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02-12-2020, 12:48 PM   #1
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William Optics RedCat 51 vs Pentax DA* 300mm F4 for Astro

Hi all,

Trying to choose between William Optics RedCat 51 (or z61) and Pentax DA* 300mm F4 for Astrophotography.

Has anybody had experience with both and could compare? On one hand I am hearing good feedback about Pentax DA* 300mm and it is faster, but on the other hand, few lens review sites sharpness rates were not particularly great:
Pentax smc DA Star 300mm F4 ED (IF) SDM - DxOMark
Pentax SMC DA* 300mm f/4 ED [IF] SDM - Review / Test Report - Analysis

I haven't found objective sharpness data on RedCat 51, but I know that they advertise themselves as the world sharpest lens and it is cheaper (but slower at f4.9).

Also, I have some experience with astro using Rokinon(Samyang) 135mm, and its performance at f2 was great. Looking at Optical limits review, Rokinon 135mm has much higher sharpness than Pentax DA* 300mm, although I am not sure if those are directly comparable due to different focal length
Samyang 135mm T2.2 ED UMC CINE DS ( 135mm f/2 ED UMC ) - Review / Test Report - Analysis
Pentax SMC DA* 300mm f/4 ED [IF] SDM - Review / Test Report - Analysis

My goal is to find a lens/scope in 250-350mm range that performs for astro at least as well as Rokinon 135mm did. Note that I am pursuing only astro at this time and not interested in other uses (at least for now).

Thanks for your advice!

02-12-2020, 05:58 PM - 2 Likes   #2
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I've got both the whitecat 51 and a DA* 300 and FA* 300. and the K-1 Mk 1


In my view the Whitecat is the better lens for astro work.


The flatter field, less CA (both the 300s have some) and the pinpoint stars are in the WO favor.

The ability to use standard 2" astro filters without having to resort to step-up/down rings an added bonus.

Here's a couple of images for comparison.



Lagoon and Trifid nebulas WO White cat 51.



North American Nebula Pentax FA* 300
02-12-2020, 06:35 PM   #3
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To throw something else into the mix, what about a 645 A* 300? Considerably cheaper than either of the other options, and probably at least as sharp at the DA 300.
02-12-2020, 07:19 PM   #4
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The optical limits tests are on completely different bodies so you can't compare results. DxOMark test are also useless unless you're comparing lenses on the same bodies. Unfortunately DXO isn't very clear about this and will give a lens a very low rating simply because it was tested on a low resolution body.

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