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New Redcat51 comparison with Canon 70-200 f2.8L

Astrophotography DSLR Equipment Imaging Optics Refractor
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#1 blueplanetphoto

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Posted 03 October 2022 - 08:58 PM

A comparison of lenses. Quite a remarkable difference, actually. I just purchased the Redcat 51 to use with my Canon R5 on the Star Adventurer GTi mount. Mostly for travel, but also for wide field work in the backyard. The image with the Canon is also likely slightly out of focus due to leaving my Bahtinov mask at home. Both images are single exposures autostretched in Siril and cropped from a screenshot. Both are from an imaging of Andromeda (M31), though the Canon image was made in Bortle 3 last week and the Redcat 51 image in my Bortle 5-6 backyard last night. 

Attached Thumbnails

  • andromeda-9-23-22-Canon70-200f28L-upperleft.jpg
  • andromeda-10-2-22-redcat51-upperleft.jpg

Edited by blueplanetphoto, 03 October 2022 - 09:00 PM.


#2 jimhoward999

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Posted 03 October 2022 - 09:13 PM

The massive coma, which appears constant with field, cannot be representative of a Canon 70-280 F/2.8 zoom.  So something is wrong.  It is as if something is tilted or wedged in the lens.

 

Also, forgive the question, but where is Andromeda?    The star field looks vaguely correct,  but is the exposure so short that Andromeda itself is not visible in a single frame?


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#3 Jim Waters

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Posted 03 October 2022 - 10:02 PM

There's something wrong with the Canon lens.



#4 blueplanetphoto

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Posted 03 October 2022 - 10:10 PM

The massive coma, which appears constant with field, cannot be representative of a Canon 70-280 F/2.8 zoom.  So something is wrong.  It is as if something is tilted or wedged in the lens.

 

Also, forgive the question, but where is Andromeda?    The star field looks vaguely correct,  but is the exposure so short that Andromeda itself is not visible in a single frame?

Both of these images are of the upper left corner of the overall field. Andromeda is in the center of both of the original full size images, but these are 100% crops of just the upper left. Both exposures are 30 sec @ 800ISO.


Edited by blueplanetphoto, 03 October 2022 - 10:10 PM.


#5 blueplanetphoto

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Posted 03 October 2022 - 10:15 PM

There's something wrong with the Canon lens.

I've been using this lens in my professional terrestrial work for 20 years and I haven't noticed any issue at the edges, even at 100%. This was actually a bit of a surprise. 



#6 Jim Waters

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Posted 03 October 2022 - 10:24 PM

Both of these images are of the upper left corner of the overall field. Andromeda is in the center of both of the original full size images, but these are 100% crops of just the upper left. Both exposures are 30 sec @ 800ISO.

OK - I feel somewhat better.  Zoom lenses and astrophotography don't belong in the same sentence.  Always use prime lenses.  Star fields are very hard on zoom lenses.

 

However, I expected better edge coma on that 'L' lens.



#7 Sacred Heart

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Posted 03 October 2022 - 11:22 PM

Last week I tried using my 70 - 200 F2.8 lens with my QHY533C camera.  Problem #1 could not get close enough for focus, needed to add my 1.4 tele-extender.  Problem #2, with that and focus set at infinity I then moved the zoom off of 200 to focus. I had to leave the focus at infinity and move the zoom to focus.

 

So, I am not using the Canon lens anymore, I am getting a focal reducer for my TV76.   

 

Proper tool for the job. 

 

My experience,   Joe


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#8 Kitfox

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Posted 04 October 2022 - 12:13 PM

I have/do own and use large format lenses that cost as much as a luxury car. Excellent lenses for fashion or landscape. But for imaging a white dot on a black background, they wouldn’t compare to a good apo with a matched flattener unless stopped down. Even then, all the air-glass transitions inside limit how much resolution is possible. Not to mention, a camera lens is optimized for focus at less than infinity, and has to be good at focus from at most few feet to infinity. Im not an optical expert, but that has to involve compromises.

 

When you know that some of history’s  best camera lenses didn’t have ANY special glass, it’s amazing the designers could do what they did. I believe your lens may have a couple of ED elements, but no SD or fluorite. 
 

Did you shoot with the canon wide open and at 200mm?  If you take the aperture down a stop or two, your results will be better.  Not for sure about that lens, but stopping down helps immensely; at least until diffraction effects from the diaphragm start to negate the improvement. Most lenses do best on off-axis point-source imaging at about f/8 to f/11. But don’t ever expect theoretical diffraction limit performance. At least I’ve never seen it…even Zeiss or Hasselblad. 


Edited by Kitfox, 04 October 2022 - 12:24 PM.


#9 SilverLitz

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Posted 04 October 2022 - 04:01 PM

My Canon EF300mm f/2.8L (non-IS) is a great "normal" lens for sports, but for AP, the much lower priced Vixen FL55ss significantly outperforms it.



#10 Jim Waters

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Posted 04 October 2022 - 04:08 PM

The Vixen FL55ss is a nice scope.



#11 poorfield

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Posted 05 October 2022 - 06:55 PM

FYI, these MTF curves quantify the resolution difference btwn the redcat51 and the Canon zoom.

 

Compare the 30 lp/mm curve, ie, the light blue (redcat) and the green (canon).

 

So the curve plots yaxis=MTF (basically contrast  in a 0 to 100% scale) and x-axis being the distance in mm from the center of the field.

 

Interpreted as,  how sharp is it (100% being the sharpest) from the center to edge of field (0-20mm from center).

Sharpness decreases for any lense if there are more "line pairs per mm", ie, finer line/spaces.

 

The redcat shows an 8% drop in sharpness while the Canon shows 47% drop from center to edge.

 

redcat51 Mtf
canon70 200f2p8L Mtf

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#12 poorfield

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Posted 05 October 2022 - 07:01 PM

This is kinda fun to look thru.

 

https://www.lensrent...cus-mtf-curves/


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#13 glancey

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Posted 06 October 2022 - 12:41 PM

Nice discussion for the technorati. But the RedCat51 ( or any other telescope) with a dedicated astro camera is the right tool for the job. No discussion needed.


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#14 Jim Waters

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Posted 06 October 2022 - 01:03 PM

But some 'high quality' prime lenses get close.



#15 Jaimo!

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Posted 06 October 2022 - 11:00 PM

But is getting close, close enough?  Considering the RC51 will be less expensive.



#16 ion

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Posted 07 October 2022 - 01:00 AM

I've been using this lens in my professional terrestrial work for 20 years and I haven't noticed any issue at the edges, even at 100%. This was actually a bit of a surprise.


What do the other corners look like when the center is in focus?
Was the Canon lens at f/2.8 and the Redcat at f/4.9? Egg shaped
stars are normal in the corners wide open but yours are too
extreme and looks like coma. When stopped down to f/5.0 like
the Redcat that lens should produce round stars, but then you
may see some blue CA. But mine was dropped too lol.gif

#17 poorfield

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Posted 07 October 2022 - 02:47 AM

Interesting research application using array of canon 400mm telephotos.

 

https://www.dragonflytelescope.org/



#18 blueplanetphoto

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Posted 18 October 2022 - 03:12 PM

To follow up, this was a simple comparison. The Canon was at f2.8, so not a fair comparison to the Redcat's f4.9. As Ion mentioned, if the apertures were similar, the results would probably be more similar. I may run another test using equal parameters, just for fun, when there's time.

 

There's no question terrestrial lenses are less suitable (and zooms less than primes) than telescopes for astrophotography. But it's useful to see the difference for those who are considering one or the other, or who are being told a terrestrial lens will work just fine (it might). Lots of recommendations for "cheaper" lenses like the Rokinon or Samyang brand lenses. I don't have direct experience with those lenses but they could be quite adequate. There isn't a comparison between the Canon and Rokinon lenses because there is no Rokinon lens with a similar focal length. Astrophotography is a demanding pursuit. The tolerances are much narrower than for terrestrial photography practices. Understanding these differences can save someone wasted time in long nights, and money in purchasing inferior equipment. Both the 70-200 and Redcat 51 are wide field lenses. Probably more often than not the corners will be cropped out anyway, eliminating any errors on the edges.




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