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OAG / QHY5L-II general questions

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#1 petemumbower

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Posted 09 October 2013 - 10:48 PM

Finally got all the pieces to move into OAG guiding via my new Orion Thin Off-Axis guider and QHY5L-II. So I am currently collecting more frames for my M27 project over the last few nights with 1000D and c925. But I do have a few questions about OAG in general (not specific to these actual components):

1) What kind of exposure time are you using to find stars without too much adjustment. Assuming a sensitive guider like this QHY or a Lodestar? I am in a rich part of the milky way currently and 1s seemed to do the job, but there are less stars than expected.

2) I am using my .63R/C with this setup and kind of expected the off-axis stars to be better shaped. But in general they have the funky distortions OAG tend to present. Is this the norm?

3) Any setting tips for using a OAG with PHD? I am using it with my c925 so the scale is pretty small. The bumps on the tracking graph pretty much take up the entire graph. But my stars in the image itself are nice a round.

Just little questions I think. Overall, I am really pleased with how this is going tonight. All my subs look fantastic, I can really tell now that I was dealing with differential flexture with the guidescope on top.

#2 mmalik

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Posted 10 October 2013 - 06:35 AM

What kind of exposure time are you using to find stars without too much adjustment


I generally use 2sec exposures for guiding with PHD defaults; same should work for OAG as well. On rare occasions you may have to go longer (up to 5sec) if the stars are dim. Regards


On a side note, story of my quest for a good guide camera here....

#3 srosenfraz

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Posted 10 October 2013 - 11:32 AM

1) What kind of exposure time are you using to find stars without too much adjustment. Assuming a sensitive guider like this QHY or a Lodestar? I am in a rich part of the milky way currently and 1s seemed to do the job, but there are less stars than expected.


I usually end up using 2 to 4 second exposures. Its not so much a function of how much time is needed to see good guide stars, but that you don't want to over correct (chase the seeing). However, if you're having trouble finding guide stars at 1 second, by all means, try increasing the exposure time.


2) I am using my .63R/C with this setup and kind of expected the off-axis stars to be better shaped. But in general they have the funky distortions OAG tend to present. Is this the norm?



With an uncorrected SCT, I would expect the off axis stars to look terrible. But, for guiding purposes, its a non-issue (to some extent, its a benefit). Phd achieves sub pixel guiding accuracy by calculating the "center of mass" of the star. If the star is out of focus, it can calculate it more accurately because it can tell how intense the star is at various points around the center of mass. If the star is in perfect focus at only one pixel (in an ideally focused star), then it only knows that its on a particular pixel until it moves to the next pixel). So, don't worry about the ugly looking stars.



3) Any setting tips for using a OAG with PHD? I am using it with my c925 so the scale is pretty small. The bumps on the tracking graph pretty much take up the entire graph. But my stars in the image itself are nice a round.



You're correct that the fundamental thing that matters is if your stars are round enough for your satisfaction. That having been said, you'll want to keep in mind that you're imaging at a much longer focal length than you probably were when using a typical guide scope. Assuming your guider is behind the FR, you're now guiding at ~1450mm. If you were previously guiding with a short tube refractor, you were probably then guiding at 400mm. So, your image scale (if using the same camera or a camera with comparable pixel sizes) is about 3.5x. If a good guide graph before was .20 RMS, then you should expect an RMS of .70 to be exactly the same quality of guiding (ignoring that you now don't have the issue of flexure to add to the equation).

For my purposes, I found with my Lodestar that my stars looked pretty good when my RMS was below about .70, and marginally acceptable up to about 1.00 pixels RMS. Using Jerry's formula for calculating pixel scale, I find this means:

Lodestar pixel size - 8.2 microns
Focal length at ONAG on C8 with F/R (f/5.8) - 1160mm = 45.7 inches

P=8*8.2/45.7 = 1.4 arc seconds per pixel.

So, my guiding tolerance is that I want my guiding to be better than 1.4 * .70 = 1.0 arc seconds RMS, and no worse than 1.4 * 1.00 = 1.4 arc seconds RMS ***at*** 1160mm.

Now, as your focal length increases, you'll want proportionately more accurate guiding. As such, if you're imaging at 1450mm, you'll want guiding accuracy of 1160mm/1450mm = .80x more accurate (smaller RMS), so you would want guiding accuracy of ideally .80 arc seconds RMS and no worse than 1.1 arc seconds RMS.

Working backwards, your QHY5L-II camera has pixels of 3.75 microns, your focal length is 1450mm (57.1 inches), so:

P = 8 * 3.75 / 57.1 = .5 arc seconds per pixel.

Since you want guiding accuracy of better than .80 arc seconds RMS, you'll want to have Phd RMS values of .better than .80 / .5 = 1.6 pixels RMS, and no worse than 1.1 / .5 = 2.2 pixels RMS.

(hopefully I didn't make any math errors - the results sound reasonable to me but it sure would be nice if someone could double check me).

Now, that is to achieve the level of guiding accuracy with which I am happy. You may find that I'm either too picky or too lax - you'll have to decide for your own purposes. But, the key to keep in mind is that you should expect what might look to you like a much "uglier" guiding graph than you were accustomed to seeing when guiding at 400mm (or whatever shorter FL you were using).

Hope this helps - have fun with your new toy!

#4 petemumbower

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Posted 10 October 2013 - 01:46 PM

Thanks mmalik and Scott for the tips. I think I will have to experiment with exposure time and guiding results and see what works best. Since I have a CGEM, the longer exposures might not be possible.

Scott - You bring up many great points. Your math seems very sound and based off your results, it works. If you recall, I like to image often at the native f/10 of my c9.25, so 2350mm. I have been collecting data of M27 the last few nights with my Celestron F/R. But the star shapes were beyond horrible in the OAG. They were all horrible in the actual subframes. I think my F/R is bad actually the more I think about it. My star images on the edges are so much better without the F/R on. That is a separate frustration, but can account for my question since I was trying the TOAG with the F/R on. Also, I have the spacing pretty close too, within a few mm.

#5 srosenfraz

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Posted 10 October 2013 - 03:08 PM

FWIW, I'd put pretty high odds that your FR isn't any worse than the rest of them. The stars look worse in the corners because you're seeing farther off-axis at f/6.3, then you are at f/10. If it helps, I've attached a corner crop of my scope when its focused for the center (this is at f/5.8 - slightly more corner than you see at f/6.3).

Attached Files



#6 petemumbower

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Posted 10 October 2013 - 03:12 PM

Well I have a typical FR then. That is exactly how my stars look off in the corners. I feel better now, sort of. I am guessing you just crop out the worst portions of the frame? Then again, I do recall you image at different focus points through out the frame right?

#7 srosenfraz

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Posted 10 October 2013 - 03:27 PM

Depends upon the target. If I'm imaging something small, I'll focus for the center and crop out the edges. If I want the full frame and want to make it simple, I focus for about halfway from the edge to the center (edges still look bad, but most of the image is usable). If I seriously want the whole field of view, then I focus for different parts of the image and then mask the groups of subs to allow the focused parts to show (focus stacking). The latter method takes a lot more time for any given image, but it can give good results.

#8 pfile

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Posted 10 October 2013 - 03:39 PM

i was looking at the QHY5L-II this morning due to the fact that it can be used with metaguide and also supposedly it's good for planetary. have you tried using it for anything else besides a guider?

rob

#9 petemumbower

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Posted 10 October 2013 - 03:52 PM

Rob - I did a test image (5min) using the QHY5L-II with MetaGuide last night. The stars were nice and round. I have not tried it with planets yet, but that is on my list for tomorrow morning. The seeing is suppose to be good according to CSC. So far I am really happy with the camera. The only issue I have is with so many devices on the back USB panel...I had to move the QHY5L-II to the front panel plugs to be on its own "USB root".

#10 pfile

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Posted 10 October 2013 - 04:03 PM

thanks pete, i look forward to hearing about what you get planetary wise.

oh shoot, that was the thread on the BYE yahoo group about it disrupting the USB bus and multiple driver versions for the QHY? i already have enough USB issues but this camera seems attractive due to the planetary + metaguide compatibility.

rob

#11 petemumbower

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Posted 10 October 2013 - 05:14 PM

Rob, I do think I have all the USB issues resolved. I have not posted my results to the BYE yahoo group yet pending a few more imaging runs to confirm this is fixed.

metaguide was a big reason I wanted this too, but I still need to learn how to use it.

#12 petemumbower

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Posted 11 October 2013 - 09:29 AM

Rob - I did get out this morning to try the QHY5L-II on Jupiter. Results are here:

http://www.cloudynig...6130620/page...






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