Quick question about Barlows

[SmokeyJoe]

What are the ramifications of using a Barlow when imaging with a Dslr, this question came about because I was trying to find some way of getting my 1.25" moon filter to fit somewhere in my imaging train and the only way I could do it is to attach it to my 2x Barlow which will connect to my T ring. 

Originally I was going to image the moon, I had forgotten about my Barlow since concentrating on AP, and now I wonder what effect it would have when imaging smaller galaxies and nebula. 

The reason I ask is I can't recall reading anything about using a Barlow and a dslr, probably because the answer is so obvious.  Going to try tonight anyway if the cloud ever goes. 

I'm using a SW 80ED and a Canon 450d. 

[Louis D]

A Barlow will increase magnification and exposure time.  It will also spread out the image circle, decreasing vignetting relative to no Barlow, assuming you use a 2" Barlow.  I'm not sure about vignetting with a 1.25" Barlow and a DSLR.  It would depend on the sensor size.

I'm not sure why you would want to put a moon filter in front of your DSLR.  Just use a higher shutter speed or lower ISO instead.

[SmokeyJoe]

3 hours ago, Louis D said:

I'm not sure why you would want to put a moon filter in front of your DSLR.  Just use a higher shutter speed or lower ISO instead.

Just playing about, took a couple of pics at very fast shutter speed of the quarter moon last month at the end of a DSO session and it was very over exposed on the edge of the sunlit half.  Didn't think about dropping down the ISO, not used to proper cameras. 

[Louis D]

56 minutes ago, SmokeyJoe said:

Just playing about, took a couple of pics at very fast shutter speed of the quarter moon last month at the end of a DSO session and it was very over exposed on the edge of the sunlit half.  Didn't think about dropping down the ISO, not used to proper cameras. 

The full moon is no brighter than f/16 at 1/125s at ISO 100.  Let's say you're shooting with an f/5.6 telescope.  That would be 3 stops faster which would necessitate either ISO 12.5 or 1/1000s shutter speed, or some combination of the two.  Perhaps ISO 50 and 1/500s.  It depends on the ISO settings available in your DSLR.

High shutter speeds are your friend in this case because they will mask mount vibrations and reduce atmospheric blurring.  In fact, some astrophotographers will take hundreds to thousands of video frames, select the best, and stack them with specialized post processing software to create ultra sharp images.

To get the earthshine illuminated half will require a vastly slower shutter speed and/or higher ISO.  You would then need to composite the two images (dark and bright parts) in post processing.

[AstroTim]

Depending upon which barlow you have, you might be able to unscrew the lens and swap in the filter. That way you wouldn’t get the magnification, but would still get an increase in light path due to the barlow housing.