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F/20 Performance with Aperture Mask

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#1 Beastmaster

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Posted 10 August 2019 - 01:37 AM

Hey all, I've been perusing old posts about the magic of 60mm F/20 scopes like the Southern Precision Instrument scopes from Royal Astro and I was wondering if one can get the same performance and experience by using an aperture mask.

 

In other words, in the real world, would one noticed a difference between an SPI 60mm / 1200mm Royal Astro Glass versus stepping down a 76mm / 1200mm Royal Astro scope?  Theoretically, it seems like it would be the same, but how does the real world compare?  Part of the line of questioning stems from me owning a 76/1200 and wondering how to up its performance.  

 

Part of me is curious if this is indeed a simple answer or will elicit a debate like the ones I enjoy reading on these boards

 

Thanks!

 

Fred 


Edited by Beastmaster, 10 August 2019 - 01:39 AM.


#2 Astro-Master

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Posted 10 August 2019 - 02:11 AM

I would think if you made a good mask, and centered it well, it should be the same.  Try it out and let us know.



#3 siriusandthepup

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Posted 10 August 2019 - 03:42 AM

It will work and perform great...for a 60mm.

 

You can get an idea of the performance without having to purchase one (I have Carton 60mm f/17 and f/20 scopes).

 

Will you be tempted to leave the mask on? No. Your full aperture good quality lens still rules.


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#4 TOMDEY

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Posted 10 August 2019 - 04:37 AM

Theoretically, an optimized 60mm will perform better than a stopped-down optimized 76mm. In actual practice, I would expect that stopping down your 76 will give a good rendition of what the 60mm would show... and that will just be a significantly dimmer image, with less resolution.    Tom


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#5 Piggyback

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Posted 10 August 2019 - 10:37 AM

Stepping down a quality 76mm refractor? Why would you want to do that when you get better planetary resolution with your 3 inch? Aperture rules!


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#6 agmoonsolns

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Posted 10 August 2019 - 07:46 PM

As above have posted, greater aperture will show much more. That being said, if you're just curious like I am, the 76mm masked down does deliver a tiny bit better performance over the 60mm. I believe this is because the edges of a lens are usually not as good as the center and the 76mm with those edges masked out delivers the best part of the lens. This might be giving it a bit of an advantage over the 60mm. Of course, you might get different results comparing different scopes than what I used. Maybe, my 76mm was just simply much better than the 60mm I was using for the comparison.

 

If you want to improve the performance of your 76mm you can do things like upgrade the focuser to 1.25" and use high quality eyepieces/diagonal, use a Takahashi 1.6x Extender Q (makes a surprising difference, but then you're stuck using really high magnifications), and depending on the correction of your lens, sometimes using a prism instead of a mirror diagonal will produce better color correction due to some canceling out of the CA.

 

Also, flocking the tube/focuser and using flat black paint on any shiny reflective surfaces helps. Making sure the baffles are correctly placed is something else you can try. Adding a better finder or some type of red dot/reflex finder. Getting tube rings from Orion, adding a dovetail, and putting it on a better mount is another popular option. There are filters one can get which are supposed to help with CA if that's a big deal to you. At almost f/16, I have never seen a need to do that, but everyone is different.

 

I am sure the experts here will have lots more great advice on improving your 76mm.


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#7 Chuck Hards

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Posted 10 August 2019 - 08:34 PM

The gain in color correction from ~f/16 to ~f/20 with 2 inch and 3 inch apertures is so slight that it's not a practical concern.  Keep the larger aperture, it will give you the best advantage in this situation.  

 

I like a dedicated f/20 scope because I get a higher power with any given eyepiece, than I do with an f/16.  I think there are other slight advantages, but to stop-down an excellent 3" f/15 or higher makes little sense.

 

My 2 cents, your mileage may vary.


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#8 ccwemyss

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Posted 10 August 2019 - 09:48 PM

The gain in color correction from ~f/16 to ~f/20 with 2 inch and 3 inch apertures is so slight that it's not a practical concern.  Keep the larger aperture, it will give you the best advantage in this situation.  

 

I like a dedicated f/20 scope because I get a higher power with any given eyepiece, than I do with an f/16.  I think there are other slight advantages, but to stop-down an excellent 3" f/15 or higher makes little sense.

 

My 2 cents, your mileage may vary.

 

Just to clarify for the OP, you meant that an f20 60mm vs. a f16 60mm gives you higher power.

 

The thread has been mixing in a 75mm f16 too, which would have the same focal length as the f20 60mm (1200mm) and would thus give the same power for the same eyepiece. 

 

Stopping down a 75mm scope with a 1200mm fl, to a 60mm aperture would only reduce the brightness and resolution. But it's a good way to see if that's still usable for someone's purposes, if they are interested in getting a lighter rig. 

 

Chip W. 


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#9 Chuck Hards

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Posted 10 August 2019 - 11:43 PM

Sure, it's an excellent way to see what to expect with a smaller scope. 

 

There are times with larger telescops when stopping it down can reduce the effects of poor seeing.  I've never found it to be much of an issue with scopes smaller than 4 or 5 inches, though.


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#10 MartinPond

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Posted 10 August 2019 - 11:59 PM

It will create an area of dark space throughout

the front of the scope.

Contrast should be a tad better. 


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#11 ccwemyss

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Posted 11 August 2019 - 09:42 AM

The mask will only cover 7.5mm at the edge of the objective -- less than 5/16".  It could improve a lens that has a turned edge, and perhaps help if the tube isn't well baffled and blackened. But going from 75mm to 60mm will most noticeably show up as a 36% loss in light gathering, and theoretical resolution drops from 1.54" to 1.93". 

 

Chip W. 



#12 Beastmaster

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Posted 11 August 2019 - 10:57 AM

Thank you, everyone, for confirming my thoughts and explaining what I was trying to intuit.  Part of the question stems from seeing in the classic ads posts mention of the SPI f/20.  I've been doing a lot of observation of Jupiter and Saturn as of late and wondered what type of view an SPI f/20 would provide after reading articles like: 

 

https://www.cloudyni...-60mm-f20-ebay/

 

My Mayflower 76x1200 has been giving me incredible views as of late - better than what my ETX-90 has been.  It's a great telescope and I'm amazed at the detail even in flawed skies.


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