Colimation nightmare

[robster]

I need help please....I'm still a newbie, and have had unsatisfactory focus issues since I started with my setup - (SW 200 P DS on EQ5(goto). I'm using the stock 28mm and a BST 8mm, and can never seem to acheive crisp focus.

I have a chesire colimator, and have followed astrobaby's and astroshed's instructions, and thought I had it lined up right, but obviously not.

I read somewhere that a de-focused star should look like a donut through a correctly colimated scope, but mine don't.....I took a photo of what I get and have pasted it below (please ignore the movement - 8 second exposure with no tracking), the shape is obviously not donut shaped......

So is it colimation that's the problem? or is there something else wrong with my setup.

How do I correct it?

Any ideas would be very much appreciated!!!!!

Anyone in the Exmoor area fancy a pint / coffee / tea / cake one weekend to teach me the laws of colimation, please PM!!!

Thanks

Rob

[Gomtuu]

I have a 200PDS on EQ5.

This is what I see down the focusser tube when my scope is collimated. Second photo is down the Cheshire.

Hope these are helpful. I don't usually do a star test, but I suppose I ought to really.

[umadog]

First of all. What do you mean by "unsatisfactory focus". What are you expecting to see that you aren't getting? Are you sure it's not just bad seeing (atmospheric turbulence) that you're experiencing?

It's not possible to tell in your photo what is going on. There is too much motion. Looking at a defocussed star to evaluate collimation is known as the "star test" You can conduct this with Polaris so there is no motion. For the star test to work you need to be at fairly high power (over about 15x or 20x per inch) and you should defocus only a little. If you have defocused so much that you see the large black circle and cross-hairs of the spider then you've gone way too far.

At the start of this movie is what it looks like when you've gone too far:

The resolution in that movie isn't good enough for you to see what a true star test should look like. It's hard to photograph, but it should look something like this: http://www.hubbleoptics.com/sitebuilder/images/SP32-20090508-130016-230x522.jpg Concentric rings is what you're after. It will look SMALL. Smaller than these images suggest.

However, what you want is to have the Cheshire views look like the post above. The star test is just to double-check that everything is good. e.g. if you centre spot is in the wrong place then the view through the Cheshire will look OK but the star test will always be off. Once you know your tools are right and everything is set up then you don't ever need to do a star test. In fact, good collimation tools will be more accurate.

[Mr Q]

robster - Doing a star test is the best way to check a scope's collimation. Laser collimators may need aligning themselves but a star test is the final true test. Of course doing a star test when the "seeing" is good is a must, otherwise the rings will be very hard to see, if at all.

In the six images above, the bottom pair is what you are trying to achieve. If there is too much shimmering in the defocused star, try one near the zenith (nearly overhead) and it should be at least of 2nd magnitude.

As noted above, use a high power EP and center the star in the field of view before de-focusing. The below links will show images of poor and good collimation as well as how to do a star test. Also are examples of what poor collimation does to views of the planets.

COLIMATION 1

ABOUT COLLIMATIOM

[Stargazer_00]

I've never once seen concentric rings. When I defocus stars all I see are floaters.

[Peter Drew]

From your photo of the collimation I wouldn't think it was far enough out to give the problems you describe. I would check whether the primary mirror is tight in its cell, a mirror should be able to just move freely. A tight mirror can easily give a distorted image.

[umadog]

There are a few problems with the star test. It needs more experience to get right than does use of a Cheshire combination tool because it's less obvious what to look for. The star test is hard to do if the seeing is bad. It's hard to read small errors with a star test and it's not as easy to figure out what to tweak. The star test can't be used to adjust the secondary.

For these reasons OP is best off getting the sight-tube and Cheshire readings correct. If these are right and the centre spot is well placed then the star test will also be right. Every time--It has to be. If he can't get a particular thing right then we can give advice about that specific aspect. We can't we do that with the star test.

In my experience, I never double-check collimation with the star test. If the auto-collimator says it's right then it's right. And an auto-collimator is easier to read and more accurate than a de-focused star. If planetary detail eventually becomes blurry but the seeing isn't worse then usually that means collimation has slipped. At that point I'll confirm this with a star test and re-collimate with a Cheshire and/or auto-collimator.

[robster]

First of all. What do you mean by "unsatisfactory focus". What are you expecting to see that you aren't getting? Are you sure it's not just bad seeing (atmospheric turbulence) that you're experiencing?

What I mean by "unsatisfactory focus" is that I can't get hard, crisp, focus on stars, planets or DSOs - this in consistant across both my EPs (SW 28mm 2" and BST 8mm) and my camera (EOS 5D MKii). Things that I try to focus on always appear to be soft - a direct comparison to a friends 200P puts mine in the shade.

Unless there is a fault with one of the mirrors, I can't think of anything other than collimation. How do I rule out all the possibilities?

Thanks for your help so far.

[robster]

And further to last post - Having just watched the vid above - I can never get the spider vane star burst effect - always stays just outside of that level of clarity.

[Stargazer_00]

Might sound stupid but can you post a picture of your eyepiece in the telescope?

Also; have you ever had the scope come to focus? They are usually delivered pretty much 'OK' at this aperture and less. 8" should be quite robust to mirror slippage.

[Tim]

Hi,

If you remove the eyepieces all together in the daylight and stare down the empty focus tube using one eye only, from about 5 feet away, what do you see in the secondary mirror?

Tim

[robster]

OK, Thanks for the replies....here's my replies.....

There's a pic of my EP setup below. I can find the focus "sweet spot", it's just not crisp. I've never really seen a crisp image through it. I was initially putting this down to cheap EPs, but now I'm not so sure. - I've had the scope for nearly a year, but shortly after I got I got posted away from home, so it hasn't got much use until now.

Removing the EP and looking down the focus tube from five feet - I see the primary, but with a black crescent on the bottom left hand edge, I would say at it's thickest part, it's somewhere between 5% to 8% across the image circle.

Hope this all makes sense!

Cheers!

[Mr Q]

Not getting a "crisp" image can be from several reasons. The most likely one is trying to get the scope focused at the delecate clear spot, which becomes harder as the EP power is increased. If all EPs give a slightly out of focus image, this can be either bad seeing conditions or a mis-collimated scope.

Because seeing conditions change from location to locations and even minuet to minuet at one location, it's hard to tell which condition is causing your problem. Since all collimating tools may have some errors in their quality or alignment, the star test is the bottom lime test IF the seeing conditions permit such a test. Unless both scopes are used at the SAME TIME/SAME LOCATION, there is no accurate way of comparing two scopes.

If the conditions are not good enough for a star test, there is no guarantee that any collimating tool will be accurate enough to align the optics properly.

[Stargazer_00]

Is that a conservatory? Has it been stored in a conservatory for a long time while you were posted away? Just wondering if excessive heat could've warped the mirror cell or the secondary mirror perhaps.

The picture looks set up all fine otherwise. I'd expect sharp images, not refractor sharp but sharp nonetheless.

FYI my earlier post about not seeing conentric rings has been bugging me all day, I just had a 10 minute or less break in the clouds near sirius and got the dob out just for that tiny star test. I found that you cant get the concentric rings at all with mediun or low power but at high power (i used a 5mm) you can get them fine. Mine are perfect, which is nice to know. I had always wondered... anyway, pop your highest magnification set up in and point to a bright star, as it defocuses keep moving really slowly and you'll see rings start to appear. It's very easy to focus right past this point so go slowly.

[robster]

Hi,

Thanks for the comments....no....not a conservatory - just a room with a lot of windows! And it's not stored there - it's usually kept in it's original box in the spare bedroom. Got me worried now about warped mirror cells - can this really happen to glass? (is the primary glass?)

Confused of Exmoor!!

[Stargazer_00]

Heat can warp plastic parts, for sure. But that would be unlikely and considering how you've stored it, not even a consideration.

Any local astro club near you that you can take it to and have someone else look at it?

[Moonshane]

are you leaving the scope outside to cool outside for maybe 30-60 minutes before you observe? if not this is the most likely cause of soft images at such low powers.

even if you put the collimation way out or put your hand over the end of the scope you should get a pretty sharp image on a cool scope at 40x or so (is it an 8" scope?).

[Moonshane]

sorry, just noticed it is an 8" scope in your OP. I'd give it a good hour to cool.

[Gomtuu]

looking down the focus tube from five feet - I see the primary, but with a black crescent on the bottom left hand edge

Any chance you can post a snapshot of it?

[robster]

Here's a pic - sorry....beem busy at work this week.

Tough photo to get - I'm sure the mirror is central under the focuser, although this shot looks like it might not be. It's quite difficult to see the black cresent, as it was black against a black blackground, but if drawn a red line about it so its easier to see....

[Galilean]

The pic doesn´t appear to be too sharp either!!!

[robster]

The bit that's important is sharp - that's good old bokeh - and on that lens is something astounding (that's £1,200 of Canon's finest glass there!!) - you don't need to see a sharp focuser

[planetdnb]

i had major troubles with my 200PDS and it turned out to be bad atmospherics! Just keep trying, it's a fiddly process but you'll soon crack it!

[Pyrodave]

I've had my 200P about 12 months but its not been used very much, every time I use it I check collimation and it looks to be OK (I think)

I've done star tests but never got concentric rings just a wobbly bubble of light I think this could be down to atmospherics. TBH I think I've spent more time collimating than actually observing

[Moonshane]

I've had my 200P about 12 months but its not been used very much, every time I use it I check collimation and it looks to be OK (I think)

I've done star tests but never got concentric rings just a wobbly bubble of light I think this could be down to atmospherics. TBH I think I've spent more time collimating than actually observing

this is why in the UK a star test is pretty much useless as a method of collimation in my experience other than on those nights of perfect seeing.