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Hi folks, im still struggling with the edgehd/lepus combination. I live in bortle 6 skies and need to use a optolong l-pro, otherwise all I get is reddish sodium glow. my struggle is exp time. It seems I need 500 secs for bright objects like the iris nebula, and 600 secs for darker like crescent nebula. However the sky glow is very close up to the objects themselves… any suggestions? What do others use for exposure rime? my camera is an ASI2600MC-PRO thanks for any advice |
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| You are just hurting your image if you do drizzling so I'd quit that for starters. At B6 you sky brightness is the main limiting factor so anything above 300s is counterproductive, This said both the Iris and the Crescent are considered to be bright subjects but even then you would need many more hours to reach a reasonable SNR unless you keep your image scale down, like a LOT. 180s seems more like the thing all things considered. For not broadband subjects (such as the Crescent) get a dual NB filter such as the L-Extreme, ALP-T or equivalent from other manufactures to improve your signal gathering and reduce the sky brightness. |
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Contrary to what some others would lead you to believe, the optimal exposure time is always a longer exposure from a noise theory perspective. Longer subs will reduce the total contribution of read noise in the integration and will reduce storage obligations and processing time. I would recommend using as long of exposures as you can get away with so long as:
Hope this helps! ~Charlie |
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| I'm not sure, read noise for the 2600 at a gain of 100 where HGC kicks in is a low 1.35 (e-RMS). This might have been good advice when shooting with CCD cameras but no longer holds with the new generation of CMOS cameras of which the 2600 is a good example. It's the light pollution that killing his S/N, forcing super long integration times to get anything. What's needed is narrowband filtering to block the light pollution. I shoot dual band OSC under bortel 8 skies. With that setup, 120 sec. subs work just fine on subjects like the crescent and that's at F/10. |
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Tony Gondola: Of course using narrowband filters for appropriate targets will result in better signal to noise ratio (though on many targets that is not applicable), and of course light pollution / shot noise is the dominant noise source - that is true even in Bortle class 1 locations - but that does not negate the fact that you will unequivocally be introducing more noise into the integration using more shorter subs. This will have more of a perceivable impact when using narrowband filters especially, but is not incorrect for broadband, nor is it any different between CCDs and CMOS. Read noise does not 'kick in' at some certain level, it is present in every exposure and every integration. In fact, when using a slower system like a SCT, read noise affects the integration even more than when using a faster system. As I mentioned, there are other benefits as well, including reduced storage obligations, fewer necessary dithers which improves acquisition efficiency and reduced stacking times which can quickly snowball if shorter subs are used. The criteria listed above should guarantee that results are not compromised in any way by increasing exposure. If any one of those criteria are violated, it is safe to decrease exposure, but otherwise, I see no compelling reason to. |
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| I think it that it is fair to say that read noise in CCD's is generally greater than the current generation of CMOS sensors. I looked at some current CCD units in the 4 to 5 k range and the read noise ran between 9 and 11 e-RMS. This is a much higher value than the 2600. And yes, the read noise can kick down to a lower value if you set the gain properly using HCG mode. At a gain of 100 the read noise drops from an already low 3 e-RMS to about 1.35 e-RMS. On my 585 sensor it drops from 3.75 e-RMS to 1.1 e-RMS. With read noise that low there just isn't any advantage to doing ultra long subs. You are only asking for lower resolution and more lost time from wasted frames. |
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Hi Hermann, I am imaging usually with a 80mm APO from a Bortle 7 area close to an airport and 300s exposure time is just fine for me (I have just recently imaged the Crescent nebula, partially from this area). I am using a dual narrowband filter (IDAS NB1, Ha&OIII) with an OSC camera (Omegon VeTEC 571C) and I usually take around 10h of integration time. I try to image as much as possible when the target is high in the sky at the zenith. |
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Tony Gondola: Its true that the value of the read noise was higher on older CCDs, but the problem still is still helped on CMOS by taking shorter exposures. And I see what you mean, I interpreted that as you saying that read noise only becomes a problem at certain exposure times, my bad. You're right in that there are certain gain modes that support higher dynamic ranges and less read noise, but I am assuming that is constant so we can discuss only the effect of longer exposures. Typically there are only one or two modes that most people will use anyway and they are local maxima of dynamic range. The contribution of the read noise on most exposures will be very small, nearly negligible if using reasonable exposure times, But why not use longer subs to reduce it? If you are getting reduced resolution or having to throw away any significant number of frames by using longer subs, I would suggest that your guiding cannot handle it and that would violate the first criterion - I think that is covered by my original suggestion. There is no reason that resolution would be lost in a 10 minute exposure with good guiding vs. a 5 minute with good guiding as long as it's consistent. Unless youre lucky imaging, you will be sampling a very similar distribution of guide error which will dictate any mechanical loss to resolution. |
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| I think that's right Charles. A lot of this is dependent on a lot of variables and unless you are running hardware that's as close as possible to perfection, you'll always be compensating somewhere. I guess it's just good to know that you have options! |
