Barlow question.....please

[Stu Wilson]

If it stays clear I've got crystal clear skies and for the 1st time I'm going to try m82 on its own with 2xBarlow to fill up the frame.

A) will I achieve much?

B do I need to double exposure times?

OR

should I stick with normal setup and take a hell of alot of subs?

Cheers

 

Stu

 

 

[Pompey Monkey]

The sky background will keep the same brightness. I'd <suggest> keeping your default exposure times. Maybe someone else has better info/experience on this?

Also, with a 2xBarlow, you'll need 4 times the total exposure to get the same number of target photons per unit area on your sensor.

But, to quote Harry Hill: "There's only one way to find out!"

[vlaiv]

Well, here it goes:

1. It would probably be beneficial to increase your exposure as background will drop by x4 if barlow is x2. How much? That depends how close are you now to ideal sub length. If you are spot on - use x4 longer subs.

This is not necessary and in principle you can even keep your current exposure length but your final SNR will suffer a bit.

2. Yes, you will need x4 total exposure to compensate for x2 barlow to get same SNR.

3. You probably don't want to do this barlow thing.

Most people are oversampling as is (very few are under sampled or properly sampled) and adding barlow will only create lower SNR per sub and in total (for same imaging time - hence recommendation above to increase both) without any real resolution benefit.

What is your current sampling rate and what is your average FWHM?

If going by equipment in your signature 200P and QHY8L - then there is some sense in adding x2 barlow if your guiding is good. But it will require special processing and you won't get much more close up.

You are probably using linear debayering at the moment and barlow will be useful only if you use super pixel mode - which will result in same sampling rate.

I'm tempted to say - don't bother with barlow.

[Stu Wilson]

OK so far so good. I understand that much. Cheers 🙂