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Widefield experiements


GordonCopestake
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I'm making a start getting into astro photography. I've had a play before but it's only recently that i want to actually start taking pictures. Tonight is the first clear night we've had here for a while so took the opportunity to snap some shots to experiment with exposure times. All these images are taken with my D70 and 18-70mm kit lens at various focal lengths and exposures.

This first shot is of Cassiopeia at 18mm nice and wide for 30 seconds:

161206_cass_30sec_18mm.jpg

I quite like this as it's nice and sharp and you cannot see any star trailing at this wide focal length even at 30 seconds. The sky is surprisingly dark too given my NELM 4 skies.

Moving on I set my camera to 70mm and used M45 as a test DSO, nice and bright and in the same sort of area as Cassiopeia. These didnt turn out as well as I would have liked and i'll be looking for tips on how to improve my focus.

First image is M45 at 70mm 10second exposure 100% crop

161206_m45_10sec_70mm.jpg

Nice and dark sky but the stars are JUST showing some trailing even at 10 seconds. I think my focus must have been off as the stars are not at all sharp. I did have the focus locked to infinity but obviously something was wrong.

Next shot is the same but at 30 seconds

161206_m45_30sec_70mm.jpg

As you can see the sky is starting to show orange and star trails are much more obvious. Again stars seem out of focus though which is a pain!

Changing target to Lyra.

First shot 30 seconds at 70mm

161206_lyra_30sec_70mm.jpg

A midnight blue sky in this area with some odd vignetting. Not sure what caused this but I assume it's a function of my (cheap) lens!

Tried a longer exposure to see how much exposure my sky can take at my longest focal length. This shot is at 170 seconds (3 ish minutes)

161206_lyra_170sec_70mm.jpg

You can see the lighter sky and more vignetting. Star trailing is very obvious.

Thought i would see at just how far I could push it. This shot is 900 seconds, about 15 minutes. I'm surprised that i didn't just get a white frame, which is what i was expecting.

161206_lyra_900sec_70mm.jpg

I think that this experiment has shown that exposure times vary not only depending on your location but very much so on which area of the sky you are pointing at. Star trails are more obvious at longer focal lengths and can be seen at 10 seconds with a 70mm lens.

I have purchased a little EQ1 driven mount for the soul purpose of removing star trails and given my lyra shots I would say that a 5 minute exposure would gather lots of light pollution but would still be able to be edited out however it's best to keep to 60 second exposures, which suits me well as I doubt the accuracy of the EQ1 is much to write home about!

More to follow!

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nice trial that Gordon , does show that LP , blimey , this is something i never have to worry about with CCD work and filters, but now i have a canon 350D i suppose i will come up against it , i did try mine when i was in Suffolk ,a lot darker skies there , i just fixed it to a normal tripod set the iso to 1600, focussed on a bright star , then let it go for 45 secs , i was surprised to see no trails ,and not a bad image a s well , it was only one of the plough , but like you i have a steep learning curve with this type of imaging , but look foward to it .

Rog

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