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Astrophotography and Sketching >> Beginning and Intermediate Imaging

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orlyandico
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Re: New Telescope new [Re: rflinn68]
      #6118649 - 10/05/13 09:02 AM

I'm dubious about CCDInspector collimation readings.

It was reporting 6" last night on my AT8IN but there were obvious non-round stars in the image. What gives?

(and I can't use Metaguide as I don't have a compatible camera)


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Madratter
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Re: New Telescope new [Re: rflinn68]
      #6118651 - 10/05/13 09:04 AM Attachment (5 downloads)

I'm no expert. I prefer the 3d plot although the one you show has more information. I think what it basically does is measure the shapes of stars throughout the frame including their elongation and FWHM values. Based on how those vary it then can estimate these other values. Reddish areas are where the field curvature strongly departs from the average. Blue is good. Black is best. Looking at your screenshot, it is telling me your best focus was about half way between the center and edge of the screen. Collimation is not REAL far off. If it was, it things would have happened with those crosses in the center. It indicates there is some tilt going on. You can also see that from the curvature. Notice that the Reddish areas are in the top corners and are about equal. Given that, not surprisingly, your Tilt is oriented almost straight up. The higher the percent, the worse it is. This stuff is all best estimated in fields with lots of stars. You had them.

Now is this good or bad. The tilt is definitely strong enough it is affecting those corners. My numbers are similar but in a different direction. It could be for a number of reasons. How extended is the focuser when you are in focus? Also, have you tightened down the tension adjustment on the focuser (the knob below it) and the focus lock above? Of course don't go overboard.

You are using a DSLR which will amplify these things because it has a somewhat bigger sensor than my 8300.

People could argue about what values are good enough. I wouldn't stress about what you see there.

The stock focuser has no way of adjusting out tilt. They make adapters that allow adjusting that. Some focusers (Moonlight I believe is one) come with that ability.

The values seen will jump around somewhat. This is because of variations in seeing, tracking, etc. Anything that affects star shape will have an influence on these numbers.

The numbers most believable to me are the ones where your FWHM are the best (because that means best seeing and tracking).

I would not believe the collimation number unless it is roughly consistent in direction from frame to frame.

Here is the screenshot from mine. Numbers are vary similar but mine shows less curvature. Sensor size may factor into that. It could also be because you didn't use the CCDT67, or it could be because the amount of spacing we have between it and the camera is different.


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rflinn68Moderator
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Re: New Telescope new [Re: Madratter]
      #6118769 - 10/05/13 10:35 AM Attachment (4 downloads)

Madratter, I was using the CCDT67. What I dont understand is that I'm getting way different results with shots just minutes apart in the same imaging session. Here's one from the NGC891 session from the other night. It is showing better numbers toward the outside edges and worse in the center.

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Madratter
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Reged: 01/14/13

Re: New Telescope new [Re: rflinn68]
      #6118793 - 10/05/13 10:47 AM

What is happening, Richard, is that the part of the frame in best focus has shifted. In this case what is actually in best focus is your upper corners which was your area of worse focus before. Real nice FWHM numbers on this one. Tilt is now showing the other direction because of that change in focus. I would not worry about that.

There is some point where for any given setup, that is the best spot to focus to optimize your results. This just allows you to see what happens as that changes. Again, the darker the blue, the better that spot is in focus.


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rflinn68Moderator
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Re: New Telescope new [Re: Madratter]
      #6118968 - 10/05/13 12:32 PM

Well, thanks Madratter. Didnt mean to hijack your thread but this has peaked my interest lately. Also been looking at PEMPro. I havent done anything with PEC on my mount yet. Dont really know anything about how to go about it but I might be looking into PEMPro & this CCDInspector a little more. Seems like they could be good tools for the trade.

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CounterWeight
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Re: New Telescope new [Re: rflinn68]
      #6119009 - 10/05/13 12:52 PM

Thanks for posting all this amplifying information and the use of CCD Inspector - I was told I might need (or would probably need) it to get my speedbucket photon hoover collimated and adjusted for any focuser tilt. I see it will integrate with MaxImDL which is great. Do you find it easy to use?

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Madratter
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Reged: 01/14/13

Re: New Telescope new [Re: CounterWeight]
      #6119116 - 10/05/13 02:31 PM

I find it very easy to use. I don't own it. This is the trial. I do find it very useful, but for my uses, since I have grading images elsewhere, it is handiest just at the start when you get a new instrument.

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Madratter
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Reged: 01/14/13

Re: New Telescope [Re: Madratter]
      #6120938 - 10/06/13 04:46 PM Attachment (6 downloads)

Here is part of why I upgraded to this scope. This is a 100% crop from the middle and corners of a shoot I started with NGC 7771. I got 30 minutes of useable luminosity data before things shut down. This is the middle and corners from the resultant stack (with the stacking artifacts cropped out). Conditions were not the best that night but the FWHM for the stack as a whole was 2.97 arc-seconds. Best frame was around 2.79 arc-seconds and worst was 3.24 arc-seconds as reported by CCD Inspector.

This was shot with the CCDT67 and image scale was .9375 arc-seconds/pixel. I know I can do better than this in good seeing (actually I did do better than this with first light and my Horsehead). Still even this is better than I EVER managed to do with my C8 and the Celestron f/6.3 focal reducer. To get sub 3.0 arc-second images with it, I had to image at f/10.

To compare the middle and corners with my C8, just go to my Astrobin account, and look at any of the old images. Whether shot at f/10 or f/6.3, they were all really bad in the corners.

The image here was a simple stretch of the stacked and calibrated data with an automatic background extraction to kill gradients. No deconvolution, sharpening, or other processes were done that would influence star shape.

Edited by Madratter (10/06/13 04:49 PM)


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Madratter
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Reged: 01/14/13

Re: New Telescope new [Re: Madratter]
      #6137908 - 10/14/13 09:38 PM Attachment (3 downloads)

I finally got some clear skies so I could test out the spacer which allows my CCDT67 to work at the telecompression for which it was designed. Actually, the Baader 28mm spacer designed for their Hyperion Eyepieces ends up bringing the focal length of the scope down to 1040mm so the telecompressor is working at .64. With this setup the scope works at f/5.12. Image scale is 1.07 arc-seconds per pixel.

What has me even more stoked is the FWHM values I am getting with this combo. My best frame so far of the night is running at 2.77 arc-seconds according to CCDInspector. That is excellent compared to what I was able to achieve with the C8 at f/6.3. And I do NOT have outstanding seeing tonight. The center is better and edges are incredibly better.

Here is the Curvature map.

Edited by Madratter (10/14/13 09:44 PM)


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Madratter
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Re: New Telescope new [Re: Madratter]
      #6137933 - 10/14/13 09:53 PM Attachment (2 downloads)

This is the analysis of FWHM across the frame from PixInsight.

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Madratter
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Reged: 01/14/13

Re: New Telescope new [Re: Madratter]
      #6139282 - 10/15/13 03:20 PM

Time to revisit the reasons I stated for getting the scope and whether they panned out. They were:

1) Better stars in the corners.
2) No mirror flop (this would especially bite me with focus after meridian flips).
3) Hopefully better overall FWHM.
4) Easier (and hopefully more stable) focusing
5) Faster focal ratio, somewhat better FOV, and better matched sampling with my camera once I get the spacers I need.
6) Less problems with dew.

Better stars in the corner - this one has definitely worked out. FWHM values are much much better in the corners. Visual inspection of the stars shows the same thing. How much better depends on the configuration. With the CCDT67 screwed directly into the nosepiece of the camera and working at 1190mm (f/5.85) stars nice and round, even in the corners as shown by the corner crop a few posts back. Using the Baader Hyperion 28mm spacer tube between the nosepiece and the CCDT67 is not quite as good. Stars are still far better than the Powerstar III C8. However, the stars definitely show signs of astigmatism. In that configuration that telescope is working at 1040mm and f/5.12. So there is a tradeoff between speed, FOV, and how nice the corners are. Even in that configuration star shapes, FOV, and speed are all considerably better than the C8 using the f/6.3 focal reducer.

No Mirror Flop: My mirror actually did very little of this in my SCT. There was some focuser shift, but it was minimal. There was some mirror flop, but it didn't cause a lot of movement between the main camera and the guider. What did drive me nuts was the focus did not hold very well because of it. A meridian flip was bound to affect it and even before a meridian flip I would get focus changes that seemed to be more than just temperature changes. At least my focus seems to be far more stable with the AT8RC. Perhaps I'm ascribing that to the wrong things. Either way, I love the more stable focus. And I have the steel tube, not the carbon tube.

Hopefully better FWHM - I have been documenting this above and that has clearly been the case. It was rare I would get FWHM below 3.0 with the Powerstar III C8 at f/6.3. I had to go to f/10 and the narrower field of view. Even nights like last night with less than pristine seeing, I'm getting frames below 3.0 even at 1040mm. It will be very interesting to see how this scope does at f/8. I haven't tried that yet.

Easier and more Stable Focusing: I have already addressed the stability of focusing under mirror flop. It is much improved over the C8. What I haven't addressed is the easier focusing. Two things really help here. First you don't need to worry about approaching focus only counter-clockwise. That means if you overshoot, you don't have to back off and start over. That alone is very worthwhile. Add to that the fine focus knob of the Crayford focuser AND the larger knob (even the fine focus knob is bigger) and it is distinctly easier to get accurate focus. The focus lock seems to work well without shifting the focus much if at all.

Faster focal ratio, somewhat better FOV, and better matched sampling with my camera: This has definitely panned out. Even if I back off to 1190mm to get the real nice stars in the corner, I have better FOV and f/ratio than the C8. At 1040mm, an exposure of 7.5 minutes for the C8 can be taken in 5m. And because FWHM is better, I actually gain resolution besides. What isn't to like?

Less Problems with Dew: While this is true, the new telescope is not completely free of this problem. Imagining last night, FOG eventually rolled in. When I went out to close up the shed around 4:30 in the morning, there was a fine misting of dew on part of the primary. And I had used a Dew Shield besides. Still, the C8 would have been unusable far before the AT8RC was affected. That corrector plate in the front is a dew magnet.

Overall, I'm very pleased with how the AT8RC is working out.


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SMigol
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Reged: 07/30/10

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Re: New Telescope new [Re: Madratter]
      #6139328 - 10/15/13 03:42 PM

Quote:

This is the analysis of FWHM across the frame from PixInsight.




How do you get this report in PI?


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Madratter
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Reged: 01/14/13

Re: New Telescope new [Re: SMigol]
      #6139376 - 10/15/13 04:10 PM

There is a script called PSFEstimator. The version I use is 0.34 and I modified it slightly so that it can report arc-seconds with the images I take in Sequence Generator Pro (the regular script does arc-seconds but uses different FITs header keywords).

You will need to download the script. It does not come with PixInsight.

http://pixinsight.com/forum/index.php?topic=4010.75

I think there is an updated version, but 0.34 is what I use.

Edited by Madratter (10/15/13 04:11 PM)


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terry59
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Re: New Telescope new [Re: Madratter]
      #6139378 - 10/15/13 04:14 PM

What is your impression of the general mechanical fit and finish of the scope and focuser?

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SMigol
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Reged: 07/30/10

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Re: New Telescope new [Re: Madratter]
      #6139387 - 10/15/13 04:18 PM

Thanks! I see that it's already in my distribution. Will be trying it out as a part of my normal process.

And grats on the great setup!


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Phil Hosey
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Re: New Telescope new [Re: Madratter]
      #6139388 - 10/15/13 04:18 PM

Hey Mad,
Glad to hear things are working out well for you with the AT8RC. I can tell there is lots of potential here for me as well once I get a few minor issues ironed out. I think because I have my CCDT67 a little too far back at f/5.6 and 1120mm. I think that if I can move it just a little closer then my corners will clear up.


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Madratter
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Re: New Telescope new [Re: Phil Hosey]
      #6139415 - 10/15/13 04:37 PM

Phil, my corners at 1040 had the stars as elongated footballs with the long axis along circles whose middle is the center of the image. If that is what you are seeing at f/5.6, then yeah, I think you'll find the stars significantly more round (as in the image above) at around 1190 or 1200mm.

I'm sure the charts from Astro-Physics on the amount of reduction are correct, but in my case the CCDT67 doesn't completely screw into the threads on my nose piece or the spacer, so my calculations when trying to setup my spacing have been off. I'll probably move back to 1190 mm for most of my imaging except when that last bit of FOV is required.

You might have noticed from my CCDInspector charts, the amount of curvature is way more at 1040mm than 1190mm.

(BTW, the way I am calculating those is by plate solving images and then using the image-scale and pixel pitch of the camera to figure out the focal length).

Edited by Madratter (10/15/13 04:38 PM)


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Madratter
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Reged: 01/14/13

Re: New Telescope new [Re: SMigol]
      #6139422 - 10/15/13 04:40 PM

Quote:

Thanks! I see that it's already in my distribution. Will be trying it out as a part of my normal process.

And grats on the great setup!




Thanks Stephen.


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Phil Hosey
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Re: New Telescope new [Re: Madratter]
      #6139458 - 10/15/13 05:04 PM

Hey Mad,
Yea, I've been verifying my estimated reduction by plate-solving. Currently with my nose-piece I'm getting .99"/pixel which puts me at 1120mm. I've also done some preliminary testing with several .8x reducers, the William Optics version II, which wasn't very good, the Orion .8x reducer, which was better but still not as good as the CCDT67. I also tried my William Optics AFR-IV, but it is quite heavy and combined with my QSI is VERY heavy, so much so that the focuser sag is obvious and I can't do anything with it. I'm planning to try it with a DSLR to see how well it works. If I can get that reducer to work as well as it does with my refractor then there is the potential for having perfectly round stars corner to corner at 1200mm focal length, which I would be happy with.


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Madratter
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Re: New Telescope new [Re: Phil Hosey]
      #6139538 - 10/15/13 06:04 PM

Phil, I was curious about your camera so I did some looking. If I read correctly, it requires roughly 1.4" of back focus, not counting adapters. That is similar (actually a little less) than the back focus for my SBIG STF-8300m. If I calculated correctly it is 33.6mm vs 38mm. I use the Baader 2" nose piece that came with my camera. I think (but am not sure) it is this one:

http://www.buytelescopes.com/Products/15668-baader-planetarium-baader-t2-prec...

The CCDT67 threads into it. Then you would need roughly a 4mm to 5mm spacer to duplicate my spacing.

Anyway, the real point is that if that 1.4" back focus is correct, you definitely ought to be able to get the spacing I have with the CCDT67 one way or another.

Nice camera by the way. If it is the one with the integrated guider port, that is a great idea.


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