Exposure-How do you determine it?

[NorthernLight (Max)]

well its actually on the right- I mix that always up...

[Alchemy (Clive)]

Originally Posted by NorthernLight

Hi Paul,

yes I am using the Autosave.tif-file. So I guess I am doing something wrong, am I? what is the difference between the autosave and the final saved tiff?

left.

I think the autosave tif is a 32 bit file and the tif save function is a 16 bit file. Suffice to say I have only ever used the 16 bit file....... On thenleft of the screen there is a save image button there somewhere, that will give you the saved tif, my version of Photoshop only works with a 16 bit file.

[[1ponders] (Paul)]

I was just checking to see that you were using the autosave.

I convert the 32 bit file in PS to 16 bit and work from there.

[p1taylor]

Hi Max
that looks better,

Hi Clive

when I go to max-value all except very britte stars disappear or am I doing some thing wrong.

peter

[Alchemy (Clive)]

Originally Posted by p1taylor

Hi Clive

when I go to max-value all except very britte stars disappear or am I doing some thing wrong.

peter

No that's what happens when you remove the "factory settings" stretch that is applied, I personally just use curves in Photoshop to get it to a point where I can work with it, at least where the image becomes faintly visible, then do a color balance and work from there.

There are many different ways to work with images, I'm sure others do it differently.

[p1taylor]

that definitely looks better.

peter

[bmitchell82 (Brendan)]

You will always get a good dose of color noise when your talking about DSLR's this is due to the Thermal noise exciting the CMOS sensor.

The best way to control it is do 1 of 3 things i know of.

1. Use your camera in temps closer to zero
2. Use post processing noise reduction algorithims (Noiseware, Noise Ninja ect) or Gaussian blur a copy of the image and re introduce it back to the main image though Layer masking. this will be one of the hardest to implement! but can do well for dark areas with little to no details.
3. Get yourself a Cooled CCD/DSLR

Good crack of the whip though!

[Octane (Humayun)]

A good guide to exposure on bright extended objects is to try and expose for a histogram that peaks somewhere between 1/3rd to 2/3rds across the x-axis.

Given that you use a fast scope, and, a relatively sensitive imaging device, somewhere between 5-10 minutes would be ideal.

When I imaged with the 350D, I never went beyond 5-minute exposures at ISO-400. When I got the 40D, I pushed to 10-minutes at a slow f/7.5 and ISO-400, ocassionally flirting with ISO-800 on cold nights

Take as many exposures as you can; aim for an SNR of around 4-5. SNR is achieved by taking the square root of the number of exposures. Make sure you take dark frames. Dark frames with a DSLR are an absolute necessity. Again, aim for an SNR of 4-or-so. I have had success in taking a bunch of darks before I begin, during a meridian flip (or, half-way through my planned run of images), and, afterwards. Combining them will give you a good master dark as it should cover the variances in exposures. You don't need bias/offset frames as the bias is contained within the dark, anyway. Bias frames are only required when scaling non-matching (exposure length) dark frames. I think this is a bad idea.

You should also flat frame your images, too. Use the 1/3rd- to 1/2-way across the x-axis histogram peak rule and you should be right. Ensure you take flat darks, too. Again, SNR of 4-5 for both.

If you follow these simple rules for calibration, your images will shine.

H

[Bassnut (Fred)]

Auckland?, nah, the wobbly CHCH effect will wreak havoc. Emigrate.

[NorthernLight (Max)]

Thanks Brendan and Humayun for your advice. I´ll start working on it as soon as the weather permits it. But one question:
a flat is what can be done with an evenly illuminated t-shirt. is a dark flat a t-shirt-without-light flat? and what is the difference to a normal dark frame?

Hi Fred,

what is the CHCH-effect? I just moved here two months ago from Melbourne and am quite happy with the backyard in terms light pollution and free view to south and everywhere.

Cheers

[Octane (Humayun)]

Hi mate,

You are correct.

Think of your flat frames like a light frame. You're exposing a subject. Therefore, think of your flat dark frame, as you would a normal dark frame.

So, say, your flat frame (shot at your lowest available ISO -- typically ISO-100) requires an exposure of 1/30th of a second to give a peak histogram on the x-axis of about 1/3rd- to 1/2-way across, then, simply cover your telescope with its dust cover, and take 1/30th of a second exposures as a dark frame (matched to your flat lights). Easy peasy.

Any other questions, feel free to fire away.

H

[p1taylor]

Am I right do you have to do normal dark at say 300 secs and flat dark at same as light frames, or am wrong.

peter..

[stanlite (Grady)]

normal dark is meant to be the same ISO and exposure time as your lights. And your flat dark is the Same ISO and exposure time as your Flats. All should have the same ISO. Well this is what i have been told.

[Octane (Humayun)]

Peter,

Example:

Light frames = 300s @ ISO-400;
Dark frames = length of light frames = 300s @ ISO-400;
Flat light frames = 1/30s @ lowest ISO capable; typically ISO-100;
Flat dark frames = length of flat light frames = 1/30s @ lowest ISO capable; typically ISO-100.

H

[p1taylor]

Hi H Another thing is Bias frames and Flat Bias frames do you do these as fast as possible, do you need all these sets of frames or only sum of them.

peter

[Alchemy (Clive)]

Originally Posted by p1taylor

Hi H Another thing is Bias frames and Flat Bias frames do you do these as fast as possible, do you need all these sets of frames or only sum of them.

peter

Bias frames are the fastest frames you can shoot with the cap on, I just load a bunch of these into deep sky stacker..... I presume other programs are the same.

Flat bias ?????? Never done any as they would be the same as a bias. ( cap is still on)

[Bassnut (Fred)]

Originally Posted by NorthernLight

what is the CHCH-effect?
Cheers

As in Christchurch , get it?

[Octane (Humayun)]

Peter,

Please read my reply a few posts above where I mention I don't use bias frames as the bias is contained within the dark already.

H

[NorthernLight (Max)]

Alright, thanks Humayun for that explanation. I hadn´t heard of the dark flats before. But now it makes sense.

CHCH - ok, I get it. I thought it was yet another obstacle like a naturaly but only in NZ occuring permanent stratospheric turbulence or whatever....

No, alll good so far in Auckland-didn´t even feel the slightest tremor.

[p1taylor]

Thank you very much for your patience, I think the Penny as dropped,it makes sens now.

peter