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Old 04-11-2009, 06:23 AM
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Jase, for narrow band imaging, wouldn't you say that this would be a moot point. Especially with a very wide fov. With nb filters cutting down the stars, i would think that that you could easily attack 20 minute exposures without worrying about stars bloating. Especially with the 3nm filters i've got.
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Old 04-11-2009, 06:52 AM
jase (Jason)
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Frank, this is not a factor of blooming, but the dynamic range. Once the pixel wells reach their saturation on an ABG chip, the electrons bleed which stops the electronic charge spilling into surrounding pixels. The point here is that once the pixel wells are saturated and hit the 65k ADU limit (assuming 16bit AD conversion), you've lost data. Small dim stars are fine, but the large bright ones will place a bigger footprint on the chip, i.e. the star edges will also become saturated with time as can the general glow or halo. This coupled to the small pixel size (oversampled) will result in the bloating appearance.

Alex, yes, imaging through narrowband filters will reduce the impact given they cut a stars continuum energy. It doesn't solely negate the problem of limited well depth however. It will vastly depend what is in the FoV. Narrowband would lend its self to going longer. 3nm is as narrow as they come so as you suggest, you'll probably be able to 1200s subs, you'll need to experiment. Try a few scenes with bright stars to determine your limits in this configuration.
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Old 04-11-2009, 07:18 AM
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first shot will probably be the western veil or ic434, both of them have a very prominent star in the field.. I'll experiment as you say, but i can definitely say i won't be using the camera for broad band imaging at all, so i dare say the lower well depth of the 8300 won't affect my imaging much at all..
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Old 04-11-2009, 08:25 AM
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Quote:
Originally Posted by jase View Post
Indeed. The KAF-8300 probably has the lowest well depth of any CCD camera being manufactured today. At only 25.5k, the imager needs to take much shorter exposures to ensure the brighter stars don't saturate and bloat. This of course limits the amount of dim nebulosity one can obtain in a single sub exposure. Nothing stopping the imager blending both long and short subs though in post processing. 10min subs are probably way too long depending what is in the FoV. The chip has its uses, but based on the small well depth, it requires additional consideration. Personally, I see this chip is a compromise and not for me.
And bright stars saturate very quickly.
Using my ST10XME, eta carina (the star not the neb) will saturate through a V filter with a 10sec exposure. A "R" filter will saturate in 5 secs.
The reality is that you have to live with saturated bright stars if you want to also image surrounding nebula.
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Old 04-11-2009, 10:20 AM
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So imaging with this camera from suburban Sydney with my SN 8 inch (f/4) would not be advisable with anything other than NB filters?

Pete
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Old 04-11-2009, 10:45 AM
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you'd have to watch your exposures thats for sure.. At f/4 you'll puck the light down and stars could bloat quickly. You could most definitely do it, it just depends on the exposure times..
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Old 04-11-2009, 11:13 AM
jase (Jason)
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And bright stars saturate very quickly.
Using my ST10XME, eta carina (the star not the neb) will saturate through a V filter with a 10sec exposure. A "R" filter will saturate in 5 secs.
The reality is that you have to live with saturated bright stars if you want to also image surrounding nebula.
Totally different cameras Terry. Despite the obvious, back illuminated verses front illuminated, the ST10XME with the KAF-3200ME chip has a well depth of ~77k. That's three times the size of the KAF-8300 well depth! While you'll reach saturation with your camera relatively quick, you've got the benefit of picking up the faint nebulosity in the process. Particularly so when comparing 6.8u pixels verses 5.4u which is probably delivering better sampling. For your photometry work, you need a NABG chip for accurate readings, so the ST10XME wins hands down.
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Old 04-11-2009, 11:39 AM
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Dealer hat off for a bit....

I have to disagree with Jase's thoughts on the KAF8300.

I don't see the well depth causing a "star bloat" problem. Most CMOS devices have individual pixel depth half as much again ( even smaller pixels, lower fill factors) and while they do suffer from surface scattering (due the on-pixel architecture) they don't seem to suffer too much from intrinsic "bloat" due 1000x (or better) ABG protection (as on the KAF8300)

What the KAF8300 can painfully show up due its high sampling is any loss of focus, spherical error or abberations in the optics.

On the sampling side, putting this chip on a, say 10"RC would not be a great choice, but an 8"RC with say an adjustable flattner/reducer it should deliver some excellent results.

Where the ST8300 may be very cool is with camera lenses (!). Sampling would not an issue, low noise and high H-Alpha QE could lead to some delightful wide field imagery....

BTW I put my money where my mouth is, and ordered one for myself today!

Last edited by Peter Ward; 04-11-2009 at 11:50 AM. Reason: clarification
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Old 04-11-2009, 12:02 PM
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thats the exact reason i am so keen on the st-8300m. With my 480mm focal length, the sampling should be good. I was not too impressed with the 3.5 arcsec per pixel i was getting from my previous camera in this scope.. Can not wait to get a hold of one of these bad boys!
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Old 04-11-2009, 12:09 PM
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Totally different cameras Terry. Despite the obvious, back illuminated verses front illuminated, the ST10XME with the KAF-3200ME chip has a well depth of ~77k. That's three times the size of the KAF-8300 well depth! While you'll reach saturation with your camera relatively quick, you've got the benefit of picking up the faint nebulosity in the process. Particularly so when comparing 6.8u pixels verses 5.4u which is probably delivering better sampling. For your photometry work, you need a NABG chip for accurate readings, so the ST10XME wins hands down.
I agree absolutely. I was just making the point that the bright stars saturate very quickly with almost any camera. To get any reasonable signal with nebula needs much more signal than a 5 sec exposure can achieve. The colour of the bright stars will be lost with saturation and they will expand in size on the image as the area of saturation increases. I think the skill is in manageing this star size and still getting a nice image of whatever nebula is being photographed.
This is totally different to my photometry as I don't want the nebula (if present) to be visible. I just wan't the non staurated star image.
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Old 04-11-2009, 12:43 PM
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Can I just ask then if the camera would be suitable for my ED127 running at f7.5.

I don't understand the technical side of the issues involved but am reading and trying to learn.

Frank
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Old 04-11-2009, 12:59 PM
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frank, yes, the resolution would be good, i would think about 1.3 arcsec per pixel. Would have to check that when i get home, but i would say the KAF-8300 would be well suited to any optical system between 350mm and about 1700mm. Anything under 350mm will be undersampled. Although at that short a focal length you're not likely to notice. Anything over 1700mm will be heavily oversampled.. I'll be using mine at 480mm and 1000mm. If i can make a lens adapter, i might do some super wide field work at 50 to 135mm.. Would love to do a wide field view of orion at 50mm in sho pallete!
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Old 04-11-2009, 01:09 PM
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Jase and Peter, given both of your replies, I found that star bloat is an issue at 10 minutes, but like I said on the really bright ones only. I am using a flattener now but still some minor aberations that I am hoping to get taken care of in the next shipment from GSO. What are your individual recommendations for the 8" RC and the KAF8300 and exposure times? I found 5 minute subs have way too much noise and don't help with going deep either. I would need to take hundreds to pick up my signal on say NGC1365. Do I just suffer the bloated stars or do I use shorter subs but more of them?

Thoughts?
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  #74  
Old 04-11-2009, 01:21 PM
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if its only one or two stars bloating out paul, i'd just tidy them up during processing rather than trying different capture methods to avoid it.. Thats just me though, and i'm definitely not at the level of jase or peter..
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Old 04-11-2009, 02:12 PM
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........ What are your individual recommendations for the 8" RC and the KAF8300 and exposure times?
Moot point. I'd be more worried about being sky limited. My standard filtered sub is 10 minutes...regardless of aperture.

http://www.atscope.com.au/BRO/gallery34.html (ngc1566) is a low brightness galaxy similar to Ngc1365. I would have expected a better result, albeit with a narrower field, with KAF8300 due lower noise and better QE.

Last edited by Peter Ward; 04-11-2009 at 02:28 PM.
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Old 04-11-2009, 05:13 PM
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Thanks Peter, I have dark skies so that should suit fine. I will stick with 10 minute subs then, given this recommendation.

Alex, that is what I was intending to do, but it is often better to ask just in case I am making a critical error.
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Old 04-11-2009, 06:38 PM
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Fair call.. Given that thats what you were intending to do, I feel better knowing that my initial thoughts on the matter weren't outlandish

I will probably stick to 15~20 minute narrowband subs.. I'll figure out what works best with the 8300 chip and just stay with that....
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Old 04-11-2009, 07:04 PM
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I think Peter is spot on here. If you have poor seeing (as most of us do) then I don't see how having triple the well depth is going to help you get tighter stars.

Mark
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Old 04-11-2009, 07:34 PM
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I don't see the well depth causing a "star bloat" problem.
How can this not be Peter? The on pixel architecture such as the ABG LOD is identical across the KAF series. The only thing that comes to mind is the combination of well depth and pixel size (in correlation to sampling). I'm not stating that you can't produce great results with this camera, but its a horses for courses matter. You only need to review what Wolfgang Promper is doing with the U8300M to see the chip's potential, however there is no denying that the approach to imaging changes with a low well depth. Speaking with others on this matter at AIC, many take short and long subs to address the issue. There are undoubtedly other methods to address this such as post processing as previously mentioned.
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Old 04-11-2009, 07:47 PM
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How can this not be Peter? The on pixel architecture such as the ABG LOD is identical across the KAF series. ....
We'll have to agree to disagree Jase

I see this more as a scattering problem (small pixels) rather than lack of well depth....actually the KAF8300 has a "signature" that reminds me very much of TP2415.....humm wonder what the well depth was for film....
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