What is the sharpest <300mm focal lenght telescope/lens ?

Equipment Refractors
Luka Poropatandrea tasselliDan H. M.
•40 replies•1.9k views
Luka Poropat avatar
Luka Poropat avatar
Im starting this topic as a genuine question I have currently.
In a world of lenses & telescopes what is the current optically sharpest telescope on the market that fits the sub 300mm specifications. 
I am talking... full frame small pixels corrected & capable, amazing strehl widefield lens/telescope that produces pinpoint small stars across the entire field of view - basically only diffraction limited telescope by aperture. 
Ideally something like this is a native focal lenght telescope that doesnt use a reducer to achieve its focal lenght.
Feel free to pitch more ideas as OTAs and etc.


Here is the list I came up with so far: 

WO RC51 (FPL53) (sharp so far but not perfect on some copies)

WO RC61 (FPL53) (bigger than RC51 means more aperture and more achievable sharpness ?) 

Borg55 (Canon Fluorite) (very niche, sometimes good sometimes bad - corners questionable) 

Tak60CB (Fluorite) (its a tak, but who uses this with tiny pixels FF and pushes it to the limit especially with reducer) 

WO Pleiades 68 (FPL53)  ??? issues  https://www.astrobin.com/forum/c/equipment-forums/william-optics-pleiades-68/william-optics-pleiades-68/?page=3

Canon/Nikon 200mm F/2 - (ED/SED) (unknown, stopped down ? )

Sigma 135 F/1.8 (FLD/SLD) poor quality until stopped to F5 which makes it pointless https://www.astrobin.com/forum/c/astrophotography/equipment/sigma-art-135mm-f18/

Askar FRA300 (undisclosed) - softer than RC51/61

Canon 400mm F/2.8 (Canon Fluorite, ED) etc., used on Dragonfly array- focal lenght too big
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andrea tasselli avatar
andrea tasselli avatar
Nikon AF-S 300mm  VR
Valerio Pardi avatar
Valerio Pardi avatar
Perhaps not the sharpest,  but nikon 300 f/2.8 VR is very sharp
https://app.astrobin.com/equipment/explorer/telescope/1943/nikkor-af-s-vr-300mm-f28g-if-ed
Luka Poropat avatar
Luka Poropat avatar
andrea tasselli:
Nikon AF-S 300mm  VR

From images on astrobin this is far from sharp unfortunately.
andrea tasselli avatar
andrea tasselli avatar
Luka Poropat:
From images on astrobin this is far from sharp unfortunately.


*Yeah? What about this: Nikon AF-S Nikkor 300mm F/2.8 ED Test - New (andrea tasselli) - AstroBin
SemiPro avatar
SemiPro avatar
It's hard to trust images on Astrobin, especially from lenses. A lot of the time those shooting with lenses are just starting out, or they have always used a lens/DSLR combo and have no sense for tilt/backfocus adjustment or other astrophotography related calibrations.

You may be looking for a unicorn, because lenses are still not designed with small pixel astronomy cameras in mind. This is how they can get away with only a few elements of quality glass in between all the different elements shoved into a lens.

In terms of telescopes that meet your description, you have pretty much listed all the ones that I can think of.
Luka Poropat avatar
Luka Poropat avatar
andrea tasselli:
Luka Poropat:
From images on astrobin this is far from sharp unfortunately.


*Yeah? What about this: Nikon AF-S Nikkor 300mm F/2.8 ED Test - New (andrea tasselli) - AstroBin

Unfortunately still not sharp, you can see right edge of the field weird star shapes/halos around the stars (if thats not just post processing artifact) and thats on an APS-C sensor size.
Seung-Jun Kim avatar
Seung-Jun Kim avatar
What about FSQ85 with new QB 0.73 reducer?  - 330mm
Luka Poropat avatar
Luka Poropat avatar
Seung-Jun Kim:
FSQ85 with 0.73 reducer - 330mm

Reducer causes all sorts of issues, also not suitable for small pixels. Most Taks were designed/made where 5+um pixels were a thing and it was easier on the optics. Nowadays its much harder to achieve great optical performance on a sensor like IMX455. 
andrea tasselli avatar
andrea tasselli avatar
Luka Poropat:
Unfortunately still not sharp, you can see right edge of the field weird star shapes/halos around the stars (if thats not just post processing artifact) and thats on an APS-C sensor size.


*With a median FWHM of 1.8 pixels (IMX571) is as sharp as it gets at full aperture. The aesthetic doesn't really enters the equation (halos and such). I don't have a FF sensor for doing these tests reliably but there are links to them posted previously.
Luka Poropat avatar
Luka Poropat avatar
andrea tasselli:
With a median FWHM of 1.8 pixels (IMX571) is as sharp as it gets at full aperture


I think you have been doing your math completely wrong, also from seeing that in your image description. To calculate for instance seeing or the resolution you are imaging at its FWHM x pixel scale in that image above its FWHM of 4.9", which for a sampling of 2.63"/px gives you a 12.887 
When your scopes optical fwhm is like 1/4th your observed FWHM, your FWHM is accurate to the true seeing within like a percent.
Supposing there arent any significant tracking errors.
Daniel Carter avatar
Daniel Carter avatar
Luka Poropat:
Seung-Jun Kim:
FSQ85 with 0.73 reducer - 330mm

Reducer causes all sorts of issues, also not suitable for small pixels. Most Taks were designed/made where 5+um pixels were a thing and it was easier on the optics. Nowadays its much harder to achieve great optical performance on a sensor like IMX455. 

Takahashi has released a new reducer for the FSQ-85 that shows better results than the older QE version.
Well Written
andrea tasselli avatar
andrea tasselli avatar
Luka Poropat:
I think you have been doing your math completely wrong, also from seeing that in your image description. To calculate for instance seeing or the resolution you are imaging at its FWHM x pixel scale in that image above its FWHM of 4.9", which for a sampling of 2.63"/px gives you a 12.887 
When your scopes optical fwhm is like 1/4th your observed FWHM, your FWHM is accurate to the true seeing within like a percent.
Supposing there arent any significant tracking errors.


FWHM is expressed in pixels so it is 4.7". Don't where you picked up your numbers.
Luka Poropat avatar
Luka Poropat avatar
andrea tasselli:
Luka Poropat:
I think you have been doing your math completely wrong, also from seeing that in your image description. To calculate for instance seeing or the resolution you are imaging at its FWHM x pixel scale in that image above its FWHM of 4.9", which for a sampling of 2.63"/px gives you a 12.887 
When your scopes optical fwhm is like 1/4th your observed FWHM, your FWHM is accurate to the true seeing within like a percent.
Supposing there arent any significant tracking errors.


FWHM is expressed in pixels so it is 4.7". Don't where you picked up your numbers.

I picked it up directly from your description. Where you clearly state : median FWHM is 4.9" with a sampling of 2.62"/px yields a total 1.8px which is inherently wrong because you did the math wrong, it should be the other way around 4.9x2.62=12.8
Jure Menart avatar
Jure Menart avatar
Luka Poropat:
andrea tasselli:
Luka Poropat:
I think you have been doing your math completely wrong, also from seeing that in your image description. To calculate for instance seeing or the resolution you are imaging at its FWHM x pixel scale in that image above its FWHM of 4.9", which for a sampling of 2.63"/px gives you a 12.887 
When your scopes optical fwhm is like 1/4th your observed FWHM, your FWHM is accurate to the true seeing within like a percent.
Supposing there arent any significant tracking errors.


FWHM is expressed in pixels so it is 4.7". Don't where you picked up your numbers.

I picked it up directly from your description. Where you clearly state : median FWHM is 4.9" with a sampling of 2.62"/px yields a total 1.8px which is inherently wrong because you did the math wrong, it should be the other way around 4.9x2.62=12.8

You should clearly check your math again

Think of it in more logical way: with each pixel you catch 2.62", seeing is 4.9" - so how many pixels you need to cover 4.9"?

Is it 12.8 pixels or less then 2 pixels?
andrea tasselli avatar
andrea tasselli avatar
Luka Poropat:
I picked it up directly from your description. Where you clearly state : median FWHM is 4.9" with a sampling of 2.62"/px yields a total 1.8px which is inherently wrong because you did the math wrong, it should be the other way around 4.9x2.62=12.8


Suit yourself...
[deleted]
[deleted]
https://store-eu.hasselblad.com/
Enjoy
JohnHen avatar
JohnHen avatar
I have the FS-60CB for travel and attached to it a modded EOS R. I run it at native FL with an AsiAirP. It is a tiny and light-weight set-up that can go anywhere anytime (i have no images of it in my AB portfolio though).

If you also consider a class higher, an FSQ-85 is a great choice. I run it at native FL 455mm (i have several images in my AB portfolio).

Cheers, John
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Luka Poropat avatar
Luka Poropat avatar
Jure Menart:
Luka Poropat:
andrea tasselli:
Luka Poropat:
I think you have been doing your math completely wrong, also from seeing that in your image description. To calculate for instance seeing or the resolution you are imaging at its FWHM x pixel scale in that image above its FWHM of 4.9", which for a sampling of 2.63"/px gives you a 12.887 
When your scopes optical fwhm is like 1/4th your observed FWHM, your FWHM is accurate to the true seeing within like a percent.
Supposing there arent any significant tracking errors.


FWHM is expressed in pixels so it is 4.7". Don't where you picked up your numbers.

I picked it up directly from your description. Where you clearly state : median FWHM is 4.9" with a sampling of 2.62"/px yields a total 1.8px which is inherently wrong because you did the math wrong, it should be the other way around 4.9x2.62=12.8

You should clearly check your math again

Think of it in more logical way: with each pixel you catch 2.62", seeing is 4.9" - so how many pixels you need to cover 4.9"?

Is it 12.8 pixels or less then 2 pixels?

Median FWHM of 4.9" is measured on a stacked image in a FWHME Eccentricity tool in PixInsight or a similar tool in another software and this number is not the seeing. The "seeing" is calculated by multiplying that Median FWHM with the resolution/sampling (calculated from pixel size on a camera and focal lenght of the telescope/lens).  

And just by measuring the jpeg on astrobin (which is not correct but as an example) his Median FWHM is 2,509px and multiplying that with his sampling of 2,59"/px its 6,5 which is still not even near the crazy 1.8 or 1.6 values Andrea is talking about. 

Luka Poropat avatar
Luka Poropat avatar
https://store-eu.hasselblad.com/
Enjoy

Has anyone ever even tried this ? 
And if not then why put it here without actual examples of images.
Oscar H. avatar
Oscar H. avatar
Luka Poropat:
https://store-eu.hasselblad.com/
Enjoy

Has anyone ever even tried this ? 
And if not then why put it here without actual examples of images.

well there's some images here https://www.hasselblad.com/inspiration/stories/teruyasu-kitayama-astrography/

A redcat or samyang (or Nikkor 300mm) lens is still better.
John avatar
John avatar
meyer gorlitz 300/f4 . razor sharp to the edge,only you need to focus seperate on the blue or red area. https://vintagelens.nl/shop/lenses/m42/meyer-optik-gorlitz-orestegor-300mm-f4-0-n-o-s-m42/?aewcobtn=1
Karl Theberge avatar
Karl Theberge avatar
Well, I ain't know shit about math, but I am quite familiar with rude poeple and you, my dear, do not invite to discussion at all.

cheers,

K
Tom Victor avatar
Tom Victor avatar
Vixen FL55ss should probably be considered as a contender.
https://app.astrobin.com/equipment/explorer/telescope/583/vixen-fl55ss?_ga=2.49853614.943637360.1712781675-1274737048.1708889676
Luka Poropat avatar
Luka Poropat avatar
Karl Theberge:
but I am quite familiar with rude poeple and you, my dear, do not invite to discussion at all.

My intention wasn't to be rude, but to ensure accuracy in our discussion. Precision matters in technical matters like this. If there's a mistake, it's important to correct it for the sake of clarity and understanding.
Well Written