0.00
#... |
|---|
|
When I was shooting unguided, I ended up at 30s exposures, but now that I am guiding, what is advisable for exposure times using a DSLR (Canon 60D)? Also, what are some benefits and downsides of various exposure times? I see a lot of folks doing 2-3 minute exposures; sometimes longer. I imaging that longer exposures makes integrating faster because there would be less photos to process, but I would love insights so I can determine what is best for now. Thanks! |
1.20
#...
·
4
likes
|
|---|
|
Exposure time can be dependent on a number of factors, number one factor would be "bortle class". The lower the light pollution is in your area the better the night view, which yields less noise and cleaner images. Another factor is the type of filter you are using, ie. basic lpr pollution filter, enhanced filters focused on Hydrogen Alpha, Oxygen III , Sulpher, etc., all of which can assist in increased exposure time with low noise and a cleaner image. Taking advantage of more 'light frames', flats, bias and dark frames can give you a more enhanced image that you can look at and appreciate the amount of time you spent obtaining those exposures. There are many good video's out there that can give you plenty of insight to the entire process. |
1.51
#... |
|---|
|
Problems with an exposure that is too long: 1 Sky Glow saturates sensor 2 Exceeding guidance stability, oval stars 3 Blowing-out the bright parts of some brighter DSO's 4 Lost time if a sub fails for any reason (random mount wiggle, cloud, plane...) 5 Not having enough subs for effective dithering 6 Not having enough frames for Winsorized Sigma Clipping during integration Problems with an exposure that is too short 1 Too many subs create excessive processing time 2 Exposure too short to accurately capture faint DSO objects or features 3 Excessive disk space consumption 4 Lost time from delays between subs CS |
 #...
·
1
like
|
|---|
|
Hi, A 3-minute frame will have always more information than 3x 1-minute frames (for the same equipment configurations and sky situation). I've already had a 60d, and it's a great camera, with low noise, when compared to the other cropped ones. So the more exposure time you get, the better. If it takes 300 seconds it is excelent( with iso up to 800). It is good to give a 40-second interval, at least, between one photo and another to cool the sensor. If I get more time (600 seconds for example, even better, but I would reduce the iso to 400, for the same colling reason). Regards |
|
0.00
#... |
|---|
|
Thank you both! I think I’ll shoot 3 minute exposures next and see how it goes. |
0.00
#... |
|---|
ricardo leite: I have never used such long interval between exposures. If you could share some more info from your exposures it will be very interesting for me. Thanks :-) |
4.82
#...
·
10
likes
|
|---|
|
The answer depends on a lot of factors. But at the end it can be quite simple. Single subexposure time matters virtually nothing, as long as you stay within reasonable limits, and you are in the read-noise dominated regime. Than only _total integration time matters_. To determine subexposure time, it depends on:
The minimum subexposure should "swamp" the read noise, making the shot noise (from object and light pollution especially) (or dark current) the dominant noise term. For DSLRs, the best rule of thumb is to use the back of camera histogram (the one you see when displaying the image on the LCD). The peak should be somewhat around 25% from the left. Much more to the right (brighter), you are gaining very little benefit, and you might start saturating too many stars. Once you get to the minimum exposure time - let's say for you it is 1 minute. Then it *does not matter* if you do 60x1min, 30x2min, 20x3min, 6x10min (practically speaking. Longer subexposures will always have a slight, diminishing, advantage). Provided you can track accurately for any amount of time. Losing one 10min frame can be very detrimental, much more than simply using shorter exposures. Of course you should also avoid saturation - you do not want to clip all the stars (but the bright ones will almost inevitably clip), and definitely not clip the DSObject you're shooting. Usually, most systems under most skies, this number should be between 1min (fast optics, or light-polluted skies) and 5 minutes. For slow systems under dark skies, it can be as long as 10-20min. Add in narrowband filters (with which DSLRs are quite inefficient), and the "optimal" time will probably be 10-60min, which may be very impractical, and you should use shorter, 5-10min exposures and live with the small SNR loss. At the end it is a tradeoff. Once you find your optimal subexposure lenght, then the integration is what matters. Best regards, Gabriel PS: unless you have very warm nights (20ºC or more), thermal noise should not be a major concern, especially in the beginning. If your night temperatures are below 5ºC, than dark current becomes really negligible. I would recommend against "cooling" the sensor, especially if it means capturing less photons. Integration time is the most important! |
|
1.20
#...
·
1
like
|
|---|
|
I am no expert by any means! But I agree with Gabriel, and my light bulb moment came after watching this video https://youtu.be/3RH93UvP358 - This is the video of Dr. Robin Glover the creator of Sharpcap I now only shoot 75 second subs at my house due to the level of light pollution. This makes life much easier for my entry level mount! Ray |
|
1.20
#...
·
1
like
|
|---|
| https://youtu.be/3RH93UvP358 Let's try that again..... |
11.83
#...
·
1
like
|
|---|
| Gabriel is correct though I think he unintentionally made a mistake when he said that sub exposure time should be in the read noise dominated regime; your subexposure time should be such that the read noise is swamped by the background signal. This means shorter subexposure times in light polluted skies and longer ones in dark sky sites. Once your sub exposure times reach this regime, further increasing them buys you nothing. This is very well covered in the excellent Robin Glover video linked by Ray. The advice on keeping the sub exposure time so the histogram is about 25% from the left is also sound. |
|
4.82
#...
·
1
like
|
|---|
Gabriel is correct though I think he unintentionally made a mistake when he said that sub exposure time should be in the read noise dominated regime; your subexposure time should be such that the read noise is swamped by the background signal. Thank you very much for pointing that out! I'm sorry, I definitely misspelled things there - should have read the thing more carefully. I cannot edit the post, so, to be clear: The subexposure time should *NOT* be in the read-noise limited regime! RN should be swamped by the other noise sources, and you should be in the *SKYFOG* limited regime. Another errata: "I suggest you start with this recommendations." Oh, and the emoji was automatically added because of the ). |
0.00
#... |
|---|
| I always work with my Epsilon 180 ED with a Canon EOS 6 D Modified in a DAW version and always use a light polution filter, a CLS 4 and this picture was made for a year ago with 90 frames of 2 minutes on ISO 400 on a G 11 Mount. With F 2,8 you can take with 2 minutes on ISO 400 very faint objects you can see on this image. perhaps this is a solution for you and succes with your equipment. |
0.00
#... |
|---|
|
I used to go with the "30% from the left" histogram rule. Nowadays I rely on mean ADU values of the RGB channels. It's faster and more reliable then checking an histogram  How long to expose does not only depend on how many stars you're saturating, but also on how many subs you're planning to stack and store at the end of the session. When I was "newbier" than today (Nov 2020), I shot ~750 subs at 75s at a dark nebulae. Trust me, it wasn't fun (>12h with Xeon 8 core, 32gb ddr4, nvram ssd) to stack them all  also storage can become a problem quite fast, if you keep shooting tons of short subs.
|
7.61
#... |
|---|
|
In choosing the iso, how much of a factor is diminishing dynamic range? Also, with my Canon 5D IV, I've had intermittent banding issues that *seem* iso dependant. Cheers, Scott |
3.81
#... |
|---|
| In response to that question, Scott, it really depends on the camera. There is a sweet spot somewhere in the ISO /exposure time dilemma that we all face as DSLR users. I have not had any banding issues with my 6D and have found that ISO 3200 is the best value for me, allowing three minutes exposures with my L- enhance. I have actually seen my DR increase with these longer exposures and higher ISO, producing cleaner subs for me. It is just so different for each model though. |
|
7.61
#... |
|---|
|
now that I have guiding, I’ve been imaging M51 at 300s ISO 800 which seems like a good exposure but the banding is pretty prominent. The banding, though, is taken care of for the most part with darks. When I shot Bode’s without guiding at 120s ISO 2000/2500 the banding was negligent. https://www.astrobin.com/kem4rz/?nc=user |
|
11.83
#...
·
1
like
|
|---|
|
Scott - I think the reason that banding is ISO dependent has to do with the way Canon sensors are constructed. There are two components to read noise - the noise from the sensor itself and the noise introduced during analog to digital conversion. The latter is quite significant. The higher the ISO, the more the signal from the sensor is amplified before the A-D conversion is done, and hence the lower the contribution from the A-D conversion noise. Generally for Canon sensors ISO1600 and up seem to be sweet spots, though you lose one stop of DR for every doubling of ISO beyond 1600 for older Canon cameras and 400 for the Mark IV. I have used and continue to use on occasion, a Canon 5D Mark IV (unmodified). I would always get one or two very strong bands, regardless of ISO. Using darks without biases helps get rid of them. |
2.97
#... |
|---|
| Going to depend on a lot of things that effect you on a local level. I always do test runs at different exposures and try to keep my histogram (in APT) to between 40 - 60% at max for most Nebula type targets. I will also increase my exposure as the night gets darker sometimes. I am in a Bortle 8 zone so my non-filtered and broadband filtered exposures are usually between 60 - 120s at 400 ISO. My Narrowband (i.e. L-eNhanced) is between 4 - 5 min at 800 ISO. I tested this over several nights, keeping a record of my histogram settings then stacking with calibration files to see what fit my processing skill and preference best. |
|
2.97
#... |
|---|
| One note note: I use a Canon EOS Ra so I inherently have a higher sensitivity to IR / Ha eith my DSLR over an unmodified camera. |
|
7.61
#...
·
1
like
|
|---|
|
Like you said Arun, I've also found darks to take care of the banding, though I've been using biases as well. Based on another active thread, I'll try with and without biases on my current project. And thanks for the beta as to where it comes from! In any case, if I can get rid of the banding in other ways, I figure the lower ISO and better dynamic range and color sensitivity can't hurt. That said, the first thing I noticed with the IV with just regular landscape photography was how much better the dynamic range was then any other other Canon I'd had, and at all ISO's, so it gives you some latitude to adjust exposure time depending on the dark sky quality, target, passing clouds, wind, etc. Edit: Should have added that my home site is bortle 3 which gives me the option of longer subs. |
0.90
#... |
|---|
| (deleted) |
0.00
#...
·
2
likes
|
|---|
| Also, some reading on the topic https://forums.sharpcap.co.uk/viewtopic.php?t=456 |
0.90
#... |
|---|
| (deleted) |
