Exposure Times

[jtlandreneau]

I was wondering if there is a "Rule of Thumb" for single shot exposure times (assuming a static ISO, I usually default to 800) based on DSO type/magnitude/sky position/anything at all. I usually limit my shots to 120 seconds, have played around with as high as 10 minutes. It may be that I am approaching the limit of my little 130mm newt, but I feel like longer exposures have resulted in giant stars instead of pinpoints. Thanks for any input!

[greyhaven]

Can you give more information about your imaging equipment. Type of camera, mount, are you using an auto guiding set up ?  More info would be required to give a useful answer.

Grey

[jtlandreneau]

Sure thing!

Canon T3i DSLR

AstronomersWithoutBorders 130mm f/5 newtonian reflector

Orion ST80 finder

Orion SSAG

Celestron CG-4 with tracking motors

I use a polar finderscope in the CG-4 to get polar alignment, use a bahtinov mask for focusing. To get enough in-focus I have a 2x barlow lens screwed into a T-adapter, attached to the DSLR at prime focus

[jtlandreneau]

Trying M51 tonight... this will be my first galaxy attempt. any suggestions?

[ollypenrice]

The Barlow is bad news, slowing your exposures by 4x. It will also be optically bad news as well. Finding some other way of coming to focus would be far better. This might involve moving the mirror in the tube slightly.

Olly

[pete_l]

I was wondering if there is a "Rule of Thumb" for single shot exposure times (assuming a static ISO, I usually default to 800) based on DSO type/magnitude/sky position/anything at all. I usually limit my shots to 120 seconds, have played around with as high as 10 minutes. It may be that I am approaching the limit of my little 130mm newt, but I feel like longer exposures have resulted in giant stars instead of pinpoints. Thanks for any input!

It sounds like you have 2 separate questions here:

How long should I image for? and

How to reduce (or minimise) star-bloat?

In answer to the first, this can be limited by the amount of background light pollution in your location. If that is significant I'd suggest keeping your exposure times down so that your camera's histogram is mostly in the left-most third of the graph. If you don't suffer from excessive LP. you should be able to image for a lot longer: 5 or 10 minutes doesn't seem unreasonable. The figure would depend on the likelyhood of spoiling a sub-frame due to tracking issues or possibly gusts of wind affecting the positioning.

Which leads on to the second question. There are software techniques for reducing star-bloat. However, it might be worth checking some other factors as well. Apart from the other suggestions here: ensure focus is spot-on, take as much glass out of the optical path as you can, I'd also suggest checking the state of your mirror - if it needs a clean (unlikely) that could be exacerbating the problem.  I'd ask: what is your "seeing" like? If you suffer from turbulent air <!> or are imaging across rooftops or concrete that will disperse images - and the longer you image, the more dispersed they become.

[jtlandreneau]

Thanks for the input! I'm going to try 5 minute subs on m16 tonight. Tried 2 minute ones on m51 last night, first time stacking hasn't worked out for me.

I'm relatively sheltered from air gusts, but light pollution is definitely a concern