QSI Arrives

[rat156]

Now, I do take bias frames and subtract them from my darks. I do this so that the darks are scaleable.

My theory is that I only need one darkframe for each bin level, this is what CCDStack is telling me. So I have one 30minute darkframe for 1x1 and 2x2 at -15C. I can go down further in temperature in winter, by not in summer, the darks subtract out nicely so I like to keep one temperature. Apparently when the darks are made in this manner they are scaleable for both time and temperature.

I assume that what CCStack is doing is subtracting the bias from the dark so that what you're left with is "pure" dark noise. As the bias doesn't change in intensity over time, but the dark does, it scales the dark then adds the bias to the dark before subtracting. Put another way the scaled dark has lost the bias information (as it has been subtracted in the master making process), so it has to be subtracted as well as the dar from the light.

Does this make any sense?

Oh BTW Paul, welcome to deep sky imaging with a dedicated CCD. I look forward to your results, the camera looks like a cracker!

Cheers
Stuart

[Peter Ward]

Originally Posted by rat156

Now, I do take bias frames and subtract them from my darks. I do this so that the darks are scaleable.

Cheers
Stuart

Yes, you can do this, but a hack like me doesn't because.

The dark noise pattern changes slowly over time
Dark noise is not completely linear
The undelying bias pattern can be (and is often) slightly random

In effect my dark library (ie Bias + dark) is constantly updated to provide the cleanest calibrated images possible.

It also doesn't work with back illuminated chips that have spectrally dependent fringing patterns (translation: flats change with colour! arrgh!)

[rat156]

Originally Posted by Peter Ward

It also doesn't work with back illuminated chips that have spectrally dependent fringing patterns (translation: flats change with colour! arrgh!)

Lucky I don't use one of them fancy back illuminated chips...

Cheers
Stuart

[Peter Ward]

Originally Posted by rat156

Lucky I don't use one of them fancy back illuminated chips...

Cheers
Stuart

It was such a pain in the proverbial, Random bias with a low level ripple, and the fringing was so bad even a 5 degree camera rotation required a new set of median *sky* flats (dome flats proved useless) for each filter.

So.. I don't anymore either. It went back to the manufacturer!

[Octane (Humayun)]

Peter,

Can you explain what advantage the sky flats had over the dome flats? I'm assuming by dome flats you mean projecting a light source onto the roof of your observatory dome and imaging that?

Would you suggest a pauper with a DSLR (like myself) to use sky flats over artificially created ones?

Regards,
Humayun

[Peter Ward]

Originally Posted by Octane

Peter,

Can you explain what advantage the sky flats had over the dome flats? I'm assuming by dome flats you mean projecting a light source onto the roof of your observatory dome and imaging that?

Would you suggest a pauper with a DSLR (like myself) to use sky flats over artificially created ones?

Regards,
Humayun

There is nothing wrong with DSLR's. They are a great tool for wide field work.

Dome flats/T-shirt flats/sky flats are variations on the theme of providing a perfectly blank field that can be used to calibrate the camera's images.

Sky flats are easy. They have to be taken just before dawn or after sunset, adjusting the exposure to give about 30% well saturation of the sensor. (you want a high S/N but don't want ABG to kick in). Take several frames near the zenith, moving the scope around between each frame to minimize any sky gradient. Median combining them will remove any random signal.

The problem you will have with a DSLR is sky flats will not be monochrome. (Easily fixed in Photoshop!), but if done correctly you will have a nice map of the optical vignetting of your telescope (or lens) and any dust doughnuts.

Hope that helps.

[RobF (Rob)]

Thanks for showing the new baby in all its glory Paul. I'll be hanging out like the others to see what delights you show us in months to come.
(I'll just go find something to wipe this drool off my screen now....)

[AlexN]

Lots and lots of great information in this thread... Even if it has wandered slightly off the original topic...

No more bias frames for me!!

Its funny.. Once Peter said "any non-zero dark will contain the bias" it seemed painfully obvious... but prior to my reading that, I would never have thought about it... haha..

Peter - While we are on the subject of calibration.. What are your thoughts on the amount of darks/flats to be used? Do you use a proportionate amount of calibration frames to your lights? or go with the old "more is better" approach? I've been using a 1:2 ratio of Darks:Lights.. 6 darks to 12 lights and results have been good.. however there is still some noise (which may be lack of decent exposure time more than anything else..)

Thanks to everyone for their input into this thread.. Its been quite a good read..

Alex.

[Alchemy (Clive)]

looks like an interesting camera, how many megapixels, what size chip, and/pixel size.

will be waiting to see some results, nothing like some new gear to spark the interest to the next level.

[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..

I can not wait to see what Paul can make this camera do!!

[Alchemy (Clive)]

ta alex,

quick question, given its the same chip in all those cameras does the electronics give better performance in any of the models ? if so which ?

[AlexN]

The answer to that depends greatly on who you ask... Electronics wise, they are much the same... Cooling capacity and download speed is where you see the FLI cameras taking the lead... the QSI and QHY are much the same in that respect.. The QSI is initially more expensive, however with its integrated filter wheel, and only requiring 1.25" filters you do save a fair bit of the difference in the long run.. The QHY9 still comes out cheaper I think... The QSI also has an integrated OAG that guides infront of the filters... This is definitely nice... however not for everyone.. I'd go the ML8300 if I had the money and inkling to go to a mono sensor again...

[Paul Haese]

Clive,

just going to confirm that the camera is 8.3 mp and 5.4 um for pixel size. QSI electronics are very high grade. QSI has just entered the astronomy field but is best known for its imaging equipment for science. I would imagine all the top of the line camera's have very similar electronics and standards. Proof will be in the pudding though.

[gregbradley]

Not entirely true Paul. Different cameras have different quality electronics.

For example Mikes FLI PL11002 is the same chip as an STL11.
Mike posted one of his darks and I posted same temp and exposure from an STL11000M I had. The STL was way higher noise. Same chip. Difference is the read noise. The SBIG was way higher. I was surprised and I think Mike was too about how clean his FLI PL11002 is.

Perhaps the new STX will close the gap that has occurred between SBIG and Apogee and FLI and perhaps QSI (I don't know much about QSI but it looks quality). Perhaps not if they use the old electronics in a new body. I'd like to see a dark from an STX16803 before I would consider one. New guiding methods are appealling but at the end of the day you want low noise more than anything and reduced cosmetic defects which high cooling also does and the KAF series sensors tend to have (vertical lines from hot pixels and the reading process which fade with cooler temps for example).

Same between Apogee and FLI. The FLI has lower read noise of about 5 electrons compared to the Apogee of about 9 electrons.

So when you compare cooling and noise you have to factor in the read noise which varies from manufacturer to manufacturer.

Greg.

[Peter Ward]

Originally Posted by gregbradley

.... The STL was way higher noise. Same chip. Difference is the read noise.....

I disagree. (surprise surprise )

I've probably tested 2-3 dozen KAF11000 series sensors (no prizes as to which manufacturer) and have seen significant variation in the noise from chip to chip....ie all chips are *not* the same.

I have noticed Kodak seem to be continually improving the noise performance of their sensors, eg. 2009 sensors look much cleaner than those manufactured in 2006.

However, the only accurate way of testing "off chip" noise variations across manufacturers would be to place the same chip into differing heads, then measure the result.

[AlexN]

Originally Posted by Peter Ward

However, the only accurate way of testing "off chip" noise variations across manufacturers would be to place the same chip into differing heads, then measure the result.

Far too much effort - Not to mention voiding the warranty on two or three $10K+ CCDs. Better option is to take good calibration frames within 1°c of the light frames, then noise is non-existent anyway.

I know with both my ST-10XEi and ST-9E, noise in a 5 minute frame was considerably high.. however after applying a few dark frames taken within 1°c of the light frames, noise disappeared...

Everyone likes to stand up and argue the positives of their CCD over someone elses, this is all good and fair, who wouldnt want to defend the reason they bought a $10k AUD camera? At the end of the day.. Marcus, Tom Davis and Peter both take sensational images with their STL-11000M's. Mike S. Takes sensational images with his FLI PL-110002M, Greg takes sensational images with his FLI ML-8300 and Apogee U16M, Fred and Steven take sensational shots with their ST-10XME's, Ejcruz (Eddie) takes sensational shots with his QHY9, and many users (Clive, Doug, Louie etc.) take sensational images with their QHY8's

At the end of the day. CCD brand, and chip (both model and individual unit) make little difference. Its more about experience both in capture, calibration and processing. Good mount and optics always play a big part too...

Lets not turn this into a "Who's is better (or dare I say, Bigger) than who's... Rather, lets all kick back and rejoyce in the fact that there are many fantastic options available today for CCD imaging ranging in price from a meager $1800Au right the way through to whatever amount of money you're willling to spend..

Some have this option, some have that option. at the end of the day, as stated above, in the right hands, any good CCD can take great images in the right hands.

Paul's QSI will give great results, as he's made sure he has a good mount, good optics, and a great understanding of capture, guiding, processing behind him..

Thats enough from me, But I hope for once, everyone can agree on something...

Alex.

[Peter Ward]

Originally Posted by AlexN

Lets not turn this into a "Who's is better (or dare I say, Bigger) than who's... Rather, lets all kick back and rejoyce in the fact that there are many fantastic options available today....

Agreed Alex, and it was not my intention to get into a bigger/better comp...there are indeed many good choices available, but I'd only add they be made on a well informed basis.

You pays your money and takes your chances

[MrB (Simon)]

Not that I have any experience with the CCD cam's, but I do have an electronic background and would expect that there would indeed be differences between camera brands using the same Kodak sensors, and the quality of their support electronics.
I suspect the quality of the ADC(Analog to Digital converter), or anything else in the analogue signal path(like amplifiers etc) would be just as improtant as that of the CCD itself.
As with CCDs, there are different manufacturers of ADCs and different versions of ADC from each manufacturer, some are better than others, some are extremely expensive, some reasonably cheap.
The design of the support circuitry for the ADC would also have an effect on image quality... the accuracy and stability of the ADCs reference voltage for example.

Links in a chain.

[gregbradley]

At the end of the day. CCD brand, and chip (both model and individual unit) make little difference. Its more about experience both in capture, calibration and processing. Good mount and optics always play a big part too...

Alex.[/QUOTE]

I agree that is basically true. The differences between high end gear definitely get narrower and narrower. Like APO refractors. The last little bit of improvement comes at great cost. However when you are paying big dollars the little differences become bigger issues for deciding and they do tend to be little differences. You would be hard pressed to notice anything different between the same imager using an Apogee U8300, a FLI ML8300, A QSI583, a QHY9 all using the Kodak 8300 chip on the same scope and same exposure times.

Some cameras make the job a little easier though. I know imaging last weekend at -40C didn't require darks or flats really. The more noise the more you rely on accurate callibration and if that is even a bit off you have to use Photoshop etc. to try to fix the image whihc can be difficult.

There is tremendous choice now than there was when I started about 4 years ago. Now we have FLI, Apogee, QSI, Starlight Express, QHY and Sbig. The Yankee Robotics Trifid cameras seemed to drop out of the game for some reason.

Greg.

[Octane (Humayun)]

Peter,

Woops! I should have mentioned that I take flats, already. When using lenses I would use my LCD screen set to white in a fullscreen Photoshop document and use that for my flats. For a scope, I've built my own lightbox. Another option I've used is to peg a piece of foam core board about 10cm from the front of the refractor and taken flats in the middle of the day, ensuring my histogram peaks between 1/3 and 1/2 away across.

I was really just wondering as to whether taking sky flats had any obvious advantages over the, dare I say it, synthetic methods.

From what you've mentioned, though, it seems as though either way is fine.

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.

Getting back on track, I've just looked up the specs and details on the QSI and it looks to be a fantastic system. Kudos to you, Paul. Now, looking forward to what that coupled with the RC can produce.

Thanks.

Regards,
Humayun

Originally Posted by Peter Ward

There is nothing wrong with DSLR's. They are a great tool for wide field work.

Dome flats/T-shirt flats/sky flats are variations on the theme of providing a perfectly blank field that can be used to calibrate the camera's images.

Sky flats are easy. They have to be taken just before dawn or after sunset, adjusting the exposure to give about 30% well saturation of the sensor. (you want a high S/N but don't want ABG to kick in). Take several frames near the zenith, moving the scope around between each frame to minimize any sky gradient. Median combining them will remove any random signal.

The problem you will have with a DSLR is sky flats will not be monochrome. (Easily fixed in Photoshop!), but if done correctly you will have a nice map of the optical vignetting of your telescope (or lens) and any dust doughnuts.

Hope that helps.