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FORUMS Photography Talk by Genre Astronomy & Celestial Talk 
Thread started 17 Nov 2009 (Tuesday) 12:00
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New toy! William Optics Megrez 72 on my Astrotrac

 
MintMark
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Nov 17, 2009 12:00 |  #1

Here is a picture of my new telescope, a William Optics Megrez 72 which I have mounted on my Astrotrac. The telescope is mounted on the ball head, which is on the Astrotrac, which is on the geared head, which is on the tripod.

In this picture I have an extension tube, barlow lens and camera (via adapter) attached to the scope. This is its most precarious configuration! I have enough parts for three configurations

  • 344mm f4.8 with William Optics flattener III (0.8 reducer).
  • 430mm f6 prime focus with extension tube.
  • 860mm f12 with 2x barlow and extension tube.
I anticipate using the first setup mostly and trying the others for moon and (bright) planets. I haven't had enough clear weather to try everything yet, but I have some initial impressions.

The geared head (underneath) seems happy dealing with the load so far. The ball head can hold it all too, and at high elevations, but making fine adjustments is difficult. Framing the shots is hard, although I've found that using my angle finder is useful for bright targets. The whole setup, supported in the middle, is susceptible to movement so you have to be careful not to jog it during exposure. The megrez support has cork pads and the quick release plate has rubber pads, so between them it is quite springy. Still can't get used to all my equipment being held in place by tiny thumbscrews!

The second picture shows crops from a two minute exposure around Capella through the reducer/flattener. You can see centre and corner areas. My question is, is that normal for the corner stars using the flattener? I want to try the same test with prime focus but no chance yet.

I have used the scope visually and looking at Jupiter was lovely. My highest power is with a 15mm eyepiece and 2x barlow (= x57) and I could see four moons and (I think) two bands on the planet.

So, if anyone else is thinking along the same lines... it is possible and a very flexible setup. I can be outside and viewing in no time, aligned and imaging in not much longer. The hardest part is framing the shot. I can't fully endorse it for photography yet because I haven't had much experience, but I'm hopeful. I can see that I might have to improve the stability of the mount.


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Mark

  
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Adrena1in
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Nov 17, 2009 13:31 |  #2

Nice one Mark, the Megrez 72 pretty much took over from the ZenithStar 70mm that I've got...I expect they're pretty similar. Plus I always forget I've got the Focal Reducer as well, and can shorten the FL and speed the scope up...thanks for reminding me! (Never used it you see!!)


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Nighthound
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Nov 17, 2009 17:13 |  #3

Congrats on the new toy Mark. I can see where stability would be an issue. Your camera is quite a bit of weight mounted far from center. The ball head could be as point of flexure but only extended exposures would show any if it were to happen.

The test image indicates field rotation at the corners and edges. Field curvature produced by a refractor looks more like this:http://www.vikdhillon.​staff.shef.ac.uk …/telescopes/cur​vature.jpg (external link)

Was Capella very near zenith when you took this test shot? Approximately what time was it taken?

If your polar alignment was off considerably and Capella was very high in the sky, the Astrotrac could have been struggling.

Do you have a polar scope for alignment?


Steve
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MintMark
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Nov 18, 2009 12:15 as a reply to  @ Nighthound's post |  #4

Steve, thank you for your thoughts. Once you said field rotation it was obvious to me :) But, I thought that a polar alignment error would result in both drift and rotation... is it possible for the error to be such that there's little drift and mostly rotation? I'll have to think about that. I guess I thought the alignment was OK because the stars in the centre looked round. I was more worried that the flattener was not performing as it should.

Capella wasn't very high at this point... maybe 40 degrees up from the horizon. It was taken at 21:04 on the 14th.

I do have a polar scope for alignment, but I did notice one problem. I initially aligned with nothing mounted on the astrotrac. After I added the scope and camera I checked the alignment again and had to correct it. So adding the weight on top caused the geared head to move. Since then I've read all sorts of stories about mis-aligned axes and incorrect distances between things. It's a whole new set of things to learn!

Next step, practice careful alignment. That should provide an initial accuracy. Then spend some time on one target. If it gets worse through the sequence that might indicate something moving in the mount over time. After that I should be able to tell whether I need more accurate alignment (learn drift alignment?) or more stability (buy a wedge?). It all needs some clear weather though!


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Nov 18, 2009 13:37 |  #5

I'm trying to figure out how you're able to get what appears to be rotation with the Astrotrac. Rotation occurs most noticeably in the North and South and closer to zenith. Have you tried posting your test shots at an Astrotrac forum, if there is one out there?

The point of flexure I mentioned previously is something I would try to work out first. The fact that alignment changed after loading the Astrotrac with your scope and camera indicates the flexure is significant. It may be in the tripod, ball head or a combination of the two. As the Astrotrac is tracking an object balance cannot be constant if there is a flex point in the rig. It stands to reason that as the balance is fluctuating your alignment is changing slightly as well, just as it did when you added the gear (just not as suddenly). So in order to maintain a good alignment through an imaging session you'll have to get things stable.

I see that Astrotrac sells a couple of different mount heads designed to hold more weight. Those may be enough for your needs. When I saw your set up I instantly thought that a pair of rings and a small dovetail might be needed for that combo. I don't know how much weight that would add or how much the Astrotrac can tolerate.

I would lower your tripod as low as you can stand it. This will make it as stable as possible. You could suspend some weight underneath the center plate as well.


Steve
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VIGER
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Nov 18, 2009 18:40 |  #6

Would the field flatner .8X help ?


Michel
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Nov 18, 2009 21:00 |  #7

Mark, is it possible that you had your top 2 corners switched with the bottom 2?

When I looked at this again tonight and switched them it now looks like field curvature. Notice that the stars elongate out from center rather than a spinning motion:

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MintMark
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Nov 19, 2009 11:53 as a reply to  @ Nighthound's post |  #8

Steve, I just checked the images again and the corner stars definitely look concentric around the centre, so it looks more like rotation than curvature. Also, I have a 60s shot to compare with the 120s shot... if it's mis-alignment I would expect to see a greater effect in the longer shot, wouldn't I?

The corner stars in both shots look the same shape... just as elongated in both pictures. Although they are in slightly different places in the frame... something moved again?

I have a 120s pleiades shot as well and it shows the same effect. So now I think it's either normal for the flattener or maybe I haven't installed it correctly (not pushed in all the way? some spacing problem?). Anyway, I need more tests... I want to try prime focus to see what curvature really looks like, then try again with the flattener.

Michel this is with the 0.8x flattener III. They do recommend it for this scope but I don't think it was designed for it, so maybe that's just what it does.

The weight limit is currently the geared head under the astrotrac... it is close to its 5kg load. The next thing to buy would be the astrotrac wedge and have the geared head on top, hopefully making the alignment more stable and allowing fine adjustment for framing. The next weight limit is the tripod legs at 7kg. I don't know what happens then! ... £380 for the wedge... at this rate I'm building my own EQ5 for 5 times the price :)


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MintMark
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Nov 26, 2009 11:56 as a reply to  @ MintMark's post |  #9

I just wanted to report that I've tried this combination a few more times and done some research on the web. I found a few people with the same symptoms and various theories about flattener to sensor spacing and the flattener over-correcting for the lens.

If you focus with a bright star in the corner of the frame you can see the star change shape from an arc across the corner (like I originally posted) to an arc towards the centre (a bit like Steve's modified version). In between you can achieve a shape that is not round, but is not elongated either. It looks like small manta rays swimming in from each corner :)

My Aldebaran image was taken like this (see https://photography-on-the.net/forum/showthre​ad.php?t=786182 ) although the stacking has probably rounded out the corner stars, masking the effect.

I now think my original image was slightly out of focus. It was my first go and I didn't know how the corners react to the focus changes. The flattener isn't perfect, but from what I have read, they not often are.


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New toy! William Optics Megrez 72 on my Astrotrac
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