500mm Mirror Lens for Astro?

[JimFox]

Since we have so much excitement about pure Astro photography, I figured we might as well keep any gear related questions in this too. At least for now.

So my question first off is, how well would a 500mm mirror lens work for Astro photography? I have seen some shots of the moon with it and they look great. For pure landscape work the doughnut shaped bokeh wouldn’t work for me, but if it can give me a tighter shot of the stars, maybe it’s a good beginner lens?

I don’t have any long lenses, hence why I am thinking about it. I can’t spend a lot, so I was hoping I could get a workable starting lens.

 

[Colorado CJ]

I've had one in the past. The problem with them is they are an F8 lens. So without a tracking mount, you really only could use them on the moon.

They do OK as terrestrial lenses, but as you said, they have a pretty busy bokeh. Here's an example.

These were shot with arguably the best 500mm mirror lens made, the Soviet 3M-5A-MC 500mm F8 lens.

24-Nov-2012-5 by Andrew Marjama, on Flickr

24-Nov-2012-1 by Andrew Marjama, on Flickr

 

[JimFox]

Thanks Andrew! I appreciate the samples.

The couple I looked up are now f6.3, so I figure that’s not as bad as f8.

I should have explained more completely, if I can get a mirror lens to work, then I would be looking at getting one of the Astro trackers to use with it.

 

[Ben Egbert]

If I were a Nikon guy, I would get a 400 f2.8.

 

[Colorado CJ]

Thanks Andrew! I appreciate the samples.

The couple I looked up are now f6.3, so I figure that’s not as bad as f8.

I should have explained more completely, if I can get a mirror lens to work, then I would be looking at getting one of the Astro trackers to use with it.

Well, in that case, I would think it should work pretty well! After all, these 500mm mirror lenses are basically just a Catadioptric telescope.

 

[Mike Lewis]

So Jim, here is my take.

The 500mm would be a nice size for framing a lot of things, and f/8, while slow, could work fine if tracked (which would be pretty much required at 500mm in any case...) The problem likely is that the aberrations would not be very pleasing. Stars are a true torture test for lenses. Even many of the very expensive big white Canon teles struggle to render stars the way you would like. The chief culprit is coma, which turns stars into little 'kites' or 'bird' shapes in the corners of the image. I suppose if it is only a mirror optic you would at least avoid CA. But I expect you would find it lacking for astroimaging if you used it much. It would however, be a decent optic for the moon I think, as CJ stated above.

A better cheap solution can sometimes be cheaper manual focus tele lenses for older mounts (M42, or Nikon F for 2 that work on modern Canon DSLRs). They can also have some issues with coma, but I would expect them to beat out a typical 500mm mirror optic in that regard. And being manual focus (with an infinity focus stop) is a plus for astro, as getting focused on stars at night can be tricky with more modern AF lenses.

I could give you some more specific recommendations if you want to drop me a direct message.

ML

 

[Mike Lewis]

I can also pass on some direct and lots of anecdotal info on small astro-trackers, if you get to that point.

ML

 

[JimFox]

If I were a Nikon guy, I would get a 400 f2.8.

Hey Ben, thanks for that idea. The only problem was I was trying to come up with a budget system, the mirror lens I had questions about can be bought for around $200 brand new.

 

[JimFox]

So Jim, here is my take.

The 500mm would be a nice size for framing a lot of things, and f/8, while slow, could work fine if tracked (which would be pretty much required at 500mm in any case...) The problem likely is that the aberrations would not be very pleasing. Stars are a true torture test for lenses. Even many of the very expensive big white Canon teles struggle to render stars the way you would like. The chief culprit is coma, which turns stars into little 'kites' or 'bird' shapes in the corners of the image. I suppose if it is only a mirror optic you would at least avoid CA. But I expect you would find it lacking for astroimaging if you used it much. It would however, be a decent optic for the moon I think, as CJ stated above.

A better cheap solution can sometimes be cheaper manual focus tele lenses for older mounts (M42, or Nikon F for 2 that work on modern Canon DSLRs). They can also have some issues with coma, but I would expect them to beat out a typical 500mm mirror optic in that regard. And being manual focus (with an infinity focus stop) is a plus for astro, as getting focused on stars at night can be tricky with more modern AF lenses.

I could give you some more specific recommendations if you want to drop me a direct message.

ML

Thanks Mike. That’s what I was curious about the quality of the image with stars. I thought that perhaps with that doughnut bokeah, the bright stars could have an issue?

If you have alternative inexpensive options please list them here, since this could turn into a little bit of a resource thread for anyone else to who is thinking on getting into deep space astrophotography.

 

[Mike Lewis]

Thanks Mike. That’s what I was curious about the quality of the image with stars. I thought that perhaps with that doughnut bokeah, the bright stars could have an issue?

If you have alternative inexpensive options please list them here, since this could turn into a little bit of a resource thread for anyone else to who is thinking on getting into deep space astrophotography.

Jim,

I’ll try to get a few recommendations out tonight.

ML

 

[Colorado CJ]

I am using a Tair 3s 300mm f4.5 right now and it works very well. It is an m42 mount camera so it might not work with your camera.

There are a few guys using this lens over at Cloudy Nights (astronomy forum) as well.

 

[Todd H]

Hey Jim, I have two of these lenses, a Nikon & a Tamron. The Tamron is sharper of the two. Here is the horse head with the Nikon 500mm f/8 ISO 3200 for 5 minutes, single exposure, no flats, no darks. Tracked on my AstroTrac, also here is a 8 minute exposure of the andromeda galaxy ISO 2000 on a D800