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Drift Alignment - Practical Problems


gnomus

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I decided that I would grab the bull by the horns and attempt to drift align my CGEM mount last night.

I found some instructions on the internet.  I thought I understood what needed to be done, and I headed out with my (inexpensive) illuminated cross hair reticle eyepiece (12.5 mm for my 2032 mm focal length SCT scope).

I set up the CGEM in the usual way.  I did a "rough" polar alignment using my polar scope.  I did then do the 2-star alignment routine (with 3 calibration stars) - I realise I probably didn't need to do this.

I then slewed to a southern star roughly 20 degrees above the celestial equator (don't ask me which one).  I ensured that the eyepiece was aligned so that the star travelled parallel with one of the crosshairs when the mount was adjusted using the handset.  

This is when things started to go wrong.

The CGEM mount is attached to the tripod by a bolt up through the centre of the mount.  When doing the "rough" polar alignment one leaves this bolt slightly slackened until you get the altitude and azimuth sorted out.  Then one tightens up the central bolt and leg-spreader.

It seems clear why one needs to do it this way, and I assumed that I would have to do the same when doing drift alignment.  I observed the southern star.  When it drifted, I made some adjustment to the azimuth adjustment knobs.  I looked through the eyepiece whilst doing this.  The movement when adjusting seemed a little "jerky".  For example, I would make a small adjustment - but the star didn't seem to move at all.  Another small adjustment - same again.  Another adjustment - star jumps quite a bit to the right.  I worked with this as best I could for a while.  I think I got to a point where the star was barely moving over 5 minutes.  However, I now needed to tighten up the central bolt and leg-spreader again.  Doing this caused the star to move visibly in the eyepiece.  I played about for a bit but am not sure that I got the azimuth set correctly.

Since this was my first attempt, I thought I should have a go anyway with adjusting the altitude on an easterly star (I have a house to my west).  Essentially I had similar problems.

I decided to go ahead and try imaging.  I was using my Nikon piggy-backed on top of the scope with a lens length of 340 mm.  I tried a 2 minute exposure.  I was not all that impressed with the results.  If anything, I got as much trailing as I had had using the Celestron "all star" polar alignment method a few nights previously - but shooting through the 2032mm  (x 0.7 reduced) scope.

Can someone offer practical advice as to how to make these adjustments?  I assume that I do have to slacken the central bolt?  Could the jerkiness I had when adjusting be due to excessive friction between the mount plate and tripod plate (should I grease these surfaces perhaps)?

I do have the gear for guiding, but have not tried this yet.  Can I guide with a "rough" alignment, or does the polar alignment have to be pretty much spot on for guiding to be successful?

If you guys can offer me any pointers, I would be very grateful.

I attach an image of the trailing I got with a 2 min exposure using the 340 mm lens.

     

post-39248-0-62503400-1416848453.jpg

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Hi

Things are always more demanding with a long focal length... I take it you've balanced your setup for imaging first? I don't know your mount but I have an AVX which I think is similar? As regards the central bolt I have mine tight but not so tight that it's difficult to adjust in azimuth. If the adjustment bolts on the cgem are anything like those on the AVX then they are quite chunky and crude... It takes practice to get used to making the necessary adjustments. Only very fine adjustments are required once you are close to good PA. Personally, I take a computerised approach and use Astrotortilla to do my PA :). PHD2 has a drift align routine though I've never used it. You'll probably struggle to image much at 2032mm without guiding and good PA. I try to get my PA within ~1 min of arc if I'm imaging with my 150pds, fl=750mm, on my heq5. With the ST80, fl=400mm, on the AVX, I can get 180s unguided with good PA, and PA error can be relaxed somewhat with guiding. Nobody has taught me this, it's just what I've found works for me :) Um, the AVX has an ASPA routine for doing PA with the handset though I've never used it. Presumably the cgem handset also has it?

Maybe someone else that has your mount can chime in and confirm how they do there setup and PA.

Louise

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As you say friction causes the mount to jump once the bolts overcome the resistance, I had a similar problem and cut a circle of 3mm teflon to fit between the tripod and the mount. 

As Louise say you only need to slacken the centre bolt a fraction even less with teflon plate.

Dave

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I would add this to your routine.

Drift for a couple of minutes or so, then note which way you need to move the mount.

But first of all, you must re-centre the star.

Then when you loosen, adjust, and re-tighten the bolts, you will be able to see that the mount has ended up in a new position.

And you can repeat that adjustment until you end up with the amount of adjustment you actually wanted.

Then carry out another drift and repeat until happy.


Michael

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And only worry about North / South drift, ignore East / West, align one axis of the camera or reticle to RA so its square to the drift, you don't need to know which is North and South just adjust whichever direction brings it back to the N / S centre. best start with big adjustments until you reverse the drift and then refine it.

Dave

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Do you guide? PHD2 has a wonderful Drift Align wizard that's very simple to use once you get the hang of it.

Phil

I now have the gear to do so, but have not tried it yet. I have a ZWO 120 camera and I bought an adaptor to allow me to attach this to my 9x50 finderscope. I've downloaded PHD (I don't think my version is v2) and I can get a picture on screen indoors. I've not had the chance to try it in the field yet. Any pointers would be most welcome.

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I would suggest getting PHD2 as I don't think PHD has the option for the drift alignment wizard.

Phil

OK. Where might I get hold of that? I went back to the Stark Labs site. I found the version I aleady have. I also downloaded a "pre-release" version, but this was showing as v1.14.xxx (or something close to that - i'm on a different machine at the moment), but certainly not as v2.

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Number of times the objective diameter goes into the length.

Dave

Is that the focal length or the focal ratio?

I'm asking because PHD 2 wants me to enter a focal length for my guiding scope - which is my finder scope (for now anyway).  

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Hello mate,

I've tried looking this up for you but it seems you need to take your finderscope apart and do some calculations with measurements etc :/ You could just estimate it perhaps? I'm not sure how accurate it needs to be for the drift alignment.

Phil

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Is that the focal length or the focal ratio?

I'm asking because PHD 2 wants me to enter a focal length for my guiding scope - which is my finder scope (for now anyway).  

Example.  Focal length (just the tube length) =  200mm, objective dia' = 50mm, focal ratio = 200 divided by 50 = f/4

Dave

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I think they are typically about 180mm but a quick check measuring from the plane of the objective to the eyepiece will confirm.

Louise

Thanks again for everyone's help.

OK - if it is as simple as measuring, I can do that.  The only specs I could find on a similar finderscope was this http://www.amazon.co.uk/TS-Optics-Angled-Finder-scope/dp/B0077667G6/ref=sr_1_16?ie=UTF8&qid=1417042633&sr=8-16&keywords=celestron+finder+scope which indicated a focal length of 208mm.

How critical is it that I get this measurement correct for PHD to guide?

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