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Old 28-05-2008, 02:37 PM
ozstockman (Mike)
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QHY8 bias and flats

Hi guys,

I had a chance yesterday to get a first light with my QHY8 and got a wonderful result. See the image attached. It's 7x400sec light frames + 4 flats. The frames were calibrated in MaximDL and stacked in DSS and final processing was done in PS. Autoguided with 8" SCT and QHY5 using PHD, frames taken with ED80mm f/6.8 APO and QHY8

Taking lights does not seem to be a big problem especially after Theo kindly explained some basic stuff about offset and gain to me wasting his Friday night time :-)

I have however few questions about taking flats and bias frames with QHY8 because I don't think I did it right.

1)Should bias and flat frames be taken with the same gain and offset settings as for light frames? I forgot what settings I had for light frames for the image below and I guess this is why it's not the best result I could get with this great camera.

2) How to figure out the best exposure length for flats? I used to use Aperture priority mode in my Canon DSLR and the camera did it for me. But of course it's not possible to do with QHY8.

And if I get it right I shouldn't use binning for light frames if I want to get a color image, right? I used 2x2 binning and it seems that I lost all color details for this image.

thanks in advance,

Mike
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  #2  
Old 28-05-2008, 04:46 PM
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seeker372011 (Narayan)
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Quote:
Originally Posted by ozstockman View Post

I have however few questions about taking flats and bias frames with QHY8 because I don't think I did it right.

1)Should bias and flat frames be taken with the same gain and offset settings as for light frames? I forgot what settings I had for light frames for the image below and I guess this is why it's not the best result I could get with this great camera.


2) How to figure out the best exposure length for flats? I used to use Aperture priority mode in my Canon DSLR and the camera did it for me. But of course it's not possible to do with QHY8.

And if I get it right I shouldn't use binning for light frames if I want to get a color image, right? I used 2x2 binning and it seems that I lost all color details for this image.

thanks in advance,

Mike
I am sure others will pitch in but here's my take

the answer to question 1 is no !

I use a few milliseconds for bias and much much lower gain for flats than I do for lights. When I used the same gain the flats did not work..the dust marks were not divided out of the final image

one way to determine the best exposure for flats is to look at your histogram..see the exposure that moves it about halfway


oh and most certainly you should shoot in colour at 1 x 1

Narayan
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Old 28-05-2008, 04:47 PM
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Terry B
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Bias and offset are the same thing. It is just the amount added to each pixel count so that there are no negative counts. Essentially a zero integration of a dark exposure. You do a few of them and average them. If you then look at the pixel count for the image, it will be some +ve number with hopefully very little variation across the image.If the variation between pixels across the sensor is minimal you can just fill an image with the mean of the image creating a perfectly flat image and save that as your offset.
Flats need to have an exposure that roughly 1/2 fills the maximum well depth for your sensor. If the exposure is short enough then you don't needto worry about darks etc for the flats.
I have taken to using a flash aimed at a wall that the scope points at with a T shirt over the front of the scope. I then use a 1 sec exposure and fire the flash manually. This gives a lovely flat field and the exposure time is not important.
You need to subtract the bias frame from your flats or they don't work well.
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Old 31-05-2008, 10:21 AM
ozstockman (Mike)
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Thank you guys. I followed your recommendations and got a much better result. See my color image of Eta Carina in this thread http://www.iceinspace.com.au/forum/s...d.php?p=331154
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