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Old 02-09-2019, 04:20 PM
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gregbradley is offline
Join Date: Feb 2006
Location: Sydney
Posts: 15,517
Originally Posted by Astrofriend View Post
First light with Canon 300 mm lens f/4.

At last we got some darkness so I could test my "new" Canon 300 mm f/4 lens. I used my Star Adventurer mount and didn't had very big expectations about the result. But it worked very well, this is nice, it's like a small telescope. Easy to bring with you on travel. No problem to adapt field flatter. Built in focus motor that I can remote control from my smartphone.

See here:

It was a lot of things that went wrong this night, but I got 44 minutes of exposures. Next time, better polar align and better focus.

At the bright stars there are some blue bleeding, I have seen that on other 300 mm lenses too. Maybe less when I focus it better.

Nice shot Lars. You can remove the blue halos using Lightroom or Photoshop Camera RAW filter. Both of these programs have sliders to remove chromatic aberration from the images and those blue halos will go. You have to be precise though on setting which colour you want removed otherwise it can weaken other star colours.

300mm F4 is a good formula. The other possibility is get stop down ring to screw onto your lens to reduce it down to say F5 or F5.6. Then these aberrations tend to disappear, stars will focus to finer points and less fuzzy.

I did this with my Pentax 645 F4 EDIF lens and it made the images better.
You are doing long exposures anyway so losing a stop of light is not that big a deal. F5.6 is still plenty fast.

300mm though is hard to locate objects you want to image though with a tracker. I piggyback my 250mm or 300mm on a scope on a nice mount that has accurate go-tos. I spent a fair bit of time trying to locate even bright objects without go-to and it wasn't a lot of fun!


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