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Old 25-09-2009, 03:53 PM
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toryglen-boy (Duncan)
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The great QHY8 flats mystery

Yeah me again, whinging that i cant get this sorted ...

ok, taking flats, in MaximDL i take an exposure of around 0.025 secs, and i get the first image, i would have thought it would have been a bit more uniform, instead of black with white splodge, anyway, the second image is how it appears if i save it as a PNG and dont do ANYTHING to it ..

image 3 is what the flat looks like, if i take a sub of any longer than about 0.3 seconds, image 4 is a sample image i took to test with, and image 5 is what the sample picture is like with the flat filtered through it..

what am i missing? remember image 1 & 2 are the same thing, just saved to different formats. i dont mind getting slaughtered if its something stupid, as long as i get it going ....
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  #2  
Old 25-09-2009, 05:06 PM
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Gawd damn ! I can't wait for the expert replies
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Old 25-09-2009, 05:32 PM
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Duncan, what did you use for a light source? Is this a single image or a stacked master (averaging). Have you applied your dark flats/bias frames to this flat. The exposure time seems very short. It also appears maxim has applied a pretty funky stretch to that first flat.

Mark
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Old 25-09-2009, 05:52 PM
TheDecepticon
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Step 1Ditch MaxIm. Step 2:Get Nebulosity2. Step 3PM me when you have completed step 1& 2!!

(The opinions expressed by the author of this post regarding MaxIm are a personal opinion and are not open to debate or war)
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Old 25-09-2009, 06:03 PM
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mtodman (Matt)
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Hi Duncan,
Have you focused the camera before taking flats? You need to ensure that your flats setup is EXACTLY the same as your imaging setup.
Also, open your information & histogram windows. Use the histogram window to perform a screen stretch to better view the image. Your information window will tell you the value of the pixels (you need to aim for about 20,000 - 30,000).
Matt
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Old 25-09-2009, 06:06 PM
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Quote:
Originally Posted by TheDecepticon View Post
Step 1Ditch MaxIm. Step 2:Get Nebulosity2. Step 3PM me when you have completed step 1& 2!!

(The opinions expressed by the author of this post regarding MaxIm are a personal opinion and are not open to debate or war)
I am not sure that buying more software is gonning to fix Duncan's problem. He is doing something wrong, lets see if we can work out what it is first.

Mark
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Old 25-09-2009, 06:37 PM
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Tricky huh?
I'll disagree with the comment about ditching Maxim, if there is a better program out there then it must be good. My problems with maxim related to the QHY8 not playing well with it, and this to me was a software issue.
No matter. Couple of comments. Why an offset of 137? Did you arrive at this by trial?
In respect of an ADU count, I went with the "multiweb method" and kept my ADU to about 9000, worked for me.
The first shot is what I used to get, just use the screen stretch box to give a more accurate look and I think you will find it is OK. Plenty of donuts but OK. I also wonder if the slight darkening in the centre is a shadow of your secondary and spider vane??
When you say "filtered through it" in image 5, did you use "Process/Calibrate?
Gary
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Old 25-09-2009, 06:42 PM
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Pic 1 looks about right, but a bit contrasty. pic 2 is what, a PNG?, isnt that an 8 bit format?, why bother at all with PNG?. pic 3 is what happens when you overload a sensor, it goes very weird. and which flat did you use for pic 5?.
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Old 25-09-2009, 08:31 PM
Hagar (Doug)
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Duncan , I think this is one of those easy ones. Maxim has an auto stretch feature built into it's screen display. If you open an image it will always try to stretch the image to whatever the preset is. If you open the stretch display window you can select range and you might find your images don't look so bad.
Just one of the reasons I only use maxim for basic capture but better still is Nebulosity.

A few things you must remember when taking flats with a QHY8.
1. The full well depth of the QHY 8 is approx 27,000E ADU therefore your flats should be taken with a maximum pixel intensity/ ADU count of between 9000 and 13500. This is taken accross the entire image not just a point you pick.
2. To achieve reasonable results you must take Bias frames or at least darks at the same duration as your flats to calibrate the flats correctly.
3. Capture your Flats at the same Gain, Offset and Focus and camera position as your light frames.
4. Talking about Gain and offset settings. Lots has been said about both but the easiest way to set them is to use whatever gain setting you like. I personally never went above about 12. Take a bias frame and adjust your offset until the peak of the histogram is just to the right of the left side, sounds a bit Irish but it works. As long as the same gain is used, well in most cases it doesn't matter to much, your offset is now set and should never need to change unless you use some NB filters and then only a small change is required.

Good luck with it all, it's really not that hard, it just seems like it is.

Last edited by Hagar; 25-09-2009 at 08:53 PM.
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Old 25-09-2009, 08:39 PM
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multiweb (Marc)
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Quote:
Originally Posted by toryglen-boy View Post
Yeah me again, whinging that i cant get this sorted
Don't worry about what your flats look like visually (for now). Make sure you reach the right ADU levels. And remember that you will need bias frames to subtract from your bias prior to divide in the sub or you'll get inverse vignetting like your las two pics show. Doesn't matter what capture or processing software you use so don't worry about maxim or other programs. Use what you've got.
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Old 26-09-2009, 02:05 PM
TheDecepticon
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How did you go?
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Old 26-09-2009, 03:19 PM
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toryglen-boy (Duncan)
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Quote:
Originally Posted by TheDecepticon View Post
How did you go?
I think Hagar has something, i noticed that when i open anyfile, it automatically gets stretched to some degree, when i take the flat, and stretch it to "normal" it looks like how a flat would look, that explains why if i take the same image and save it as a different format, then it looks completely different from how it does in Maxim.

I think over the next 2 weeks, i need to experiment with getting, bias, lights, and flats all calibrated, although this excercise has been a good way for finding my way around Maxim.

although i might invest in Nebulosity.

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Old 26-09-2009, 03:32 PM
TheDecepticon
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Cool, as long as your moving forward!
I must admit, MaxIm does have some not so well understood ways of doing things. I wouldn't say Nebulosity is "better" but it is "easier", and this can be the difference at times. I have used MaxIm quite a lot over time, trying to utilize its features, and while it is very good and essentially quite a complete package, Nebulosity and PHD are still "easier".
Cheers.
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