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Why A High ISO With Astrophotography?

G2EWS

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G2EWS

G2EWS

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Hi All,

I have asked this a few times but don't seem to get an answer.

In photography if you are on a tripod then the lowest ISO makes sense as it gives you the highest quality image. It is irrelevant how long the shutter is open as the camera is obviously steady. I have spent a lot on my tripod (Gitzo G3530LS) and head (Manfrotto 405 geared head) so when I take a shot of a landscape the light intensity does not affect my image.

Why is it in astrophotography you talk about 800 ISO and sometimes higher?

I just don't understand the reason for this.

Best regards

Chris

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rowan46

rowan46

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It is analogous to putting a more sensitive film in the camera. in reality its not quite as simple as turning up the iso as high as it will go as increasing iso also introduces increased noise, but especially for scopes whose tracking is not as good, you may want to get as much light in your 30 sec shot as possible.

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G2EWS

G2EWS

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Hi Rowan,

Many thanks for that.

As a photographer I am obviously aware of the implications of using a higher ISO. As mentioned elsewhere I have a Nikon D3X for landscape and portrait work and Nikon D3S for sports, gigs and low light situations.

But if we have a telescope on a mount, with the camera on the end, surely we can set the exposure to as long as we want can't we? Thereby ending up with a better quality image.

Best regards

Chris

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G2EWS

G2EWS

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Hi Billy,

That was a very interesting if deep paper.

It is interesting that both Canon and Nikon have admitted that you should not use your camera beyond the normal ISO parameters. You know that both have an 'extended' range, well this is a pr/advertising gimmick, nothing more.

What intrigues me is the tests that Dr Stark have done seem to show that in the Canon's case ISO 800 has the lowest noise.

I look forward to testing this on the D3X which I think will be my camera of choice once I get my kit. At 25mp I will have some room for error!!

Once again many thanks for the link and my apologies for not spotting it on the other thread.

Best regards

Chris

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Psychobilly

Psychobilly

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No worries Chris...

You also want to get as many "subs" as possible to "statistically" remove the random noise from the camera and if you have enough even the trails from Satellites and planes "magically" disappear...

I tend to shoot most targets at ISO800 but will drop to ISO400 or ISO200 for very bright targets or when i want a shallower image... and will push to ISO1600 if I am chasign the faint stuff thats on the edge of being out of reach of a DSLR...

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G2EWS

G2EWS

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Hi Billy,

That is useful information.

I have just put a link to that article on one of my photography forums. It will be interesting to see their reaction. We are so set on the fact that the lowest ISO is best when you can. Dr Stark appears to be telling us that this is not the case on a Canon anyway.

Sorry to be a newbie, but subs are the amount of shots you take?

So the more you take the more you can stack and the less noise and better clarity you get?

Best regards

Chris

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G2EWS

G2EWS

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Hi Billy,

Thanks. I am enjoying the book and doing my best to take it in. Am aware that these things take a few reads before a newbie has any idea what they mean of course!

Off tomorrow night to the lecture at Seend and then Astrofest on Friday. Hope to see and be able to discuss various aspects. The only thing that bothers me is I don't want to get too many conflicting suggestions. However as I used to teach sales people I think I can see through the bull at Astrofest!!

Oh and all this patience from someone who normally buys on a whim! I have been known to spend a lot of money on a car at first drive even though I was not intending buying at the time!!!

Must be my age!

Best regards

Chris

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AlexB

AlexB

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Hi Chris,

Other than the below...

All electronic imaging devices will build up some amount

of charge over time. This buildup of charge is called

dark current and it is driven significantly by the temperature of the sensor. After all, infra-red photons are still

photons. So, unless you're at absolute zero (-273 C),

your image is going to get brighter and noisier over time

even with the lens-cap on. Twice the exposure duration

will typically yield twice the dark current and the rate at

which it builds up typically doubles for every 6 degrees

Centigrade increase in the temperature of the sensor

...the other consideration is, mounting a D3/D3S/D3X on a scope will cause you balance issues. D3's are heavy monsters, I wouldn't put my other half's D3 on my scope.

If you've got a Nikon D90 / 3000 / 5000 / 7000 or similar, I would advice using that instead. Balance will affect the scopes ability to track as far as I'm aware! Also, due to the D3s full frame sensor, you will lose some of the magnification the crop factor gives you on smaller DSLRs. Obviously, if you were to use the D3X then you would reap the benefits are a huge crisp image.

I'm not an expert but I hope that helps.

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jgs001

jgs001

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Alex, it looks like with the Canon range, the Amp is kept off for long exposures from the 400d upward. You can cause the Amp to be on and active during the long exposure if you use liveview to trigger, but if not, then it's not an issue.

I don't know much about this, having no experience of Nikon, but I've read that at (or it might be above) ISO800, Nikon use a filter (median ??) to help reduce the noise and this affects the star images, and it's even applied to the Nikon raw data.

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G2EWS

G2EWS

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Hi Chris,

Other than the below...

...the other consideration is, mounting a D3/D3S/D3X on a scope will cause you balance issues. D3's are heavy monsters, I wouldn't put my other half's D3 on my scope.

If you've got a Nikon D90 / 3000 / 5000 / 7000 or similar, I would advice using that instead. Balance will affect the scopes ability to track as far as I'm aware! Also, due to the D3s full frame sensor, you will lose some of the magnification the crop factor gives you on smaller DSLRs. Obviously, if you were to use the D3X then you would reap the benefits are a huge crisp image.

Like I said, I'm not an expert, hope that helps.

Hi Alex,

Many thanks for your input in this very confusing matter!

I have chosen a screw thread so this should not a problem with stability and I also understand that the EQ6 should not be a problem in terms of weight. But this is only what I have been told on the forum.

Hi jgs001,

Not really sure about that, but time will tell when I get my kit I suppose!

Best regards

Chris

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G2EWS

G2EWS

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Hi John,

Many thanks anyway for your information.

Not likely to change to Canon from the far superior Nikon alternative.

Oops this is not a photography forum were we have these arguments I mean debates in the joke section all the time!!! ;)

Best regards

Chris

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AlexB

AlexB

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Hi Alex,

Many thanks for your input in this very confusing matter!

I have chosen a screw thread so this should not a problem with stability and I also understand that the EQ6 should not be a problem in terms of weight. But this is only what I have been told on the forum.

Hi again,

You're welcome, but as I said I'm no expert :p.

On the note of balancing... while the EQ6 is more than enough to handle the weight the problem (as I understand it) you will face is getting the balance right. With that amount of weight you will find that it will pull on the declination axis a bit (I think that's right.) What will end up happening is you will have to rotate the tube (if using a Newtonian) to put the focuser in the top most position so the weight pushes down directly. Otherwise you will have some torsion affects or flexure maybe which may or may not affect collimation (depending on quality of the tube) and you may introduce some periodic error in the motion of your axes due to the imbalance.

This is what I've gained from reading various tidbits of information across the net. You may find you have no issue at all! :D

Not likely to change to Canon from the far superior Nikon alternative.

;)

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jgs001

jgs001

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Chris, I think the 100ED is f/9, whereas the 80 and 120 are both f/7.5. That will make a difference... 5 minutes at f/7.5 is 7.5 minutes at f/9 (It took me long enough to work that out ;), but you get the ratio from it. f/9 takes half as much again as f/7.5 for the equivalent exposure) The 80 is a little corker..

As for that Nikon thing I mentioned... Apparently in the newest releases you can turn it off (just seen that in another post).

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G2EWS

G2EWS

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Hi John,

So I may be better going for the 120ED if I want to go better then? Will ask the question at Astrofest on Friday if I can find someone interesting to talk to!!

Thanks for the Nikon information. Don't suppose you have a link to the thread to save me searching for it?

Best regards

Chris

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