High iso, or Longer exposure

[Daniel]

Right now, I have a predicament, until I get my guide scope sorted im not really able to expose any longer than 30s otherwise i tend to get some trailing on the stars, Because of this Im shooting as high iso as I can (1600 and sometimes 3200) what im wondering is would I be better off shooting at a lower iso for stacking and therefore getting rid of as much noise as possible, or can I keep the high iso's and get rid of that in stacking?

Oh, and one other thing, is this weather really good for the scope, last night i had frost on the scope and mount and ice crystals forming on the lens!!!

Thanks

Daniel
:O)

[Mike]

Daniel,

A low an ISO as possible will be best. However, you will need longer exposure lengths to get the best signal to noise ratio. Have you tried doing a really accurate polar alignment? I have had up to 4 minutes unguided by spending half an hour on drift aligning. If your mount has GoTo there are easier techniques.

Frost and ice won't do any damage to your scope at all. We've all had completely iced up scopes at deep sky camp. Just make sure when you pack it away you leave the caps off and the box lid open to allow it to dry off.

[Rick]

Oh, and one other thing, is this weather really good for the scope, last night i had frost on the scope and mount and ice crystals forming on the lens!!!

Just take care to let the moisture dry off any optical surfaces naturally, preferably somewhere where dust won't settle on them.

[Daniel]

Thanks guy's, Must confess I've never done a really accurate polar alignment, Just ordered an illuminated eyepiece, so I shall try that as soon as i can. I've got a goto mount but tried the polar alignment routine once with it and it sent the mount completely off track, shall have another go with that tonight weather permitting.

Good to hear the equipment stands up to the cold, was terrified i'd start hearing cracking noises as i brought the scope in from the cold!

[Mike]

Daniel - try the iterative method to get accurate polar alignment using your goto.

[MarkS]

Daniel,

Even if your mount is perfectly polar aligned you will still see some smearing in the equatorial direction because of periodic error in the mount.  On my setup this becomes objectionable after 30 seconds but Mike has managed 4 minutes. The answer is to try it and see.  TonyG also got a wonderful picture of the Andromeda Galaxy using 5 minutes unguided.

On the ISO question I have a different viewpoint to Mike.  The CCD on a camera is simply collecting and counting photons.  All the ISO does is to change how this is converted into a 12bit or 16bit pixel value in your image (the technical term for this conversion is gain).  My Canon at ISO 800 gives more or less 1 digital unit per photon.

Here's a simplistic example using my camera to illustrate the concept:
Suppose successive pixels collect  1, 2, 3, 4, 5, 6, 7, 8, 9, 10 photons.

ISO 3200 multiplies the photon count by 4 so I would see the following pixel values in the final image:
4, 8, 12, 16, 20, 24, 28, 32, 36, 40

ISO 1600 multiplies by 2 so I would see the following pixel values:
2, 4, 6, 8, 10, 12, 14, 16, 18, 20

ISO 800 multiplies by 1 so I would see the following pixel values:
1, 2, 3, 4, 5, 6, 7, 8, 9, 10

ISO 400 multiplies by 0.5:
0, 1, 1, 2, 2, 3, 3, 4, 4, 5

ISO 200 multiplies by 0.25:
0, 0, 0, 1, 1, 1, 1, 2, 2, 2

ISO 100 multiplies by 0.125:
0, 0, 0, 0, 0, 0, 0, 1, 1, 1

So, at ISO 100, lots of different photon counts get converted into the same pixel value e.g. any photon count in the range 16-23 will be converted into a pixel count of 2.  This loss in granularity can be a big problem for very faint objects.  On the other hand ISO 1600 & 3200 are very wasteful of space (in the example above some numbers are never used as pixel values because you never get fractions of a photon!).

On the other hand, ISO 100 will reduce the problem of star saturating because it keeps the pixel values low.

So the decision of which ISO to use depends on what you are trying to achieve.
If you want to prevent star saturation and you don't care about faint objects then use the lowest ISO possible.
If you care about faint objects instead (at the expense of star saturation) then use an ISO that give a gain of approx 1 - this is ISO 800 for my Canon.

How do you work out the gain for your camera? Well, if you're lucky then someone has already done it for your camera and put the results on the web.  Otherwise ISO 400 or 800 normally does the trick for faint objects.

In practice, CCD read noise is an additional factor that obscures the direct relationship illustrated above, but the general principle still holds.

[Daniel]

Thanks guy's, for all the help, Just processed an image of andromeda and the black eye galaxy I did last night, and your messages just answered why the image came out looking the way it did, I did them both on 20s ISO 3200 (which i don't like using too much) and they showed more noise than image even though I took 200 of each.

As soon as this fog clears I'm definetly getting out there to try a proper polar alignment, and I'll try a much longer exposure on a lower ISO.

Mark, sounds like I should do some experiments on ISO 400 and 800

One other problem Im getting here is obviously light pollution, right now, i know there's not much i can do with it seeing as the moon is near full, but i was also wondering how far you let your images go before there over exposed, do you ever take images that are that murky brown colour and clean up in photoshop, or would you always keep the background relatively dark?


Thanks again guy's Im really learning loads here!

Daniel
:O)

[JohnP]

Daniel,

Any chance you could post a few shots so we can take a look? We may be able to help with post processing etc. I find that with my DSLR 800 iso is optimum (EOS300D).

John

[Fay]

Daniel,

Easiest way to polar align the mount is a free programme called POLARFINDER by Jason Dale. A couple of us use this method.

Fay 

[Mike]

That will only give an approximate alignment though Fay. Good enough for guided images, but not for unguided. For unguided you would need to use either the drift method or the iterative method (if you haev goto).

My 4 minutes was also using PEC by the way.

[Fay]

I forgot he is not guiding at the moment

[Daniel]

Well, if all goes to plan, i will be guiding by tonight (well at least be set up for it, im not holding my breath with this weather the way it is at the moment)

Does that mean that if im guiding my polar alignment doesn't have to be quite as accurate to get the same results?

[Mike]

For guiding your polar alignmetn will need to be reasonably accurate and the mroe accurate the less your mount will need guiding corrections. However, it isn't as critical and to be honest I usually only use the mounts polar scope and then use the Kochab's Clock method. This gives a good enough alignment for guiding and takes 30 seconds.

[Daniel]

Hi Mike, Thanks for the advice, I've been looking up the Kochab clock method of aligning, and it definetly seems the most user friendly to me. I've got a polar alignment scope so the offset shouldn't be a problem either.

One thing I am wondering is how do i sight Kochab and Polaris together? the polar alignment scope FOV is too narrow to show both and Im not entirely sure how I can be positive my Declination is centered correctly if i was to line them up in my scope viewfinder (i've just bought an illuminated reticule)

[Mike]

You use both eyes. I use my left eye to sight Kochab and the right to look down the polar scope at Polaris (or the other way round depending on what  time of year it is).