Star testing for collimation.

[sheeprug]

As people might have noticed from my recent welcome message, I'm getting somewhat disappointing resolution from my Orion  Optics 14" dob.  I rather suspect it's a collimation issue, though I've set it up with a cheshire  pretty well  - to my untrained eye at any rate.   However, it does look like there's a little coma and astigmatism.  The mirror is stamped at 1/4 pv.

So I'm wondering if trying the sort of star testing described here might be worth trying: 

http://www.backyardastronomy.com/Backyard_Astronomy/Downloads_files/Appendix%20A-Testing.pdf

or

http://www.telescope-optics.net/star_testing_telescope.htm

It also looks like a pretty useful sort of skill to acquire. 

Big problem is the necessary magnification: 40-50 x the aperture in inches.   That's 560x to 700x for my scope.  I can't imagine i'd use that in normal use (how do you track a dob at 700x?), but a 3x barlow would get me to that range with my current eyepieces.

So, question is does star testing work in our skys in the UK.  Do many people use it, or is the atmosphere usually too disturbed. And if so, is it worth splashing out on a 3x barlow which would give me magnifications i'd probably very rarely use for normal viewing.   Or should I try some other collimaion technique?

Best,

SR.

[Dave In Vermont]

In the case of a mammoth 14" mirror, I would forgo the 40 - 50X guidelines. And I'd suggest a 2nd magnitude star that doesn't move around much - Polaris is the usual pick-of-the-litter. For magnification, 300X is what I'd try for. This depends on seeing conditions. You want to use what will give you the most detailing in the out-of-focus zones in the de-focused disk.

Do try to get as close to perfect collimation prior to conducting the star-test. It's best to work on mirrors with screwdrivers in a well-lit area. You don't want to drop one down onto your primary. And try to work with the tube level, or pointed downwards, in case the screwdriver does jump out of your hand.

Artificial stars are available these days. They add another layer of ways to go about the star-test.

Go slowly and breathe - you'll get there!

Dave

[Laurie61]

Hi, star testing does work and its my prefer method after first using a collimation cap. If you can see Polaris you don't have to worry about tracking, just re centre star after adjustment. I guess the performance problems can't be caused by cool down times ? 

[cjdawson]

Collomation is a key to getting the most out of your optics.

First of all, a scope that size is going to take time to cool down.   I don't use Cheshire's myself (have an SCT so they don't really work)  So let's jump straight to the star test.

As others have pointed out, be careful not to drop stuff on your scope and break things - makes for a very bad night.

Looking through the eyepiece, get your scope on target - polaris is a good place to go.

Then defocus the scope.  Take a look at the rings that form.  What you are looking for is a round donught shape.  The more circular the better.  When you first get the scope out, it's likely that you'll see wobbles and spikes on the doughnut shape.  These will be caused by your scope needing to cool down, and by atmospheric conditions.  Leave the scope for about 30 mins and see if it gets any better.  You should notice that there will be fewer spikes and things.  For a 14" scope it might take a few hours for everything to cool properly.  (This is why you see professional observatories opening their domes about an hour before sunset - it lets their optics cool down.

Once you get to a good clean doughnut, the idea is to make sure that the hole in the middle (caused by your secondary mirror) is centered.  Make small adjustments to center it, then change to the next higher power eyepiece and repeat.  Once you get to your eyepieces maximum, and still have a centered whole, then your collomation is good.

You can do this with an artifical star, which will get you alot of the way there, but nothing beats an actual star test with final tweaks.

Hope this is useful.

[StargeezerTim]

Hi,

I did this for the first time this week. When I defocused by shortening the focuser, a nice circular donut was visible. When i defocused by extending the focuser, the donut slightly flattened on one side. When I looked up the result, it says that it is typical of poor collimation and small adjustments of primary was needed.

I didn't try adjusting because I thought that if I get a round donut on the extended focuser, it might ruin the good result when the focuser is shortened.

Should I have worried and is it OK to adjust the primary in situ (i.e. whilst looking at the defocused image?)

Sorry if this doesn't make sense! Tim.

[cjdawson]

If by extended focusser you make a Barlow lens -   basically a lens that you put into the scope to double the power of your eyepieces.  I do use one when collimating, again this is increasing the power and the idea is to get the best collimation at the highest power.

[StargeezerTim]

If by extended focusser you make a Barlow lens -   basically a lens that you put into the scope to double the power of your eyepieces.  I do use one when collimating, again this is increasing the power and the idea is to get the best collimation at the highest power.

No... by extending the focuser I mean just turning the focus knob outwards and shortening is getting the donut by turning the focusing knob inwards (shortening the lenght of protruding barrell). I knew I would have trouble explaining this Lol.

[sheeprug]

In the case of a mammoth 14" mirror, I would forgo the 40 - 50X guidelines. And I'd suggest a 2nd magnitude star that doesn't move around much - Polaris is the usual pick-of-the-litter. For magnification, 300X is what I'd try for. This depends on seeing conditions. You want to use what will give you the most detailing in the out-of-focus zones in the de-focused disk.

<snip>

Dave

Thanks Dave. 

That sounds more sensible, but the way I read it you needed to use the high magnifications to see the diffraction patterns properly?

SR.

[Dave In Vermont]

Yes - that's why high-mag. is used. Sort of like cleanly splitting close double-stars. But if the mag. is so high you can't see anything but a muddy blur - what good will that do you?

Common sense rules the roost. Go high - but still discernible.

Dave

[Moonshane]

Ensure that the star is dead centre though or collimation will always look off. Quite how people do this to within say 1% error I have no idea so I never star test, just rely on my eyes and accurate collimation.

[cjdawson]

No... by extending the focuser I mean just turning the focus knob outwards and shortening is getting the donut by turning the focusing knob inwards (shortening the lenght of protruding barrell). I knew I would have trouble explaining this Lol.

Ah got-cha.   In that case, what I'd suggest is find the focal point.  This is what the collimation is all about.  Then turn the focus knob out until you get a doughnut - as you pull out the doughnut will get bigger, but dimmer.  The point of this is that the black spot in the middle is in the centre of the doughnut and not off to one side.    If your scope is collimated correctly, the doughnut should stay centred over the entire travel of the focuser.   When I do this, I like to choose a fairly bright star (for my 8" scope) I like to use a star like Vega as it's nice and bright.  Then I can run the focuser out a long way to produce a big unfocussed image with a big black spot that I makes it easy to how far off centre the collimation is.   The higher power eyepieces allow for more sensitive adjustments (fine tuning)

[sheeprug]

Yes - that's why high-mag. is used. Sort of like cleanly splitting close double-stars. But if the mag. is so high you can't see anything but a muddy blur - what good will that do you?

Common sense rules the roost. Go high - but still discernible.

Dave

Thanks Dave, All 

I've added a Baader 2.25x barlow to my armory, giving me new magnifications of 330x an 540x.   Looking at polaris last night if I de-focused slightly I could see a reasonably neat doughnut showing the collimation was pretty close, but here was too much disturbance to see any diffraction rings.  This did improve as the night went on  (mirror cooling  down?).          Later i moved to try splitting the doubles in epsilon lyrae, which were more or less overhead.  I couldn't quite split them cleanly due to, i think, the atmospheric conditions.   On the other hand, it might be the limit of the scope - it's a 1/4 PV mirror.   

So,  a couple of questions:  

1. are there any websites out there which predict/forecast/monitor seeing conditions?  At this stage I'n not clear what good or bad seeing conditions look like.

2. is it possible to relate mirror specifications to telescope resolution?  I'm assuming yes, but it's probably very complicated.  If anybody else out there has similar quality mirrors it would be interesting to see what resolution you achieve.

[skybadger]

Hi

Ive read this with an open mind (honest!) and feel I have a few pointers.

To keep the star centred for good collimation only turn half the adjustment you ever need, recenter and do again,

Don;t be afraid after some adjustments to re-check the secondary and re-adjust that. Its an iterative process.

The doughnut should collapse from out of focus rings to a central point and then to out of focus rings on the other side. The pattern should be symmetric. If its not its either astigmatism or misalignment,

There are other causes of fuzzy stars, like mirror roughness as well as low suface accuracy, thermal conditions and collimation. So open the scope early and watch the air currents over the star slowly decay. Watch out that the ground isn;t concrete, tarmac or paving slab which will also have its own air currents.

Avoid collimating at anything less than 60 degrees altitude - you need round stars, in focus as well as out of focus. IT ha been shown that refraction will cause noticeable flattening of stars (see Metaguide website) Collimation is also prone to shift over 60 degrees of altitude if you collimated first by eye horizontally so check it hasn't moved. This is the key problem if you want to use a synthetic star. Synthetic stars are great if you mount them at 60 degrees altitude where you use your scope..

High powers are only required for fainter stars, in focus, when you can see the airy disk rings in good to perfect seeing,

Centering out of focus rings on bright is a starter process that gets you on the way but you need to go the fainter rings to be able to see the close-in diffraction rings which are overwhelmed on bright stars, Mag 4 to 6 is good, maybe fainter on a bucket!

Check that equal amounts of out-of-focus result in symmetric images either side of focus - the central hole of the doughnut should be the same size and the rings should have the same emphasis. This is a test for spherical aberration.

Use moonlit nights for collimating  - I hate missing dark ones.

Finally, as you get close, very small movements are critical for perfect collimation - which is why scopes that retain collimation well are prized. That last ounce of tweaking can make all the difference betwwen a Skybadger and a Peach.

I'm currently re-assembling my 12" cat and spent last night going through the same rigmarole. It is easier on a newt though.

HTH

Mike

[sheeprug]

Thanks for taking the time to reply Mike.   There's some interesting thoughts there, particularly about the type of ground.   I'm setting up on my patio at the moment, so if concrete slabs take a lot of time to cool down that might be an issue.  

SR.

[cjdawson]

Thanks Dave, All 

I've added a Baader 2.25x barlow to my armory, giving me new magnifications of 330x an 540x.   Looking at polaris last night if I de-focused slightly I could see a reasonably neat doughnut showing the collimation was pretty close, but here was too much disturbance to see any diffraction rings.  This did improve as the night went on  (mirror cooling  down?).          Later i moved to try splitting the doubles in epsilon lyrae, which were more or less overhead.  I couldn't quite split them cleanly due to, i think, the atmospheric conditions.   On the other hand, it might be the limit of the scope - it's a 1/4 PV mirror.   

So,  a couple of questions:  

1. are there any websites out there which predict/forecast/monitor seeing conditions?  At this stage I'n not clear what good or bad seeing conditions look like.

2. is it possible to relate mirror specifications to telescope resolution?  I'm assuming yes, but it's probably very complicated.  If anybody else out there has similar quality mirrors it would be interesting to see what resolution you achieve.

The disturbance is likely to be a combination of factors.  Mirror cooling down is the fist, I've hears of 14" mirrors taking hours of cool down.   Closed optics like and SCT or refractor also take longer to cool.    I've been thinking for a while about making a fan blower that I can put into the eyepiece hole of my Sct, to force airflow inside the OTA.  Basically a couple of piece of pipe - one inside the other, the smaller one, blows air into the scope (holes in the side of the pipe to let the air out) and the larger to allow the air out again.   I'm not sure of all the practicalities of this.  I've not done this project yet, as I'm aware that there could be an issue with blowing dust into the closed part of the scope optics - which is the last thing I want.

As skybadger said, atmospheric conditions, and local conditions can also contribute to distortion.  These tend to result in the image jumping around rather than the image itself distorting.  Image distorting, tends to be a localised scope issue (from my experience).    If you are at the point where you are using high power eyepieces, and find the distortion is the main factor holding you back, I'd say that you've pretty much reached the limits of your setup (including location) and would be happy to say collimation completed

It's worth checking on another night, when the sky conditions are different as you might find that you can improve the collimation, but on the other hand you might find that you can't even get to the same point you were before as the overall conditions are not as good as the previous time.   At this point, the changes that we're talking about are tiny and will give you that final slip of adjustment that makes people look at your scope with envy

[sheeprug]

Thanks Nebula.  Intereswting your comments about fans.   The OO14" mount does have a fan, but it's actually a rather puny thing. I'm feeling rather tempted to beef it up a little.  On the other hand, the primary did get a little dew on it the other night, so it can't be too warm!

SR

[sheeprug]

Thanks Nebula.  Intereswting your comments about fans.   The OO14" mount does have a fan, but it's actually a rather puny thing. I'm feeling rather tempted to beef it up a little.  On the other hand, the primary did get a little dew on it the other night, so it can't be too warm!

SR

Apologies  - I mistook your group status for a username! 

[Stu]

Apologies - I mistook your group status for a username!

At least they weren't a Vacuum

More seriously, the more I read about thermal equilibrium and cooling issues, the more difficult it seems to be! I used to have an 8" Mak which was a devil to cool. Amazing when it was, but tricky to achieve. The SCT I have now is likely to be similar.

I think sucking air out of the scope is probably better than blowing in, less chance of blowing dust and dirt inside. The cool air should find its way in through other vents I think.

I've recently seen a comment about using flexible ice packs strapped around the mirror location on a Mak or SCT. This makes a lot of sense in that if you can get the mirror down to, or possibly even below ambient then it will stabilize more easily. Similar to the water cooled Newt which Olly mentioned recently.