short or long exposures

[malc-c]

Im out in the garden with the canon 400d attached to my f5 sw200p - skies are ok but typically light polluted - is it better to do say 100 x 60 second exposures at 800 iso, and then stack them, or 50 x 120 second exposues and let dss do its work. Also how mand darks woukd be ideal to overcom the lp

a quick reply would be appreciated

malc

[NickK]

I may be totally wrong but: darks show the sensor noise as the image is basically the scope with the lens cap on.

If the LP is present in the image, it will be constant as a long term noise as a signal rather than changing random noise between each frame.

Unless you create a gradient then you're looking at filters.

[malc-c]

nick, thanks for the reply. I have the camera dirctly connected to the scope via a tee mount - what sort of lp filter could i use. the only lp filter i have is for 1.25" that fits eyepieces.

[NickK]

nick, thanks for the reply. I have the camera dirctly connected to the scope via a tee mount - what sort of lp filter could i use. the only lp filter i have is for 1.25" that fits eyepieces.

Filtering out LP (sodium wavelengths), you're looking at something like the Baader skyglow filter.

I, correct or not, stack the skyglow and IR/UV filters onto the end of the 1.25" adaptor piece as there's a thread inside the nose piece of the adaptor that can take the filter thread.

[malc-c]

just tried using the 2" to 1.25" adapter with the lp filter attached, but this moves the camera around an inch and a half further out and i cant then get focus as i run out of travel. need to come up with some other filter arrangement. would a 2" filter screw directly to the 2" ring that the t ring then attaches to ?

[RogerTheDodger]

I haven't got a dslr yet, but this is what I'm intending to use to get a 2" LP filter on it Adaptors - FLO 2-inch T mount camera adapter

[RikM]

I use a 2" MaxDSLR adaptor instead of the T-ring and the thread on the scope. This is basically a T-ring with the 2" nosepiece attached and is threaded to accept 2" filters. I use a SkyWatcher LPR filter, which is similar to (but cheaper than) the Baader neodinium filter. This has made a big difference to my images.

As Nick says, darks don't have anything to do with light pollution. They only help to mask the electronic noise of the sensor. Add filters or move to somewhere with no LP are your only options.

[RikM]

It sounds wrong, but you need to use longer subs with light pollution. You can get rid of a lot of the orangey background in the processing and with the longer subs you will have captured more image signal.

[malc-c]

Thanks guys for your input.

Rik, what length of subs would you suggest. 3, 4, 5 mins ??

[RikM]

Check the histogram display on your camera and go for as long as you can without the peak reaching the right hand end of the range. I think having the big peak 1/2 - 2/3 of the way along is what is recommended, but I may well be mistaken, sorry. Since my mount is unguided, I can only get a maximum of 2 minutes on a good night, 90sec typically, before the stars start trailing, so I am limited by that.