QSI Arrives

[mithrandir (Andrew)]

Originally Posted by AlexN

The QSI583 that Paul bought is a KAF-8300 chip, same physical chip size to a QHY8, 8.3mp, and 5.4um Pixel size (from memory) Same chip as used in the FLI ML8300, QHY9 etc..

They are similar but different:

QHY8
Sony ICX-453
Total pixel : 3110 x 2030
Active pixels: 3032 x 2016
Pixel Size: 7.8um x 7.8um square
Effective sensor area: 23.65mm x 15.72mm (28.4 mm diagonal)

QSI583
Kodak KAF-8300
Total pixel : 3348x2574
Active pixels: 3326x2504
Pixel Size: 5.4µm x 5.4µm
Effective sensor area: 17.96mm x 13.52mm (22.48 diagonal)

[AlexN]

Originally Posted by mithrandir

They are similar but different:

QHY8
Sony ICX-453
Total pixel : 3110 x 2030
Active pixels: 3032 x 2016
Pixel Size: 7.8um x 7.8um square
Effective sensor area: 23.65mm x 15.72mm (28.4 mm diagonal)

QSI583
Kodak KAF-8300
Total pixel : 3348x2574
Active pixels: 3326x2504
Pixel Size: 5.4µm x 5.4µm
Effective sensor area: 17.96mm x 13.52mm (22.48 diagonal)

I mentioned that the QSI was the same as the QHY9 not QHY8.
The QHY9 uses the same KAF-8300 sensor.

[AlexN]

oh, and effective chip area and chip physical size are not the same measurement... effective chip area is the physical chip minus the are occupied by the pixels used for the antiblooming gate from what I can discern... They are both APS-C sized sensors...

[gregbradley]

Re: bias frames. I've never used them in DSLR work. I just don't see the point considering the bias is included in the darks for the lights as well as the darks for the flat lights. In my experience, they've just introduced further noise into the final image.

Humayan


Do you subtract them from your flat when you are making a master flat or have you subtracted them from each individual light exposure?

The point here is subtracting them from your flats not your light exposures. If you subtract a dark or use ICNR when taking your flat then you are doing what IU am saying here which is to subtract bias when making a flat master not subtracting bias from a light exposure.
Flats are normalised and then divided into the image, darks are subtracted. So mathematically you can see that if you didn't remove the wrong numbers in the pixel grid caused by bias noise then some numbers that is divided into the image when applying a flat would be falsely too high or too low. The point of a flat is to bring about evenness of illumination of the light. This then would reduce its effectiveness. How much less effective? Perhaps not a lot. Probably depends on how noisy your bias is I suppose. Some cameras are cleaner than others.

However I agree subtracting a bias from an image can cause more noise and unwanted effects as you are subtracting the same thing twice if you subtracted a dark. The use referred to here is subtracting it from the flat images before making a master flat. If you subtracted a dark from the flats then you wouldn't subtract the bias (it'd be much the same anyway - a 3 second dark would look the same as a bias).

I think personally it is also a minor improvement and nothing important in the scheme of things. I have also noticed a slight improvement in noisy images by subtracting a pedestal (100 subtracted from the pixel count assumed to be bias type noise) instead of a bias. Again its subtle but when processing a galaxy image every little bit helps.

I prefer to keep my camera and filters clean and to run the camera as cold as possible to reduce noise so all this callibrating is less important in the fist place.

My TEC180 for example is very evenly illuminated in the first place. So is the AP140. Flats then are almost not needed.


Greg.

[Octane (Humayun)]

Hi Greg,

I calibrate each of my flat lights against a flat master dark (flat darks having been median combined and cosmetic defects noted), effectively removing the crud from each flat light. Only once the individual flats have been calibrated, do I median combine them into a master flat.

That has worked for me from day dot. I initially did use bias frames but got rid of them quick fast when they were leaving my images in artifactland.

So, in essence, no, I don't use bias frames to remove any bias from flat lights or darks (noting that the bias is included in the flat dark, just as it is in the dark for the lights).

Once I move to CCDland, it might be a different story. But, that won't be happening for many years as I need to exhaust what I currently have.

Regards,
Humayun

[gregbradley]

Then we are basically doing the same thing. I am not sure how a bias is done on a DSLR anyway - is it a really short exposure?

I once had CCDsoft allow me to do auto dark reduction on flats. It normally doesn't come up as an option and I haven't been able to fluke it since but that was handy.

Greg.

[rat156]

We seem to have strayed somewhat off topic, but I'll just push us alittle further away...

I do twilight flats, CCDSoft allows me to do auto darks for the lights, so I get calibrated flats.

This is when I come into problems. The exposure is so that the initial lightframe is about halfway to full, but once the darkframe is removed the maximum pixel count can be as low as 5000. Which exposure do I use, the initial or the dark subtracted.

Cheers
Stuart

[Octane (Humayun)]

Greg,

Yep, spot on. It's whatever short exposure the camera will allow. In some of the older cameras, this was 1/4000sec, nowadays, the higher end cameras allow you to do 1/8000sec.

Regards,
Humayun

Originally Posted by gregbradley

Then we are basically doing the same thing. I am not sure how a bias is done on a DSLR anyway - is it a really short exposure?

I once had CCDsoft allow me to do auto dark reduction on flats. It normally doesn't come up as an option and I haven't been able to fluke it since but that was handy.

Greg.

[Peter Ward]

Originally Posted by rat156

Which exposure do I use, the initial or the dark subtracted.

Cheers
Stuart

Initial.