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Hi all, I want to know being Under Bortle 7/8/9 how long is the exposure needed or enough to image the stars only say either with RGB filters or OSC camera? I am trying to think about exposing the stars alone with separate equipment/gear so i save time for other data such as SHO and Lum, so i want to know how long is the stars can be exposed under heavy light pollution, and what is a good start exposure time to use, 1 minute or 2 or all the way up to 5 minutes per exposure? |
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Dear Tareq greetings from Italy. My equipment is ZWO Asi 1600mm pro, telescope William optics redcat 51 on Eqm35 pro as mount, and I used to record from home balcony under a Bortle 6 sky. After trying some combination I found very good scores (to me) following @Jon Rista short time exposure for dso. Mr. Rista approach in few words and by what I can understand consists in taking a lot of short exposure subframes instead of less long time exposure shots. please Cfr. Here for starting deeper look to the question: https://www.cloudynights.com/topic/573886-sub-exposure-tables-for-asi-1600-and-maybe-qhy163/ actually my LRGB subframes takes 60” at gain 173 while SHO and Hb subframes at 180” still at gain 173. Maybe I could set higher time with lower gain, but actually I’m pretty satisfied with these results. hope I helped somehow! wishing clear clear skies Stefano |
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Dear Stefano, Thank you very much for your reply and answer, and greetings from UAE here. I do have IMX571 cameras, two of them, one with mono and one with color, and with both cameras i will always use Gain 100 low or high mode, so this is fixed. For narrowbanding i always try to go longer, like 2 minutes and more, mainly i try to stay with 5 minutes exposures, but that for SHO, i was talking about LRGB, or say RGB only for stars, because i saw many images where people replaced the stars with RGB ones, so i am thinking about shooting stars RGB alone separate from LumSHO data. Tareq |
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Dear Tareq, As Stefano mentioned, the best strategy for stars only is to keep low exposures. If dealing with a lot of data isn’t a problem, I would suggest max 30sec. If you still see overexposed stars then bring it down to 15sec and gather around 1 hour of exposure time. that should be way enough to have beautifully colored stars. regards, Devoll |
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Dear @Devoll Duka and @Tareq Abdulla thanks for reply! Yes I completely agree with 30" or lower exposure for just stars subframes records. When there was ZTF comet available I used exposure from 10 to 20 seconds il LRGB and stars resulted very well pin-pointed and sharped, with no halo (at 60" wider and brighter stars present halos) and very uniform dark background. In a starless(SHO and/or LRGB) && stars( [L] RGB) approach I think it could be very very good, and as introduced upper, lower lrgb timing exposure for stars dedicated subframes can save precious time to be focused about more narrowband recording and/or calibration frames. Regards and wishing everybody the best Stefano |
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| Thank you very much again Stefano and thank you Devoll |
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I do 30 seconds exposure for RGB stars. I use a FSQ106ED at F3.6. Slower systems may require longer sub exposures. For my system, I found any longer will saturate the stars. I use a QHY268M (IMX571). Any background noise or gradients will not be an issue because you will only be using the high signal stars in the final image. 20 to 30 sub frames is adequate. I use StarXteminator to create a star only image. LYNN K. |
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Lynn K: Excellent, will do that, thank you very much |
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Dear Tareq, With a 5", f7 refractor I have satisfactory results with 30sec exposures under a Bortle 7-8 sky. I use a ZWO ASI 2600MM-Pro and usally do 30min for R,G,B each and spend the rest of the nights for L, S H or O. Greetings to UAE from Dortmund. Dirk |
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Dear Tareq, Dear Dirk, Thank you very much for your post, it will help me once i get back to imaging, i will give it a try and see, but i was asking so i can decide between a mono or a color for RGB alone for stars, if i buy a mono camera it means i must buy a filter and then i am forced to buy a cheap scope for Ha/OIII. Greetings also to Dortmund from Ajman[UAE] Tareq |
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It does depend on your f/ratio and your gain. The higher your gain (above unity) the smaller your dynamic range/higher the sensitivity and the quicker your stars will saturate. The lower your f/ratio, the faster your stars will saturate. After much trial and error I pretty much always shoot around unity gain. I've personally just been more successful there. With my 294MM/MC cameras and a f/6 scope, I found I don't saturate stars with 60 second RGB exposures. If I had an f/4 scope (a little more than twice as much light per unit of time) then I would be better off with 30 second exposures at the same gain. With a f/2.8 scope, you'd be limited to closer to 15 seconds per subframe. When I did shoot Lum (I don't anymore), I would cut my Lum subframe exposures in half, since they are taking in so much more light. So if my RGB frames were 60 seconds, my Lum subframes would be 30 seconds. Ultimately you'll need to look at your subframe histograms at the far right and find an exposure that doesn't leave a huge spike against the right side. |
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It does depend on your f/ratio and your gain. The higher your gain (above unity) the smaller your dynamic range/higher the sensitivity and the quicker your stars will saturate. The lower your f/ratio, the faster your stars will saturate. Good tips, i will keep that in mind, i have to find a scope that will be perfect at 420-432mm for RGB stars so it can match my two 90mm triplets, i try to stay with 90mm and larger not smaller, i could go with 70mm or 80mm scopes, unless i go larger for NB and use one of those 90mm triplet dedicated for RGB stars. Also don't forget about the light pollution, but i will try to go shorter, i am thinking to use IMX571 sensor rather than smaller ones such as 1600 or 294. Thank you |
