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01-24-2013, 02:34 AM   #451
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QuoteOriginally posted by smigol Quote
Here's what I have seen:

- - - - -

After using this cooling, I have noted that the camera will track much closer to ambient. I've not seen it drop below ambient. Another thing that happens is that thermal equilibrium is reached faster and stays steady. Having the steady cooling seems to help with the noise profile and I am getting much smoother results.

- - - - -.
Just a loose idea: When you have thermal equilibrium between camera, scope and surroundings, you will certainly not have any turbulent air flows due to temperature differences in your optical path. Could that have any or only negligible impact on your image quality?

01-24-2013, 09:11 AM   #452
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QuoteOriginally posted by Stone G. Quote
If GWARmacine is right and your scope has a 24.5mm / 0.965" eyepiece draw tube, the direct solution would be to get a 24.5mm nosepiece that screws directly into the T2-ring (c.f. 1 below).

However, such adapters seem to have become quite rare (as are 24.5mm eyepieces themselves) and an easier solution might be to get a 1.25"-to-T2 nosepiece (2 below) and a 24.5mm-to-1.25" eyepiece adapter (3 below). The adater shown in '2' can be found "everywhere" and he latter adapter type, '3' is still widely available.
I grabbed a tape measure and checked - the tube where the eyepiece goes is definitely just shy of under an inch, so I'm going to figure the 0.965" is what to go for. I'll try to figure the best route between now and whenever to try this out. I'm thinking more of using the scope for terrestrial stuff (birds! I live almost on the banks of the Kennebec River so get lots of cormorants, gulls, osprey and other stuff) anyways, which means I won't really need to do anything until warmish weather. Being central Maine, that means at least late March towards April for me.
01-24-2013, 10:37 AM   #453
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QuoteOriginally posted by Stone G. Quote
Just a loose idea: When you have thermal equilibrium between camera, scope and surroundings, you will certainly not have any turbulent air flows due to temperature differences in your optical path. Could that have any or only negligible impact on your image quality?
I'm aware that tube currents and other temperature differentials can have bad effects on image quality. I use the fan on the heatsink all the time, so I'm not sure if there's a noticeable effect.

When I use the scope, I have an extension of the dew shield by about another 6 inches. This helps reduce the formation of dew and keeps stray light out. It may also act as a shield for these thermal plumes.

I have noticed that when I run the dew heater at maximum settings that the star shapes tend to get a little ugly. I'm experimenting with how low I can set them and still keep condensation away.

There are so many variables in play with this pastime that I could take just one and spend weeks fixing it.
01-25-2013, 12:24 AM   #454
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QuoteOriginally posted by smigol Quote
Native ISO is not necessarily the lowest setting. You would need to do some testing to find the unity gain value (where the value of electrons vs ADU is about 1). This also assumes that read noise is also amplified which is common among CMOS sensors.

There's been extensive testing of Canon and the general consensus is that ISO 400 to 800 is the ideal range because of the balance of unity gain vs the shenanigans that happens to the RAW files by the in camera software. Someone would have to do the same testing on the Pentax cameras to determine the proper settings.

In my testing for the K10D, I've settled on ISO 400 because it's a reasonable balance of noise and exposure length for my locations. A 10 minute exposure usually shows skyglow and thus there is no need to go deeper.

I've done some long exposure testing with my K10D and a lot of long exposure work with a Nikon D50. In theory if you get good results at 10 minutes with ISO 400, you should get better results - lower noise per image with no loss in bright areas by decreasing ISO to 100 and increasing exposure time per sub. Here's some experimental work that I've done with my K10D:
https://picasaweb.google.com/102361289894036464287/Astrophotos
Images 3, 4, 5, and 6 (left to right) were done with a K10D, the rest with a Nikon D50. These were done at an extremely dark location.

The 14bit K5 should always use ISO100, IMHO because it gives the best possible dynamic range and SNR.

01-25-2013, 10:20 AM   #455
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Pentax K10D noise characteristics for Astrophotography

QuoteOriginally posted by duncan1 Quote
I've done some long exposure testing with my K10D and a lot of long exposure work with a Nikon D50. In theory if you get good results at 10 minutes with ISO 400, you should get better results - lower noise per image with no loss in bright areas by decreasing ISO to 100 and increasing exposure time per sub. Here's some experimental work that I've done with my K10D:
https://picasaweb.google.com/102361289894036464287/Astrophotos
Images 3, 4, 5, and 6 (left to right) were done with a K10D, the rest with a Nikon D50. These were done at an extremely dark location.

The 14bit K5 should always use ISO100, IMHO because it gives the best possible dynamic range and SNR.
I understand your thoughts about the lower ISO setting giving better headroom. I tried this for a while as well. Used 200 ISO for a long while and then finally moved to 400 ISO for my purposes after I did some testing.

I did a range of tests with my K10D where I checked response and noise via a script in PixInsight. The procedure is as noted by the script developer Georg Viehoever to follow "the method described in described in "The Handbook of Astronomical Image Processing" (HAIP) by Richard Berry and James Burnell, Second Printing, Errata for chapter "8.2 Basic CCD Testing" (see http://www.willbell.com/aip4win/Errata%20to%202nd%20%20Printing%202nd%20Edition%20of%20HAIP.pdf). All you need are 2 bias frames, 2 flat frames and 1 dark frame."











This was in the attempt to understand where unity gain should be (.7 e/ADU). The closest I was able to show was at 1600 ISO(!).
What was more interesting was the read noise. For a CMOS chip, it's very important to note the point where the signal will be higher than the readout noise. This difference becomes important once you stat stretching the image. The readnoise for the K10D remains fairly constant across the whole range at 3-4 e. Consider what happens when this noise floor is stretched along with signal. A much less aggressive stretch will be required of a higher ISO setting vs a lower ISO setting. The aggressive stretch will pull this noise floor up as it competes with the signal.

I've also tested another concept:
The sub length is the critical component as the amount of light that falls on the sensor is the same no matter what ISO setting. The test that I did compared a shot of a specified length at ISO 100 through ISO 1600. Then I stretched the results just as I would in astrophotography. What I found was that the noise levels were pretty much the same.




Look at how much the bias shows up in the stretched 100 ISO vs the 1600 ISO. The bias signal is the source of the blue streaks.

It's important to note that it's unknown what kind of optimizations are performed on the raw file as it passes through the camera's software. Canon has been well-tested and shown that they are messing with the raws before being written. DSLRs are meant to be used in bright conditions so they are setup to give good results rather than looking at the blackness of space.

I have shot at 100 and 200 ISO before and it does seem clean. However, I feel strongly that a lot of signal is "lopped off" the low end at 100 ISO in noise reduction either in the camera or in post. Much of the headroom of the lower ISO is wasted except on bright stars. The background skyglow doesn't climb out of the lowest parts of the histogram unless using longer subs.

This leads to another point which is part of the limits of the rest of the hardware: mount capability.
I shoot 10 minute subs because my mount can handle them very well. It could handle 20 minute subs once I fix the remaining flexure issues. When I shoot widefield, there is no flexure problem so I could use a longer sub.

With all this in mind, there are other limits to the camera other than read noise and ISO setting. The K10D is plagued by strong amp glow that is non-linear in temperature response. Thus, dark frames only go so far in removing this glow. Software that is not able to handle the non-linear glow has significant problems with removing this glow. Deep Sky Stacker has known problems with amp glow and is only able to scale darks a little ways. Because the amp glow is non-linear, scaled darks can't touch it.

I finally stopped using DSS for calibration because of this amp glow. Instead, I've found that Maxim does a much better job of calibrating. Unfortunately, Maxim does not do a great job of debayer (and I've yet to get Pix Insight to do a decent job of it). So I'm still stacking in DSS of the calibrated FITs from Maxim to give a debayered FIT with good hot pixel rejection and mosaic.

FWIW, the QHY10 and Starlight Xpress SXVR‐M26C astrophotography cameras use the same Sony ICX493AQA SuperHAD chip as the K10D. There are further tests about readout and gain levels for these cameras that help to shed light on the way this chip works. Eventually, once I have exhausted the capabilities of my K10D -- or it breaks -- I'll get one of these devices to replace it.

Last edited by smigol; 01-25-2013 at 10:24 AM. Reason: Added introduction
01-26-2013, 06:47 AM   #456
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QuoteOriginally posted by smigol Quote
I understand your thoughts about the lower ISO setting giving better headroom. I tried this for a while as well. Used 200 ISO for a long while and then finally moved to 400 ISO for my purposes after I did some testing.

I did a range of tests with my K10D where I checked response and noise via a script in PixInsight. The procedure is as noted by the script developer Georg Viehoever to follow "the method described in described in "The Handbook of Astronomical Image Processing" (HAIP) by Richard Berry and James Burnell, Second Printing, Errata for chapter "8.2 Basic CCD Testing" (see http://www.willbell.com/aip4win/Errata%20to%202nd%20%20Printing%202nd%20Edition%20of%20HAIP.pdf). All you need are 2 bias frames, 2 flat frames and 1 dark frame."











This was in the attempt to understand where unity gain should be (.7 e/ADU). The closest I was able to show was at 1600 ISO(!).
What was more interesting was the read noise. For a CMOS chip, it's very important to note the point where the signal will be higher than the readout noise. This difference becomes important once you stat stretching the image. The readnoise for the K10D remains fairly constant across the whole range at 3-4 e. Consider what happens when this noise floor is stretched along with signal. A much less aggressive stretch will be required of a higher ISO setting vs a lower ISO setting. The aggressive stretch will pull this noise floor up as it competes with the signal.

I've also tested another concept:
The sub length is the critical component as the amount of light that falls on the sensor is the same no matter what ISO setting. The test that I did compared a shot of a specified length at ISO 100 through ISO 1600. Then I stretched the results just as I would in astrophotography. What I found was that the noise levels were pretty much the same.




Look at how much the bias shows up in the stretched 100 ISO vs the 1600 ISO. The bias signal is the source of the blue streaks.

It's important to note that it's unknown what kind of optimizations are performed on the raw file as it passes through the camera's software. Canon has been well-tested and shown that they are messing with the raws before being written. DSLRs are meant to be used in bright conditions so they are setup to give good results rather than looking at the blackness of space.

I have shot at 100 and 200 ISO before and it does seem clean. However, I feel strongly that a lot of signal is "lopped off" the low end at 100 ISO in noise reduction either in the camera or in post. Much of the headroom of the lower ISO is wasted except on bright stars. The background skyglow doesn't climb out of the lowest parts of the histogram unless using longer subs.

This leads to another point which is part of the limits of the rest of the hardware: mount capability.
I shoot 10 minute subs because my mount can handle them very well. It could handle 20 minute subs once I fix the remaining flexure issues. When I shoot widefield, there is no flexure problem so I could use a longer sub.

With all this in mind, there are other limits to the camera other than read noise and ISO setting. The K10D is plagued by strong amp glow that is non-linear in temperature response. Thus, dark frames only go so far in removing this glow. Software that is not able to handle the non-linear glow has significant problems with removing this glow. Deep Sky Stacker has known problems with amp glow and is only able to scale darks a little ways. Because the amp glow is non-linear, scaled darks can't touch it.

I finally stopped using DSS for calibration because of this amp glow. Instead, I've found that Maxim does a much better job of calibrating. Unfortunately, Maxim does not do a great job of debayer (and I've yet to get Pix Insight to do a decent job of it). So I'm still stacking in DSS of the calibrated FITs from Maxim to give a debayered FIT with good hot pixel rejection and mosaic.

FWIW, the QHY10 and Starlight Xpress SXVR‐M26C astrophotography cameras use the same Sony ICX493AQA SuperHAD chip as the K10D. There are further tests about readout and gain levels for these cameras that help to shed light on the way this chip works. Eventually, once I have exhausted the capabilities of my K10D -- or it breaks -- I'll get one of these devices to replace it.
Very agree with your point of view. Based on my practical experience, as long as can guarantee under the conditions of exposure but ISO1600 is first selection!
01-26-2013, 02:50 PM   #457
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My best shot with a non-guided mount and my DA* 300/4 on a K-5, sadly pretty boring.
1/10
F4
ISO 12800
40 Light frames and 40 Dark frames stacked in deep sky stacker.

01-26-2013, 09:05 PM   #458
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QuoteOriginally posted by VisualDarkness Quote
My best shot with a non-guided mount and my DA* 300/4 on a K-5, sadly pretty boring.
1/10
F4
ISO 12800
40 Light frames and 40 Dark frames stacked in deep sky stacker.

With a 300mm lens, you should be able to get away with a 2.5 second exposure with minimal trailing. That would capture a lot more stars as long as you have a steady tripod.

01-26-2013, 09:30 PM   #459
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QuoteOriginally posted by krp Quote
With a 300mm lens, you should be able to get away with a 2.5 second exposure with minimal trailing. That would capture a lot more stars as long as you have a steady tripod.
Yeah, I realized it when uploading the pic (6oo/FL). I don't know where I got the 1/10s from but 4am may have played a part in it.
01-26-2013, 11:51 PM   #460
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Thor's Helmet with Pentax K10D

Here's one more from last week taken on the nights from Jan 18 to the 20th.

This is a view of "Thor's Helmet" or NGC 2359.




This is a large region of nebulosity surrounding a Wolf-Rayet star. WR stars are massive, energetic stars that spend time near the end of their lives throwing off large amounts of gas. The envelope of gas from this star is remarkable because of the appearance of a helmet. This can be better seen in a flipped and cropped version of the same shot:




This is a near-100% view of the image. It shows the flaws of the sky background and noise reduction efforts.

This is the stack of 83 frame at 600 seconds at 400 ISO making nearly 14 hours of integration.
Scope used: Stellarvue SV4 with flattener and IDAS LPR v2 filter
Camera: Full spectrum modified Pentax K10D with cooler giving temperatures of 4-6 C.

I'm hoping the weather will hold and in two weeks I can get back out under the stars for a widefield of this area.
01-27-2013, 07:58 AM   #461
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Nice work smigol.
01-27-2013, 12:13 PM   #462
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QuoteOriginally posted by gs2c Quote
Very agree with your point of view. Based on my practical experience, as long as can guarantee under the conditions of exposure but ISO1600 is first selection!
Smigol, I appreciate your detailed and thoughtful reply but in my experience, it is necessary to use longer subs, especially on faint objects, since the probability of capturing faint objects is better on fewer longer subs, than on more but shorter subs - however skyglow is an issue, that I don't worry about while imaging at my preferred sight, since it has virtually no artificial skyglow. Anyways your images are extremely well done. What mount are you using?
01-27-2013, 08:52 PM   #463
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QuoteOriginally posted by duncan1 Quote
Smigol, I appreciate your detailed and thoughtful reply but in my experience, it is necessary to use longer subs, especially on faint objects, since the probability of capturing faint objects is better on fewer longer subs, than on more but shorter subs - however skyglow is an issue, that I don't worry about while imaging at my preferred sight, since it has virtually no artificial skyglow. Anyways your images are extremely well done. What mount are you using?
I also agree about the longer subs point for faint objects. Since I've been mostly imaging at home or near a suburban dark sky site, the 10 minute subs were my preferred duration. Skyglow pushes well above the 25% point on the histogram at 10 minutes, so all a longer duration would provide would be more saturation.

The mount I use is a Losmandy G11 and that works very well. I do have some flexure issues that also limit the sub duration as well, but I'm getting this slowly solved. I could probably push 20 minutes once I get out under skies that support that kind of duration.

Also, I have an extensive library of darks at temperatures ranging from 0C to 32C at the 10 minute duration. I've started building a library of 20 minute darks for possible experiments.
02-01-2013, 05:10 PM   #464
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My best attempt at the Big Dipper and more with K-5 + Sigma 20/1.8
20s F4 ISO640






Two with landscape, note the brutal flare in the sky from lighting.






Last edited by VisualDarkness; 02-01-2013 at 05:46 PM.
02-02-2013, 11:42 PM   #465
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QuoteOriginally posted by VisualDarkness Quote

...snip...
note the brutal flare in the sky from lighting.



Always remove any clear filter from the lenses when using them at night for the flare reason. Also, hoods make a great protection from off-axis glare reduction and dew prevention.
I will use a light pollution filter on my lenses when I do widefield, but these are special cases --- and they still produce flare from bright stars.
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