Open Cluster NGC3766 - I think

[JohnH]

I was testing out GuideMaster again last night and with some pleasing results too I must say. I am limited still by the short exposure support for the LPI to tracking on stars of Mag 6 or brighter approx however I found this pretty object while hunting for trackable targets. Exposures are 2 minutes guided at ISO400. Processed in IP and PS.

A couple of qusetions have occured to me as I get deeper into this:

1. What is the best ISO setting to use for DSOs - ISO 1600 would appear to be best - but I see folks using 800, 400 even talk of 200. What advantage do the lower ISOs bring?

2. I appear to have more pronounced vignetting on the longer, lower ISO shots, is that normal - so I need to take flats now something I have not done yet. To do this I should shoot a clear dawn sky at ISO100 and a fast exposure and greyscale the result, yes?

3. Darks and flats must be taken (RAW or JPEG) to match the lights ?

[iceman (Mike)]

Very pretty! Very nice!

[Striker (Tony)]

John,

nice image.

Regarding flats...I was of the knowledge that you take the flats with the same ISO your imaging using a long enough exposure to get around halfway on the histogram.....also I use a white pillow case over the scope....dont know about this grey scale...I'm sure their is other methods yours may be 1.

No such thing regarding Best ISO...it depends on your target....some are using as low as 200 but most between 400-800...if your only taking short 30 second exposures you will find the higher ISO is better catching more light but the longer you go the lower the ISO the better.

I am between 400-800 ISO atm.

Take all your images in raws including Flats, Darks, Bias frames...then you can combine them in IP and make a master file.

Hope this help John.

[Itchy]

Hi John

Originally Posted by JohnH

1. What is the best ISO setting to use for DSOs - ISO 1600 would appear to be best - but I see folks using 800, 400 even talk of 200. What advantage do the lower ISOs bring?

There has been much discussion about the optimal ISO. The definitive work was done by Terry Lovejoy from Queensland.
http://www.pbase.com/terrylovejoy/dy...itivity_curves

As you will see it is a trade off between sensitivty and dynamic range.

2. I appear to have more pronounced vignetting on the longer, lower ISO shots, is that normal - so I need to take flats now something I have not done yet. To do this I should shoot a clear dawn sky at ISO100 and a fast exposure and greyscale the result, yes?

Yes that is one method. Lower ISO's in the flats reduce noise. You want as little noise as possible in your flats so as to not "add" more noise to your light frames. I usually use around 20 frames to make a master flat. The greyscaling will depend on what version of IP you have. Earlier versions required you to greyscale the flat before calibration, but the latest version (2.75) does this for you automatically. I take my flats indoors in daylight with a white cloth over the objective and the camera on P mode.

3. Darks and flats must be taken (RAW or JPEG) to match the lights ?

As Striker said (gosh he is right again ), take everything in RAW. Don't even think of JPEG.

Cheers

[JohnH]

Great wisdom from the forum as always. Thanks for the tips and pointers.