ISO 1600 v ISO 800

[nmoushon]

I was trying to say the benefit of the larger aperture is not wasted, it is going into helping maintain the brightness of the galaxy, even though the galaxy is a bigger scale than when imaged with the 80mm.

I disagree that the galaxy is a point light source, though maybe I'm missing your point? If it was point source, wouldn't that mean it would look the same size when doing visual astronomy no matter what magnification is used?

I'm not sure I understand what you are saying about the amount of light. I would say that the amount of light / number of photons from the galaxy reaching the sensor IS increased with the bigger aperture telescope (assuming the galaxy fits within the sensor). Those extra photons help to maintain image brightness because the photons from the galaxy cover a larger area of the sensor - they are being spread over more sensor pixels, but there are more of the galaxy photons, so each pixel that the galaxy covers ends up the same average brightness as with the 80mm scope.

You could be right about the point light source. I don't know enough about light and physic to properly continue with a discussion on that. I'm an architect not a physicist. If you need an obsy built I'm your man But point out equations on light and pixels I can't do that proper and not going to begin to try. Would love to know exacts on whats what though.

The extra aperture is not wasted in terms of light gathering power. If it wasn't for the scope collecting more light from the galaxy, then the galaxy would be fainter with the increased image scale, just like if you barlow the 80mm scope. Get the smaller aperture scope using a barlow to image the galaxy at the same scale, then the bigger aperture scope wins, that's where its extra light gathering power counts.

True, more aperture would make the object brighter...but only for visual purposes. Putting a barlow on any scope henders the brightness because it increases the FL, which in turn changes your f/ratio. More obvious in smaller aperture scope than smaller ones. That part hold true for both visual and AP. If you were to get the same FL on than 80mm scope to equal that of the 200mm scope it would go from f/5 to f/12.5 so yes it would appear much much dimmer. Both visually and through an image, which would give the appearance that the large aperture scope must make the image brighter. Which in fact it doesn't. The f/ratio is the determinig factor to the 'brightness' of your image. You can test this in a pretty simple way. Take your 80mm and take a single 60sec shot of M42 (At f/5). Then put on a .7x reducer and take another 60sec shot of M42 (f/3.5). Then take both images and open them in PS. Since the scales are not the same zoom out on the first image till you get M42 as close to the same size as the other as possible. Have them open in two windows so you see both simultaniously. If aperture was what let you gather more light then the two images would be the same brightness. If one is brighter than the other then that shows that the f/ratio is what make an image brighter and not aperture.

Again this is just what I've read/learned over the last 10months or so. I have no way of proving anything because I'm just going off of what other people have posted here and other place on the internet.

That's my thoughts anyhow, or am I wrong? Quite a confusing area!! :-o

This is VERY CONFUSING lol I could be wrong too which would suck because then I'd need to rethink my entire set up and 10months of lots of reading. But would be nice if the record could be set straight once and for all.

[Merlin66]

Guys,

I think to continue this "discussion" we should start a new thread...

We seem to have drifted away from the original issue of 800 ISO v's 1600 ISO.

[Ibbo!]

Good topic and discussion

and please take Merlins advice and start new thread please

Steve

[mytola]

DmOMark claims to test dslr sensors, and according to the result they display the 1000D sensor improves in dynamic range, tonal range and color range all the way down to ISO 100. I guess dynamic range is less important for AP since we can stack different exposures, but the two other properties could maybe be noticable.

[mytola]

That shoul be DxOMark

http://www.dxomark.com/index.php/Cameras/Camera-Sensor-Database/Canon/EOS-1000D

[dph1nm]

DmOMark claims to test dslr sensors, and according to the result they display the 1000D sensor improves in dynamic range, tonal range and color range all the way down to ISO 100. I guess dynamic range is less important for AP since we can stack different exposures, but the two other properties could maybe be noticable.

I don't think these quantities (particularly S/N) are measured in a way which is meaningful for astronomy. I suspect their definition of "Grey Level" is the problem.

NigelM

[Ags]

Well, this discussion has gotten under my skin, so I wrote a program to examine the noise and quantization for my camera at different ISO levels.

I conclude that noise does not increase up to ISOs of 1600 on a 1100D, but it does thereafter. The noise gets brighter, but the total number of noise pixels does not increase. (Hot pixels do increase marginally).

I also conclude that higher ISOs do quantize weak signal better, and contrary to what I have read this applies even above ISO 1600.

Here is the thread I created for my app:

[Gina]

I have always wondered if the sensor gets hotter is the ISO is higher... anybody know?

It doesn't from my my tests.

[Gina]

I also conclude that higher ISOs do quantize weak signal better, and contrary to what I have read this applies even above ISO 1600.

Yes, that's my findings too. In this respect ISO6400 is better than 3200 which is better than 1600. For very faint images and/or NB I find ISO 6400 useful. You do need a large number of subs to get the noise down but using a cooled 1100D I find I can drag out some very faint nebulosity.