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Just getting accustomed to my new RASA 8, settled on 30 sec images with ASI553MC to avoid blowing out stars, Gain 100. But should I go shorter or longer? I have seen some imagers using 120 plus secs, this seems very long for such a fast scope. Others have gone for 15 secs or less, which will create a huge amount of images. I plan on shooting 240 x 30 secs, to give 2 hours integration time. Given that this is an F2 scope this should collect around 9x more photons than an F6 of the same fl. That's about 12 hours equivalent, which seems huge. Is that right? Max |
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| Yep, that's about right. I'm shooting 90 seconds with Ha filters, 60 seconds for unfiltered exposures with the 1600mm. My 2600MC exposures were around 60 seconds also if I recall. |
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| Yes, 30s seems about right, depending on sky brightness, of course. You could also turn down the gain to 0 to be able to go longer and not deal with as many subs. |
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| I think that's what I'm using, gain 0. |
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Hi i use 100 gain When the gain value is 100, I believe the HCG high gain mode is turned on, the readout noise is greatly reduced, and the dynamic range is basically unchanged according to the technical specs. You can see this effect in the performance graphs |
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| Yes, that is correct and often desirable. But at f/2 you might want to trade read noise for higher well depth so you can expose longer. This would mean you can reduce the amount of subs. Maybe not as much a concern with the IMX533 but with larger sensors it‘s quite time consuming to stack 800 or more subs. |
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| Oop, actually I had it reversed, I'm doing gain 100 or whatever is default. |
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I think you are in the ballpark. I am not sure if gain 100 is unity for your camera, but it makes sense to get to the firmware sweet spot depending on the target you are shooting. But I think there is value in using other settings depending on the target, etc. As Torben stated, there may be objects with a very high dynamic range that could blow out and thereby be unmanageable during processing. Just because gain 100 gets you back to full dynamic range, it does not necessarily (I didn't look up your camera graphs) mean that you are going to get full well depth at that location. Gain zero usually offers max dynamic range and pixel well depth. But for two of my cameras, both dramatically lose full well depth regardless of whether the firmware gives you full dynamic range. Either way, it might be worth trying different settings in situations that might seem where it might make sense, like super bright details that otherwise max out under very short exposure times. In general, even the gain zero settings need not require super long sub lengths. I generally stick close to recommendations from the Sharpcap exposures. I have to say that with the RASAs, it is amazing how well they can capture the delicate wisps of the IFN using near unity gain and the type of exposures that you are using. In comparison longer scopes that show the same images require such long exposures! Alan |
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Hi Max, I looked through your images and see that you almost went the same way that I did, first the Explore Scientific and now the RASA 8 😊. From my experience I could advice 120s with the RASA (it is actually ~f/2.8 when you count the obstruction in the middle) if you use narrow band. BUT ... this depends very much on your Bortle scale as well, in high LP you might be potentially right with 30s, only trials will tell. I would not worry so much about bloated stars, you can most of the time handle those in processing, took me at least a year to get a grip on that though and I am still trying to do it better. I looked up your camera and gain 100 is what I would use with it, depending on the target though, but for most this should be the best choice. My recommendation: aim for a target that is well known (eastern veil, horsehead, and maybe best and since it is coming up very soon: Rosette) and spend some time on it (btw ... your Iris is a challenging target and what you obtained is very good already). Take one night only 30s exposures and then the next clear night 120s, and yes, try also 300s!! Spend some time on the processing, listen to some YouTube videos to see if you can learn something, with Adobe Lightroom you have plenty to choose from. Hope that helps! Keep on going!! Uwe |
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Thanks guys these are really helpful comments. So what’s your view on total integration time with a RASA 8? Even a conservative estimate puts the RASA at 4-5 times the light gathering. So if 2hrs of RASA is equivalent to 8 hours plus at F6, I had assumed that 2 hrs was more than plenty. Max |
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In general (even with the RASA) an integration time of less than 5 hours is usually not good enough to make your life easier at processing time (depends of course on your exposure times, 30 second ones give you more images so that the noise reduction is better with less total integration time). I know, there is always the desire to get one target done in one night so that one can get the next one in the next night, I believe nobody behaved differently at the beginning of the astro photography journey. Hence 5 hours would be great for 120s exposures, maybe with 30s 3 hours? This is another trial that is on your plate 😊. P.S.: When you look at all the Top Picks and IOTDs you will notice that a lot of them used a LOT of integration time. |
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With my RASA 11, I have gotten good results with 3 hrs or more, depending on the subject. In some cases, less. The caveat from my opinion is that I have not had that much time with my 11, since I live in a weather restricted location. And my processing has changed over time. However, the real test for me is that I have gone back to subjects that I did two years ago and find that with proper sub exposure and gain settings (I do believe that you can use a single setting for most all subjects and do just fine, but that making adjustments based on subject can improve things) it is remarkable what you can do with relatively short total integration times. I suggest that you do the experiment yourself. Easy to do. Next time you image, take subs on a single subject over the whole night, say 6-7 hours. You can then integrate full time, and a couple less times, say down to 2 hrs., even 1 hr, using a properly chosen subset of subs for each integration time. Compare what you get. Then decide for yourself how you want to spend your time... And that may change as you get more discriminating of your own images and compare to like images from others using the same gear. The going conventional wisdom is that more subs are better. Well, that is true. 1,000 will be better than 10. However, the useful question takes into account the fact that the improvement with more subs results in diminishing returns per an asymptotic approach to some ideal. Collect the subs you need to meet your standard. I have seen those who publish RASA images that use integration times that meet or exceed what others here publish using long fl telescopes. But I would ask, why did you buy a RASA? Its almost like there are some awards for total integration time bragging rights! RASAs take the "macho" out of imaging! But most importantly, for us weather and location restricted imagers, it allows us to do this hobby where it otherwise would be very impractical. Alan |
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| FWIW, when I started my RASA-11 journey, I did a lot of "initial survey" work, taking 30-60 minute exposures at 1/2 unity gain (I'm using an ASI183mcPro). I was pretty happy with those initial images for a while. However, I've recently found myself returning to many of those targets and exposing for 3-4 hours of integration time (dark site). This of course produces much better images (and maybe my post-processing skills are improving? ;-) ). Anyway, maybe next year I'll decide I need4-5 hours/target, but I doubt it. Maybe for really deep exposures (distant galaxy clusters) I'll want to go longer. As for exposure duration, I currently use 45 second exposures to keep the total # of subs manageable, except for open/globular clusters where I use 30s subs and just bear the post-processing pain. I like gain at the sweet spot on the camera specs graph for most objects, and haven't bothered to even try going below 1/2 unity even for the high dynamic range objects. |
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Doug, I think I am with you on integration times. I have done some recently at ~3hrs and 4 hrs total (certainly not 3 hours attempted, but just 3 hrs after dumping bad subs). But also I have been doing a number at 5 hours lately. Oddly, the 3hr targets have been those that consist of mostly dark nebulae and IFN. You would think that these would require the most time. For these, I have adopted a gain setting of ~0.4 e-/ADU, so even hotter than unity gain for my camera. My camera QHY268C is somewhat unique in having multiple readout modes, which greatly affect readout noise, dynamic range, etc. So my choice readout mode for those very low dynamic range objects was to go for the lowest readout noise and high dynamic range, even though the fullwell was only ~25% of maximum found in other modes/gain settings. The benefit was sub lengths of only 90 seconds. As an example, see  LBN 483, Two Reflection Nebulae But for other subjects with much higher dynamic range, I fall back to unity or even go to gain 0 with subs up to 240 sec. For me gain 0 is above 1.7 e-/ADU. With a very deep well, I never fully saturate a pixel, yet get just about everything else. I could go shorter, I guess, but my skies (low Bortle 4) allow me to go as long as I do and I like to get signal as high over shot noise as I can. Oddly, with these higher dynamic range objects, I have gone to more hours, such as 5 plus, when I can... But sometimes I can't, and there are other qualities in my subs that I reject subs (i.e. integration time!) for as being more important than forcing myself to have to use them to boost the integration time. And where I live, it would not be uncommon for me to have to wait for a year to complete a project. I'm too old to be that patient! CS Alan |
