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#1 Madratter

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Posted 03 October 2013 - 05:41 PM

I have been wanting a new Telescope to replace my C8 for Astrophotography for some time. After much thinking (probably too much), I ended up getting an Astro-tech AT8RC Ritchey-Chretien. I also purchased a Astrophysics CCDT67 reducer. Together they can turn it from f/8 into a f/5.36. However, I'll need an additional spacer for that. Currently it turns it into a f/6.4, which is very similar to my C8 with the Celestron f/6.3 focal reducer.

Reasons for wanting the new scope:

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.

We shall see how that all pans out. However, I have looked at a lot of pictures produced by this combination of telescope and reducer and I like what I see.

Initial impressions are the mechanicals are really nice compared to the Celestron Powerstar III. I do plan to replace the focuser with either a motorized Feather Touch or Moonlight but that will have to wait for now. My impression is the stock focuser is going to be serviceable until then.

I checked collimation with a Cheshire and I did need to adjust the secondary. The primary looked at least ballpark good.

Forecast is partially cloudy tonight. Hmm...

Here is a photo of the new setup:

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#2 rflinn68

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Posted 03 October 2013 - 05:49 PM

Excellent!! I think you are really going to enjoy it. I'm liking my Levenhuk Ra 8RCCF more and more since getting the AP CCDT67. Looking forward to seeing what you can do with it and the 8300M! :D

#3 terry59

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Posted 03 October 2013 - 05:55 PM

That looks good. I hope collimation treats you well. I know some have issues with it

#4 rigel123

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Posted 03 October 2013 - 07:52 PM

Congrats! I love my 6" and I'm sure you will enjoy the 8!

#5 proteus5

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Posted 03 October 2013 - 08:19 PM

Congrats!!!! can't wait to see first light.

#6 CounterWeight

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Posted 03 October 2013 - 09:05 PM

Nice image :) congrat's on the new gear. Hope you get clear skies soon to get it dialed in and photon hoovering.

#7 Ranger Tim

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Posted 03 October 2013 - 09:48 PM

You may find that you need the additional focuser collimation ring. Did you get one? My AT6RC has issues with the focuser alignment and this adapter ring should fix it. Waiting patiently for the package, along with waiting for the AT65EDQ....

#8 Madratter

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Posted 03 October 2013 - 10:29 PM

Thanks for the encouragement and well wishes!

As for the collimation ring, my understanding is that one comes with the moonlight so I'll probably wait until I get that.

Here is first light. It isn't much but it is what I have since the clouds rolled in. Given the forecast and the weather satellite, I was fortunate to get this.

It is a 1 second exposure used for focusing and is binned 2x2. I actually ran it through PixInsight, and it says that my FWHM on this frame as a whole was 2.38. Given the weather, I'll definitely take that. It looks like stars are pretty good across the frame.

I was able to calculate my image scale. It is .938 seconds of an arc/pixel. That gives me an effective focal length of roughly 1190mm and an f/ratio of 5.85. So it is already faster than my C8 even without any additional extension between the CCDT67 and the camera.

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#9 Wouter D'hoye

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Posted 04 October 2013 - 12:24 AM

Looks like some great gear. Have fun!

Wouter.

#10 Madratter

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Posted 04 October 2013 - 09:10 AM

Thanks Wouter.

I woke up around 3 in the morning and was having a hard time going back to sleep. Although I wasn't expecting good news given the weather report and what things looked like when I went to bed, I checked outside. It was clear!

I set things up to take the Horse Head. This morning the data was waiting for me. I love automation.

I'll be processing that data later. In the meantime, here is what CCDInspector is showing. I'm pretty happy with this result, given it says my collimation is off by 6" and that this data was binned 2x2 which reduces my FWHM when undersampled (which this data was). Most importantly, note how relatively flat the field is when using the Astrophysics CCDT67. That is quite believable as stars look good in the corners, something that was never true with the Celestron C8.

I should mention this was just the first frame from the series and was not the best.

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#11 Madratter

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Posted 04 October 2013 - 09:28 AM

By way of contrast here is my best sub with the Powerstar III C8 from the NGC 7771 image I posted yesterday. This was binned 1x1 and was shot at f/10 which is where I get my best FWHM values with that scope.

The curvature is horrible. It isn't any wonder the corners look so bad.

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#12 Madratter

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Posted 04 October 2013 - 09:35 AM

Here is the best frame with the C8 from that NGC 7771 shoot binned 2x2.

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#13 Madratter

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Posted 04 October 2013 - 09:47 AM

Here is about as directly comparable shot with the C8 as I can manage. This is the best Ha sub from the Dragon, Sharpless 2-132. This was taken at f/6.3 with the Celestron focal reducer/flatterer and binned 2x2.

Again curvature is horrible, and the corners are real real bad.

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#14 Madratter

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Posted 04 October 2013 - 10:01 AM

Now that all the data is transferred, here is the best frame from the Horse Head shoot with the new AT8RC. Again, this is HA binned 2x2 with the Astrophysics CCDT67. FWHM would be even better binned 1x1 since this is under sampled. CCDInspector is inconsistent with itself, but reports FWHM for the frame as a whole as 2.66 arc-seconds.

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#15 Madratter

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Posted 04 October 2013 - 10:10 AM

An independent view of this best frame from the AT8RC. This is a map of the FWHM using the PSF Estimator Script in PixInsight. It produced a very similar reading to CCD Inspector. The PSF Estimator value for the entire frame is 2.64 arc-seconds.

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#16 Phil Hosey

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Posted 04 October 2013 - 02:07 PM

Nice scope, I just ordered the OPT branded version while they had them on sale for under $800, mine should be here Monday.

#17 TimN

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Posted 04 October 2013 - 02:21 PM

Congratulations! I was looking at the same scope. I figured it would get me a bit deeper and I also liked some of the images that I've seen with it. However, I decided to wait awhile and put some more money into my observatory instead. Will be interested in watching your images.

#18 Madratter

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Posted 04 October 2013 - 02:31 PM

Phil, if I recall, you had one in the past. Enjoy it!

Tim, not sure what you are doing with you observatory, but I consider mine my most important piece of equipment, so that certainly makes sense.

#19 Phil Hosey

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Posted 04 October 2013 - 09:29 PM

Phil, if I recall, you had one in the past. Enjoy it!


Yup, I used to have the CF version, in fact I bought it from Joel (Buckeyestargazer). I was a nice scope and I regret selling it (which is why I'm getting another one).

#20 rflinn68

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Posted 04 October 2013 - 10:48 PM

Madratter, I just downloaded CCDInspector and I'm a noob at this. Can you explain what this stuff means? Good? Bad? :lol:
This is from a single sub from my RC.

EDIT: Weird because its all over the map depending on what sub I input into it. I tried one from my apo and it shows worse collimation and curvature than my RC. I know that the apo is spot on. I just dont know what I'm doing with this program I guess. :question:

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#21 orlyandico

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Posted 05 October 2013 - 08:02 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)

#22 Madratter

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Posted 05 October 2013 - 08:04 AM

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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#23 rflinn68

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Posted 05 October 2013 - 09:35 AM

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. :shrug:

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#24 Madratter

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Posted 05 October 2013 - 09:47 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.

#25 rflinn68

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Posted 05 October 2013 - 11:32 AM

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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