Best time to image circumpolar objects?

[BrendanC]

So, I was just going through my list of targets, and landed upon spiral galaxy, IC 342.

The way I've formulated my list is by calculating when they'll be at the meridian at midnight, the idea being that generally, this gives me the most amount of time to image it, with the least amount of air mass, before and after the meridian flip.

I looked at it on Stellarium, got my plan together etc  etc. Then a thought suddenly struck me: IC 342 is circumpolar, so it can really be imaged ANY time of the year. In fact, you could argue that the WORST time to image it is when you need to do a meridian flip right in the middle of the shoot!

I can see that I would still benefit from having less air mass, and I probably won't change things around for the sake of losing 20 minutes doing a flip, but I was just wondering what thoughts other people had on this? Basically, as per the title, what's the best way to approach circumpolar objects? Perhaps they're just nice objects that you can pick and choose any time of the year?

Your thoughts/opinions/comments please!

Thanks, Brendan

Edited December 20, 2020 by BrendanC

[vlaiv]

I would throw some math on that problem

For my location difference between IC342 being highest in the sky and being at "9 o'clock" (west/east of Polaris) is 0.3 magnitudes in extinction. It is about mag25 in surface brightness.

Let's say that I'm imaging it at 1.3"/px with 8" aperture in mag20 skies. What would be the difference in SNR or imaging time for same SNR if we assume 0.3 difference in magnitude?

In above conditions for two hours of exposure we get loss of 24% of SNR due to object position - or equivalent of additional about 1.5h of imaging - or 2h of imaging near meridian flip would be equivalent of 3.5h of imaging when target is at 6/9 o'clock with respect to Polaris.

This is however - not general rule and things change depending on declination of the object and your latitude and brightness of the target as well as light pollution (and as you imagine - all other factors ).

[BrendanC]

Brilliant. Just...  brilliant. 

Thank you again @vlaiv. I hadn't considered the impact on magnitude and therefore SNR and therefore imaging time. So, now I need to find out how to make those calculations to help me make my decisions -  any/all pointers welcome. 

[ollypenrice]

There's theory and there's IC342! What do you need to capture IC342? Exposure time. Squared and then squared again. It is a delightful but very demanding target. The two problems are that 1) it is very faint and 2) the field is infested with stars. Optimizing the altitude and transparency at capture will help control the stars but, in the real world, what you need first is signal. Personally I would get all the signal you can and, while doing that, bone up on how to control stars. I'd look at Starnet++ for removing them and then at some way of putting them back. I do so by pasting the linear image over the de-starred one in Photoshop in blend mode lighten and then gently stretching it till the stars appear.

When I did this target I didn't have Starnet so I don't knw how it would have played out. https://www.astrobin.com/327910/?image_list_page=2&nc=&nce=

Olly

[vlaiv]

27 minutes ago, BrendanC said:

Brilliant. Just...  brilliant. 

Thank you again @vlaiv. I hadn't considered the impact on magnitude and therefore SNR and therefore imaging time. So, now I need to find out how to make those calculations to help me make my decisions -  any/all pointers welcome. 

Impact is not negligible at all.

More than altitude of the target - there is issue of transparency. In fact these two are combined - each air mass attributes to extinction based on AOD - aerosol optical depth as well.

Here is good article on the subject:

https://skyandtelescope.org/astronomy-resources/transparency-and-atmospheric-extinction/

AOD can range between less than 0.1 to something like 0.5. Conversion between the two is 1.086 - so AOD of 0.4 is equal to 0.4344 mag of difference.

When everything adds up, there can be even whole magnitude of difference in brightness of the target - that is x2.512 less signal or about 40% of original signal. Yes - this means that for same conditions you can cut imaging time to at least 40% in great transparency over poor one (effects compound with presence of LP as LP does not seem to be much reduced because of poor transparency - most of it is in bottom layers and gets compounded with poor transparency).

For good forecast of AOD - check here:

https://atmosphere.copernicus.eu/charts/cams/aerosol-forecasts

Red means - more than whole magnitude per air mass!

 

[BrendanC]

Thank you again!