6 ms·

> don't you need a certain "minimum" time to capture the faintest of stars, because their signal otherwise wouldn't rise above the noise floor? I'm not an expe

by jxcl 9y ago

> don't you need a certain "minimum" time to capture the faintest of stars, because their signal otherwise wouldn't rise above the noise floor? I'm not an expert or even very knowledgeable on image stacking, but I'll share my experience. I've taken a stacked picture of the Orion constellation. 15 exposures at 5 seconds each, and combined them to get this photo: https://www.flickr.com/photos/abliskovsky/31756623694/ https://www.flickr.com/photos/abliskovsky/31756623694/ In none of the individual exposures is the Orion Nebula visible, but in this photo it's pretty clear. So in the end I'm fairly sure that your statement is true, but the signal required is very, very faint.

craftyguy 9y ago
Do you know of any good resources for "getting into" image stacking for a complete newbie? 2 years ago I tried stacking images I took of Jupiter in my 16" dob (taken with a Canon DSLR), but it was terrible and I'm not confident I used the app I found (I think it was AstroStack..) for image stacking correctly.
- jxcl 9y ago
  I don't have any particular advice. I searched the web for astrophotography stacking and used some software I don't remember the name of to do it. Sorry I can't be more helpful!
- jpk 9y ago
  RegiStax is popular for solar and planetary imaging. I've used it several times on H-alpha solar photography. It has a fairly well defined workflow and there are lots of video tutorials on how to use it.
  - craftyguy 9y ago
    Awesome, I will check it out. Thanks! Also looks like it runs under Wine on Linux, which is good news for me!
vanderZwan 9y ago
Nice result! Ok, just for fun: I'll try to make a quick back-of-an-envelope calculation and see if we can make an educated guess... Let's use the old Sunny 16 rule, and its equivalent for moonlight, which also gives us an estimate for difference in light between day and night[0]: > Daylight (sunny day): Correct exposure for this case is given by the Sunny 16 Rule: 1 / ISO [seconds] @ f/16. So at ISO 100, a typical exposure is 1/125 @ f/16 (that's 1/3 stop less exposure than the rule calls for, but it's the closest standard shutter speed.) > Full moon: to get an equivalent exposure at night with a full moon, the rule is: 1 / ISO [days] @ f/4. That's right, DAYS. For ISO 100, that means 1/100 of a day @ f/4. > That works out to something like 14.7 minutes; I just round it up to 15 minutes. So 15 minutes @ f/4, or FOUR HOURS @ f/16. > That's 21 stops difference from sunlight to moonlight. So on a sunny day you need f/16, 100 ISO, shutter speed 1/100. flickr says you shot at f/5.6, unknown ISO (I'll go with quasi-conservative 6400 ISO), 5 seconds. Aperture increased by 1.5 stops ISO increase by 6 stops 500x longer shutter time => log2(500) -> 2^9 = 512 => bit less than 9 stops. So that's between 16-17 stops of extra light sensitivity That's five stops short of the 21 stops mentioned above. However, we're not aiming for a properly exposed single picture, we're aiming for minimum exposure to capture a signal! Even the first digital SLRs had a dynamic range of five to six stops, so theoretically should be able to capture something in the shadow regions. Nowadays ten stops is about the norm I believe, with some of the better cameras going up to twelve. So that should clearly be above the minimum signal required Also, the moonlight-rule above is very conservative: it's about exposing a scene in moonlight, not about capturing stars. Stars are light sources themselves, and probably brighter than objects reflecting moon light, even in a full moon. Also, the rule-of-thumb was based on film, which actually suffered from something called "reciprocity failure"[1] so it might exaggerate the required exposure. So yes, this should work out fine, and as your picture shows it clearly does :) [0] https://www.flickr.com/groups/11947580@N00/discuss/72157620787125918/ https://www.flickr.com/groups/11947580@N00/discuss/721576207... [1] https://en.wikipedia.org/wiki/Reciprocity_(photography)#Reciprocity_failure https://en.wikipedia.org/wiki/Reciprocity_(photography)#Reci...

by jxcl 9y ago

craftyguy 9y ago

Do you know of any good resources for "getting into" image stacking for a complete newbie? 2 years ago I tried stacking images I took of Jupiter in my 16" dob (taken with a Canon DSLR), but it was terrible and I'm not confident I used the app I found (I think it was AstroStack..) for image stacking correctly.

jxcl 9y ago
I don't have any particular advice. I searched the web for astrophotography stacking and used some software I don't remember the name of to do it. Sorry I can't be more helpful!
jpk 9y ago
RegiStax is popular for solar and planetary imaging. I've used it several times on H-alpha solar photography. It has a fairly well defined workflow and there are lots of video tutorials on how to use it.
- craftyguy 9y ago
  Awesome, I will check it out. Thanks! Also looks like it runs under Wine on Linux, which is good news for me!

vanderZwan 9y ago

Nice result! Ok, just for fun: I'll try to make a quick back-of-an-envelope calculation and see if we can make an educated guess... Let's use the old Sunny 16 rule, and its equivalent for moonlight, which also gives us an estimate for difference in light between day and night[0]: > Daylight (sunny day): Correct exposure for this case is given by the Sunny 16 Rule: 1 / ISO [seconds] @ f/16. So at ISO 100, a typical exposure is 1/125 @ f/16 (that's 1/3 stop less exposure than the rule calls for, but it's the closest standard shutter speed.) > Full moon: to get an equivalent exposure at night with a full moon, the rule is: 1 / ISO [days] @ f/4. That's right, DAYS. For ISO 100, that means 1/100 of a day @ f/4. > That works out to something like 14.7 minutes; I just round it up to 15 minutes. So 15 minutes @ f/4, or FOUR HOURS @ f/16. > That's 21 stops difference from sunlight to moonlight. So on a sunny day you need f/16, 100 ISO, shutter speed 1/100. flickr says you shot at f/5.6, unknown ISO (I'll go with quasi-conservative 6400 ISO), 5 seconds. Aperture increased by 1.5 stops ISO increase by 6 stops 500x longer shutter time => log2(500) -> 2^9 = 512 => bit less than 9 stops. So that's between 16-17 stops of extra light sensitivity That's five stops short of the 21 stops mentioned above. However, we're not aiming for a properly exposed single picture, we're aiming for minimum exposure to capture a signal! Even the first digital SLRs had a dynamic range of five to six stops, so theoretically should be able to capture something in the shadow regions. Nowadays ten stops is about the norm I believe, with some of the better cameras going up to twelve. So that should clearly be above the minimum signal required Also, the moonlight-rule above is very conservative: it's about exposing a scene in moonlight, not about capturing stars. Stars are light sources themselves, and probably brighter than objects reflecting moon light, even in a full moon. Also, the rule-of-thumb was based on film, which actually suffered from something called "reciprocity failure"[1] so it might exaggerate the required exposure. So yes, this should work out fine, and as your picture shows it clearly does :) [0] https://www.flickr.com/groups/11947580@N00/discuss/72157620787125918/ https://www.flickr.com/groups/11947580@N00/discuss/721576207... [1] https://en.wikipedia.org/wiki/Reciprocity_(photography)#Reciprocity_failure https://en.wikipedia.org/wiki/Reciprocity_(photography)#Reci...