A few wide field questions

[MyRomeo]

Hi

I hope you can help me improve my wide field imaging.

I live in a heavily light polluted town centre so my imaging suffers from light pollution limitations.

Im shooting with an unmodified nikon d3100 with various lens choices including 18-55 kit, 70-300 f4.5-5.6 tamron, nikon 35mm f1.8 and 50mm f1.8 and on a motorised eq3-2 mount. Decent polar alignment but not perfect (long enough for 15s at 300mm with no trails).

For the questions let's assume 35mm lens is used and mount is motorised and aligned correctly. End goal is pin sharp stars with depth and some star colour shining through.

Question 1 - if shooting a load of subs is it better to shoot longer exposures at lower iso (say 20s at iso 200 vs 5s at iso 800)?

Question 2 - similar to above, is it better to shoot with aperture wide open and lower iso or stepped down a few stops with either longer exposure or higher iso?

Here is a sample from last night, 35mm, f1.8, 5s, iso 400, 20 frames, 20 darks, 14 flats stacked and post edited in lightroom 5.2, mount aligned and motor driving it although for 5s subs probably not needed! It had horrible lens flare thanks to me forgetting to remove the uv filter! Mostly edited out tho.

Thank you in advance!

[Physicist13]

Q1: mount tracking/polar aligning are main factors tht determine the exp time, for a given fl lens. Adj the ISO so that for this exp time no point in the image is saturated. (a LP may help if you are LP limited). Then, as always, take as many subs as possible.

Q2: lenses usually perform best when at about 1-2 stops closed down from their max aperature..just due to design/economics. you want widest aperature at which the lens "errrors" are acceptable to you.

P

[dph1nm]

Question 1 - if shooting a load of subs is it better to shoot longer exposures at lower iso (say 20s at iso 200 vs 5s at iso 800)?

Changing ISO doesn't actually  alter the sensitivity of the camera, so in that sense ISO doesn't matter (so 5s at ISO800 will capture the same number of photons as 5s at ISO200). However, with wide field shots saturation might be an issue at higher ISOs, so you might want to avoid these.

NigelM

[frugal]

 It had horrible lens flare thanks to me forgetting to remove the uv filter! Mostly edited out tho.

I thought that it was better to have a UV filter on the lens to reduce star bloat? Have I been doing it wrong?

I always use a UV filter as i thought it was a good idea (and it means I can wipe the dew off without risking damaging the lens glass )

[MyRomeo]

Frugal... I would agree (I think!) however thanks to the close proximity of street lights and the angle from the lens I find that the nikon 35mm prime when fitted with a uv filter suffers horrific flare. I even had the hood fitted too!

I'll hopefully get to repeat the shots of the Big Dipper soon without a uv filter for comparison.