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Fast refractor with good optics?

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#1 Borodog

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Posted 11 January 2021 - 06:56 PM

I’d like a short (<240mm) fast (~F/2, or less) refractor that has decent optics. An option is a slower scope with a reducer that doesn’t create horrible aberration.

What would people recommend in terms of scopes and reducers?

#2 MikeMiller

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Posted 11 January 2021 - 07:01 PM

f/2 refractor? Maybe a camera lens. I have a few f/2 or faster lenses. They are all pretty lousy when wide open, some worse than others. But at f/3.5 or f/4 they are ok.. And none have the backfocus to put an eyepiece in it. The Rokinon/Samyang 135mm f/2 is pretty well recommended. 

 

What are you trying to do?



#3 Scott in NC

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Posted 11 January 2021 - 07:13 PM

I don’t think this exists.  And if it did, it would most likely be unaffordable to the vast majority of us.


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#4 Borodog

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Posted 11 January 2021 - 07:17 PM

Large object fast EAA in Bortle 5. Worked surprisingly well with a 60mm F/1.6 C-mount lens, but of course the lens is not astro quality.

What about a 120mm F/4 like the ZWO or SVBONY mini guide scopes? Could I reduce them without too much aberration? This is purely for fast EAA. Not tryin to shoot super high quality AP.

#5 Borodog

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Posted 11 January 2021 - 07:30 PM

Will be upgrading the camera to a larger sensor with larger pixels and higher QE as well. Both will help. You can trade pixel size for focal ratio.

Let me ask another way. What’s the lowest reasonable focal ratio you can get out of a refractor reducer pair, in terms of reasonable price and low aberration?

Do I have to go straight to a RASA to get this fast?

#6 Sandy Swede

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Posted 11 January 2021 - 07:42 PM

For some answers go here  https://www.cloudyni...71-the-fastest/

 

I think one APO was f/3.6


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#7 drd715

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Posted 11 January 2021 - 07:42 PM

What you are asking for is very dificult to do in a refractor. Chromatic aberrations would be extreamly difficult to correct at such a small Focal ratio. The other problem would be correcting a highly curved focal plain to a flat infocus image to match the flat imaging sensor.

You might try something like a RASA design reflector (imaging only) that do operate in the low focal ratio environment. Also I think the Hyperstar system from Starizona is similar to RASA. The RASA guys do complain about the critical focal depth range to hit the infocus point - Its rather narrow. They seem to have some difficulties with "tilt" in the camera to focal plane mechanical setup and in addition the limited use of filters via a slider holder. And the narrow band Ha filters have to be specifically designed for use in a fast sytem because the passband is shifted off of the desired signal frequency by a steep angle light cone. But it "might" do what you want to accomplish. Everything is an experiment.

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#8 astrodom

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Posted 11 January 2021 - 07:43 PM

 A good camera lens such as the Sigma Art series, ie. the 105mm F/1.4 is a great lens, even at F2 or F2.5.  You can do astrophotography with it, but limited in focal length.  The aformentioned Rokinon 135mm F2 is also very sharp.  What is the critical application that requires F2?   The RASA of course, is your best bet at that F-stop.


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#9 bobzeq25

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Posted 11 January 2021 - 07:56 PM

Will be upgrading the camera to a larger sensor with larger pixels and higher QE as well. Both will help. You can trade pixel size for focal ratio.

Let me ask another way. What’s the lowest reasonable focal ratio you can get out of a refractor reducer pair, in terms of reasonable price and low aberration?

Do I have to go straight to a RASA to get this fast?

Depends on how you define reasonable price.  But there's no question that the RASA will more cost effective than a really fast refractor.  Generally true of reflectors versus refractors.

 

Mind you, F2 comes with a set of problems.  I don't recommend that for beginners.

 

The F5.5 Esprit 80 is an excellent combination of price, reasonably fast, excellent quality, and easy to live with and use.  A truly excellent imaging scope.  The options you've suggested have traded off quality for price, they wouldn't be nearly as good.

 

An option is a Newtonian.  F4 is tricky, F5 is easier.

 

We're friends right?  You've barely started.  Don't get so focused on hardware (common beginner mistake), focus on learning the trade.   To quote an infamous American, it's not about the bike.  In other words, Dustin Hoffman's clubs will not put you on the PGA tour.  <smile>

 

The Esprit 80 can make outstanding images.  There are no magic scopes that are low price, low F number, and high quality.


Edited by bobzeq25, 11 January 2021 - 08:04 PM.

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#10 Borodog

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Posted 11 January 2021 - 07:59 PM

Thanks, gents. It sounds like camera lenses are the way to go. I’m comfortable with that. Heck, if I have to stick with the C-mount lens to stay this fast I will. It’s amazing how fast it is.
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#11 rk2k2

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Posted 11 January 2021 - 08:45 PM

Not < f/2 but an FRA400 with reducer you get 280mm f/3.9.  If you want the < f/2 at 240mm, the closest  lens I know of is 200mm f/1.8 for $4000-$7000 (don't know of longer, but Nikon had a 300mm F/2 back in the 80's- $29,000 list)..   In which case as long as it isn't impatience dictating a need for a fast f/ , I'd get one of the numerous small APO's that will give you f/4.x's along with a mount that can easily achieve the tracking.


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#12 CHASLX200

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Posted 12 January 2021 - 07:46 AM

F2 would be F/C heaven. No way i would want one for free unless there is some way to get rid of the F/C.



#13 junomike

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Posted 12 January 2021 - 08:13 AM

F2 would be F/C heaven. No way i would want one for free unless there is some way to get rid of the F/C.

I'm sure it's use would be AP with a FF.



#14 Borodog

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Posted 12 January 2021 - 09:50 AM

What is F/C?



#15 Sarkikos

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Posted 12 January 2021 - 10:24 AM

F2 would be F/C heaven. No way i would want one for free unless there is some way to get rid of the F/C.

More like FC hell.  mrevil.gif

 

Mike



#16 doctordub

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Posted 12 January 2021 - 10:25 AM

Field Curvature.

CS

Jonathan


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#17 Sarkikos

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Posted 12 January 2021 - 10:27 AM

What is F/C?

Field Curvature.  If the instrument cannot be focused sharply simultaneously at center of field and edge of field, but can be focused sharply at center of field or edge of field, there is field curvature.  The faster the refractor the more the FC, unless it is corrected somehow.  The correction for field curvature can be internal - as in a Petzval - or external by adding a field flattener.

 

Mike


Edited by Sarkikos, 12 January 2021 - 10:56 AM.

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#18 Sarkikos

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Posted 12 January 2021 - 10:53 AM

My fastest refractor - that isn't a finder scope - is a Bresser AR-102xs f/4.5.  It is an achromat.  A TSFLAT2 field flattener does a nice job of flattening the field.  Sharp stars from center to edge.  But it still has CA (chromatic aberration).  I just don't use it for the Moon or planets.

 

Mike


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#19 Wouter1981

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Posted 12 January 2021 - 10:57 AM

You can find Nikkor 180mm f2.8 lenses from the pre electronic era rather cheapily second hand and I've seen great pictures from them. Just be carefull, yoy want the one with the gold band. There is also a non-ED version which is cheaper, but older and not as good.
https://www.ebay.com...7EAAOSw~Wpf4Laa

 

 Here you can find a review of one:

 

https://www.astropix...ikon_180mm.html

 

i bought one myself to try some EAA but lost interest in it. But it's a great lens. It's not under f2 but that is realy hard and expensive to find.

 



#20 Sarkikos

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Posted 12 January 2021 - 11:19 AM

You can find Nikkor 180mm f2.8 lenses from the pre electronic era rather cheapily second hand and I've seen great pictures from them. Just be carefull, yoy want the one with the gold band. There is also a non-ED version which is cheaper, but older and not as good.
https://www.ebay.com...7EAAOSw~Wpf4Laa

 

 Here you can find a review of one:

 

https://www.astropix...ikon_180mm.html

 

i bought one myself to try some EAA but lost interest in it. But it's a great lens. It's not under f2 but that is realy hard and expensive to find.

I know nothing about photography.  This is a 180mm f2.8.  180mm is the focal length, right?  So then what is the aperture?  Does anyone know?  Does it matter? 

 

If this were a telescope, wouldn't the aperture be 504mm?  Aperture / Focal Length = f number thinking1.gif

 

Seems like photography mostly cares about focal length and f number.  Aperture?  Eh, who cares.

 

grin.gif

Mike


Edited by Sarkikos, 12 January 2021 - 11:20 AM.


#21 mrsjeff

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Posted 12 January 2021 - 11:50 AM

Maybe try your question in the EAA forum?



#22 Sarkikos

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Posted 12 January 2021 - 11:56 AM

My question or the topic starter's? :grin:

 

Mike


Edited by Sarkikos, 12 January 2021 - 11:57 AM.


#23 bobzeq25

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Posted 12 January 2021 - 12:22 PM

I know nothing about photography.  This is a 180mm f2.8.  180mm is the focal length, right?  So then what is the aperture?  Does anyone know?  Does it matter? 

 

If this were a telescope, wouldn't the aperture be 504mm?  Aperture / Focal Length = f number thinking1.gif

 

Seems like photography mostly cares about focal length and f number.  Aperture?  Eh, who cares.

 

grin.gif

Mike

The aperture would be about 60mm (180/2.8, you've got it upside down).  Aperture matters some in imaging, but a lot less than it does for visual.  Long exposures do most of the heavy lifting.  <smile>  Your short exposure eyes don't have that capability.

 

The image below was taken with a 70mm scope (F4.8 as configured).  Better version than the crummy (required) CN jpg here.

 

https://www.astrobin.com/367734/C/

 

NGC6992 HaO(III)RGB V4.jpg


Edited by bobzeq25, 12 January 2021 - 12:23 PM.

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#24 MikeMiller

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Posted 12 January 2021 - 12:23 PM

I know nothing about photography.  This is a 180mm f2.8.  180mm is the focal length, right?  So then what is the aperture?  Does anyone know?  Does it matter? 

 

If this were a telescope, wouldn't the aperture be 504mm?  Aperture / Focal Length = f number thinking1.gif

 

Seems like photography mostly cares about focal length and f number.  Aperture?  Eh, who cares.

 

It's focal length / aperture = f number :)

 

So its 180mm / 2.8 = 64ish mm.

 

But its more complicated than that. Camera lenses basically have an "aperture stop", which is the adjustable ring. So that is what constrains the light rather than the size of the front element.

 

So you can "stop down" the lens to f/4 or slower to get a sharper image.


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#25 Sarkikos

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Posted 12 January 2021 - 12:26 PM

The aperture would be about 60mm (180/2.8, you've got it upside down).  Aperture matters some in imaging, but a lot less than it does for visual.  Long exposures do most of the heavy lifting.  <smile>  Your short exposure eyes don't have that capability.

 

The image below was taken with a 70mm scope (F4.8 as configured).  Better version than the crummy (required) CN jpg here.

 

https://www.astrobin.com/367734/C/

 

attachicon.gifNGC6992 HaO(III)RGB V4.jpg

Sorry about that. foreheadslap.gif  I must be tired today.  I should have remembered f number = focal length / aperture.  Yep, that makes much more sense. 180 / 2.8 = 64.3.  So the aperture should be around 64mm.

 

Mike


Edited by Sarkikos, 12 January 2021 - 12:27 PM.

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