed was that there was a lot of coma  and this was eccentric ie much worse in one corner of the pictures. I guessed this was an effect of poor collimation, so had a look at that.

First thing I notice is that a coupe of the spider vanes are twisted, so they arent showing the minimum section. I took the spider out and found that a couple of the vane mounts were indeed damaged - maybe the spider got bashed somehow. Not too bad though, and I managed to straighten these nicely and recenter the spider no problems.

Secondary mirror alignement is a barrel of laughs, isnt it?  I folllowed a couple of guides including Astro Baby's - and I have a curious problem. No amount of repositiioning will bring all three of the primary clips into view. I've tried extending the focus tube, and using a cap instead of the cheshire, but I cant get all 3 in view. Its as though the clips are too small or something, though they look OK.

I havent taken the primary off - I'll look at that later today just in case the clips aren't correctly positioned on the primary.

I think I can work around it by simply making sure that all 3 clips are equally displaced from the visibly field - if that makes sense.

But I wonder if anyone else has had this problem, or has any ideas what might cause it??

[Tommohawk]

Oh dear - just noticed the typo in the subject. "Collicmation" - its the stomache ache you get when trying to collimate.

[Tinker1947]

Have you tried moving the focuser inwards to see all 3 clips.....having the tube horizontal so dropping the secondary means it does hit the primary, loosen the centre screw and move the secondary in and out and at various angles so you get a idea where the secondary need to be to see the clips, this should give you a starting point.........

[Spikey]

Oh dear - just noticed the typo in the subject. "Collicmation" - its the stomache ache you get when trying to collimate.

Don't worry, it's easily fixed

[RichM63]

I still can't see the clips holding the 200P primary, but the centre spot is bang in the middle of the Cheshire.

For the secondary, my vanes were also twisted and way out.

Don't give up. When I bought my second hand 200P the PO had moved just about every screw he could have even down to mucking up the spider position. I'll post you a pic or two in a mo'.

Just take your time, maybe have Dion's collimation videos running and hit pause whilst you check against your scope.

A couple of bits of coloured card, bright light, a point and shoot, and download Al's Collimation Aid to overlay concentric circles over your image will get you to be very accurate in next to no time.

Before, note the North/South, West/East differences.

After + overlay of collimation aid.

A good and informative collimation thread can be found here.

But as a kind person pointed out to me when I started measuring pixels, seek medical help.

Best of luck,

Rich

[Tommohawk]

Hey thanks all for superquick responses and spellcheck!!

Have you tried moving the focuser inwards to see all 3 clips.....having the tube horizontal so dropping the secondary means it does hit the primary, loosen the centre screw and move the secondary in and out and at various angles so you get a idea where the secondary need to be to see the clips, this should give you a starting point.........

Tried moving focuser in and out and tried repositioning the 2ary using the centre screw - still cant see all 3 at once. I tried loosening everything and carefully repositioning the 2ary by hand but no position seems to fix it.

I still can't see the clips holding the 200P primary, but the centre spot is bang in the middle of the Cheshire.

Best of luck,

Rich

Thanks for that - I think I've got to grips with the principles having read quite a few threads on this, but will have a look at those too... more data and all that.

But interestingly you seem to be saying that you cant seen all 3 clips either?? In which case that may simply be the way it is and theres no point turning myself inside out trying to fix it?

AstroBaby has same scope I think and seems to see them OK.

[RichM63]

But interestingly you seem to be saying that you cant seen all 3 clips either?? In which case that may simply be the way it is and theres no point turning myself inside out trying to fix it?

Pretty much. I tried everything you have followed and in the end decided that I was never going to see the 3 clips.

From the guides I wondered if SW had made changes to the focuser or the clips themselves, different travel, smaller clips???

In the end I gave up and follow the coloured card and Cheshire center spot method.

The real test is under the sky, I'm happy with pin sharp stars rather than eggy blobs.

I lost count of the number of guides and papers I read on the subject, and decided that as I was following the procedures then I must have been on the right track.

This link also gives yet another theoretical overview of good and bad collimation and outlines a Star Test. Collimation.

BTW, in the 150 I see the clips perfectly, just not in the 200.

hth,

Rich

[Tommohawk]

Hi Rich

Yep, that helps for sure. I reckon it's not possible to see all 3 and I'll just work around it.

I had the impression with my 150 - with which all clips were visible - that there was distortion in the very out margin of the mirror, maybe due to the clamping of the clips. So maybe SW reduced the 2ary to effectively cut the very outer margin of the 1ary.

Not sure how I feel about paying for a 200 and only using 190!

I'll crack on and try and finish the job - thanks all

[Tommohawk]

Ok all done!! In fact I can see the 3 clips with my home made cap, provided I have the focuser all the way in. the image through the Chechire is now identical with that posted by Astrobaby for a fast scope. Result! Well.... on paper anyhow!!

Hopefully get a chance to check it out soon.

Tom

[Ouroboros]

I can't see all three clips through my Cheshire on my 200P. This is simply because the Cheshire tube is too long.

But I can easily see all three clips through my homemade eyepiece consisting of a small, centrally drilled hole. I assume this is equivalent to your cap, Tommohawk. So, I'm rather surprised you can't see them. I'm assuming you can see some of them at least? One of them isn't being obscured by the reflection of the focusing tube is it?

One thing I find tricky is adjusting the secondary mirror angle to get all three clips into view. All three screws seem rather tight, so I feel I'm twisting the spiders. Their motion are not independent either, but I've managed to line it up to get all three clips into view.

I don't feel I can move the secondary far enough from the primary to place it dead centre beneath the focuser. But it must be close enough.

I also feel my Cheshire is rather sloppy in the holder so its cross wires are not central. Nevertheless, I seem to get satisfactory collimation with perseverance.

[Ouroboros]

Ok all done!! In fact I can see the 3 clips with my home made cap, provided I have the focuser all the way in. the image through the Chechire is now identical with that posted by Astrobaby for a fast scope. Result! Well.... on paper anyhow!!

Hopefully get a chance to check it out soon.

Tom

Oh, well done. I must have missed your post.

It'll be interesting to hear you get better results when you get the chance to use it. Hopefully.

[Moonshane]

seeing the clips when collimating is a complete red herring. it's just a steer to where the edges of the two mirrors are in relation to each other. some scopes don't even have clips.

to see the clips and/or edge of the secondary with a Cheshire, just use an extension tube to bring it out.

http://stargazerslounge.com/topic/186348-collimation-tip/

[Tommohawk]

seeing the clips when collimating is a complete red herring. it's just a steer to where the edges of the two mirrors are in relation to each other. some scopes don't even have clips.

to see the clips and/or edge of the secondary with a Cheshire, just use an extension tube to bring it out.

http://stargazerslounge.com/topic/186348-collimation-tip/

Take your point about the clips - I did realise this was just for centring purpose.

Oddly the solution to not seeing the clips seems to be either use a cap and get closer, or, counter-intuitively perhaps, extend the collimator so that the collimator tube doesn't restrict the field.

Anyhow - I fired off a few subs with the newly collimated setup and all looks good. Thanks for the feedback

[michaeldurban]

I have the 250P and didn't even know it had clips..(..eh..joke..)

I can't see them with my Cheshire, but like the above posts point out

there is no need for it...

I too was afraid my secondary was too small, but after checking it turns out

it isn't...or maybe it is..a few millimeters..not sure..

anyway..

I used the Cheshire with a self centering adapter...but that doesn't really work either.

so now I ordered a Glatter Parallizer..I'm confident that will give me repeatable results.

[Ouroboros]

Are the clips a complete red herring? If all three clips are not visible through a cap, which centralises the eye on the axis of the focusing tube, doesn't this indicate that light from the edges of the primary mirror are not being transferred satisfactorily through the optical train to the eyepiece? I'm not suggesting clip visibility is the whole story. But it's a good starting point isn't it?

[itmo]

Try rotating the secondary. you might have a small rotational error in the secondary which you have compensated by tilting it with the collimation screws. Also the classis snafu in skywatchers is that the collimation screws have dug small pits in the secondary holder , so each time you touch them they will rotate the secondary in addition to tilting. See internet for milk washer mods to counter this. 

[astronymonkey]

Not seeing the3 full mirror or clips can also be a symptom of the mirror having the collimation bolts too far in . This moves the mirror up the tube meaning the edges can jo longer be seen. Ive noticed this on ky lightbridge but the collimation bolts give around a centimetre or so of travel and I think the 200p is similar. As shane has said though but not seeing the whole mirror is no big problem.

Cheers

[Moonshane]

in terms of secondary size, the things that matter are

primary size

focal length/ratio

size of field largest eyepiece field stop

distance of secondary from focal plane

size of drawtube diameter

the main thing is that the scope fully illuminates the bit you see through the eyepiece. if using a 20mm plossl and a 20mm Nagler in the same scope, the field stop sizes will differ and therefore the required fully illuminated field will differ.

read

http://www.catseyecollimation.com/mccluneytext.html

especially from

"A widely misunderstood but extremely important optical effect caused by the secondary mirror is focal plane illumination (also known as field illumination). Focal plane illumination is the ratio (shown as a percentage) of how much light actually arrives at the focal plane compared to the amount that should have arrived. The ideal arrangement in any telescope is that every incoming light ray reaches the focal plane. However, this cannot happen with the classic Newtonian design for several reasons: 1) the size of the secondary mirror would have to be undesirably large; 2) the secondary mirror blocks some of the incoming light; and 3) reflecting surfaces aren't 100% reflective. Technically speaking, light losses from number 2 and 3 above do not 'count' when calculating focal plane illumination but will be discussed here anyway since they do reduce the amount of light delivered to the focal plane.

including especially point 1

this explains all this very well I think. as always it's about compromise.

[Tommohawk]

Thanks for further thoughts on this.

Clearly there is nothing magical about Seeing The Clips. Its just a way of visualising the 1ary for alignment purposes. That said, I guess its true that if the periphery of the 1ary cant be seen from the image plane then you arent using all the 1ary.

Anyhow, in my case the main reason the 3 clips weren't visible initially is because with the cheshire, the tube itself blocks the light path. With a cap its OK -the clips can be seen, though you have to be quite close which is what caught me out - I thought being further away would be better.

I think that whilst SW are made to a price, the basic design should be OK. So the issue about 2ary mirror size really shouldnt be a concern.

i guess it must be true that the 1ary position will affect this - the closer it is to the 2ary the less likely you wil see the whole thing. But this is all fine tuning I think.

Adjusting the 2ary is a right pain - once any of the adjustment screws are loosened in order to tighten the opposing one, the whole assembly is free to rotate. Anyhow... after a lot of perserverance I got it straight and am happy with the optical result. Posted M27 here.

[Moonshane]

if you follow the link on post 12 above you should be able to see the clips no matter how big or small your secondary. that said, if you centre your secondary ignoring the primary and then centre the cross hairs of the Cheshire on the central donut taking into account the primary you will be centralised.

[Tommohawk]

Moonshane - I can see how pulling out the focusser with an extender would help to see more of the 2ary, but does this help see more of the 1ary too??

I say this because I could see more the closer I got