Resolution of the 110 lens with 3.76 micron pixel size


Roland Christen
 

Hi fellow Astronuts,

Had a chance to test the 110mm lens today to see what kind of resolution it can achieve under near perfect seeing conditions. These conditions are not common here during a typical night when astrophotography is normally done. But there is about a 1 hour time window when the skies switch from daytime scintillation to nighttime seeing conditions. That happens just when the sun begins to set at dusk, and the air becomes very still and there is not yet a nighttime radiation of earth's heat into space.

Today we are in the middle of a high pressure dome with the jet stream far away to the north. Good time to test optics on real stars. I chose Arcturus since it is quite high near the zenith right now. I wanted to see what kind of resolution can be achieved at various wavelengths. Theoretically the red wavelengths have less resolution than green or blue. I chose the 3 narrowband filters because normal wide band filters were completely saturated while the sky was still bright at dusk. I added an original Traveler field flattener which brought the 110mm lens up to F6.3. This flattener is not ideally matched, but close enough to use with this scope for now until we finish the proper mating flattener/reducer for testing. I used fast exposures to minimize seeing effects and to prevent saturation. At F6.3 the pixel scale is 1.116 arc sec per pixel (at F7 it would have been exactly 1 arc sec/pixel).

It turns out that the theoretical FWHM resolution of a 110mm aperture is right around 1.1 arc seconds at the green visual peak wavelength, which happens to be quite near the OIII filter passband. The Ha red wavelength has theoretically lower resolution of approximately 1.2 arc sec because red waves have longer wavelength. SII is farther into the deep red, basically at the edge of the infrared and has close to 1.3 arc sec FWHM theoretical resolution. As the images below show, the 110 lens produces almost exactly that theoretical resolution for these 3 wavelengths.

The interesting thing for me is that at best focus the 3.76 micron pixel size produces pixelated stars if the seeing is absolutely perfect. It appears that a lens of around F6 focal ratio could actually use smaller pixel cameras to produce non-pixelated round stars under perfect seeing conditions. Since we don't have such a thing around these parts, I guess that I can do with the larger pixels of the 6200MM camera.

Tonight the seeing is supposed to be average, and I plan on doing some narrowband imaging of a couple of objects, just to see what a wide field scope can produce as far as high res imaging of small objects.

Rolando






--
Roland Christen
Astro-Physics


Chris White
 

Exciting to hear Roland!  

Funny to hear you say the "larger" pixels of the 6200 will have to do. 

When you say you are finalizing the reducer/flattener are you talking about just the f5 reducer or have you also settled on a flattener design?

Nice to star seeing more information come out. I was just thinking about this scope earlier today. 


Chris White
 

Looks like you are really horsing that camera cooler too!  I've found that 0 degrees is excellent and benefits below 0 are incremental.  It's a really nice sensor!


Roland Christen
 

I have not yet settled on a flattener F-ratio. Something between F6 and F7 for larger format coverage and better resolution for small objects. depends somewhat on what users are looking for.

Roland

-----Original Message-----
From: Chris White <chris.white@...>
To: main@ap-gto.groups.io
Sent: Mon, Aug 1, 2022 9:54 pm
Subject: Re: [ap-gto] Resolution of the 110 lens with 3.76 micron pixel size

Exciting to hear Roland!  

Funny to hear you say the "larger" pixels of the 6200 will have to do. 

When you say you are finalizing the reducer/flattener are you talking about just the f5 reducer or have you also settled on a flattener design?

Nice to star seeing more information come out. I was just thinking about this scope earlier today. 

--
Roland Christen
Astro-Physics


Roland Christen
 

Heck, my CCD cameras are always running at -25 to -30. I thought these CMOS cameras should be able to get down to at least -15.

Rolando

-----Original Message-----
From: Chris White <chris.white@...>
To: main@ap-gto.groups.io
Sent: Mon, Aug 1, 2022 9:57 pm
Subject: Re: [ap-gto] Resolution of the 110 lens with 3.76 micron pixel size

Looks like you are really horsing that camera cooler too!  I've found that 0 degrees is excellent and benefits below 0 are incremental.  It's a really nice sensor!

--
Roland Christen
Astro-Physics


Roland Christen
 

This is what the seeing looks like right now from Clear Sky Clock: https://www.cleardarksky.com/c/AstrPhyILkey.html
Seeing is marked as 3 out of 5, average. Stars in a 60 sec exposure measure between 1.5 and 1.7 arc sec FWHM in Ha. So, I'm not getting the theoretical resolution now. But I think the image will be quite good anyhow.

Rolando

-----Original Message-----
From: Chris White <chris.white@...>
To: main@ap-gto.groups.io
Sent: Mon, Aug 1, 2022 9:54 pm
Subject: Re: [ap-gto] Resolution of the 110 lens with 3.76 micron pixel size

Exciting to hear Roland!  

Funny to hear you say the "larger" pixels of the 6200 will have to do. 

When you say you are finalizing the reducer/flattener are you talking about just the f5 reducer or have you also settled on a flattener design?

Nice to star seeing more information come out. I was just thinking about this scope earlier today. 

--
Roland Christen
Astro-Physics


Bill Long
 

That is awesome. :) Sounds like the scope is a winner! 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> on behalf of Roland Christen via groups.io <chris1011@...>
Sent: Monday, August 1, 2022 8:31 PM
To: main@ap-gto.groups.io <main@ap-gto.groups.io>
Subject: Re: [ap-gto] Resolution of the 110 lens with 3.76 micron pixel size
 
This is what the seeing looks like right now from Clear Sky Clock: https://www.cleardarksky.com/c/AstrPhyILkey.html
Seeing is marked as 3 out of 5, average. Stars in a 60 sec exposure measure between 1.5 and 1.7 arc sec FWHM in Ha. So, I'm not getting the theoretical resolution now. But I think the image will be quite good anyhow.

Rolando

-----Original Message-----
From: Chris White <chris.white@...>
To: main@ap-gto.groups.io
Sent: Mon, Aug 1, 2022 9:54 pm
Subject: Re: [ap-gto] Resolution of the 110 lens with 3.76 micron pixel size

Exciting to hear Roland!  

Funny to hear you say the "larger" pixels of the 6200 will have to do. 

When you say you are finalizing the reducer/flattener are you talking about just the f5 reducer or have you also settled on a flattener design?

Nice to star seeing more information come out. I was just thinking about this scope earlier today. 

--
Roland Christen
Astro-Physics


Bill Long
 

I cool my QHY600 to -5C, from their dark current graphs, there is no real gain from going lower.

From: main@ap-gto.groups.io <main@ap-gto.groups.io> on behalf of Roland Christen via groups.io <chris1011@...>
Sent: Monday, August 1, 2022 8:12 PM
To: main@ap-gto.groups.io <main@ap-gto.groups.io>
Subject: Re: [ap-gto] Resolution of the 110 lens with 3.76 micron pixel size
 
Heck, my CCD cameras are always running at -25 to -30. I thought these CMOS cameras should be able to get down to at least -15.

Rolando

-----Original Message-----
From: Chris White <chris.white@...>
To: main@ap-gto.groups.io
Sent: Mon, Aug 1, 2022 9:57 pm
Subject: Re: [ap-gto] Resolution of the 110 lens with 3.76 micron pixel size

Looks like you are really horsing that camera cooler too!  I've found that 0 degrees is excellent and benefits below 0 are incremental.  It's a really nice sensor!

--
Roland Christen
Astro-Physics


Eric Weiner
 

Thanks for the 110 update Roland. Very much looking forward to hearing more as info becomes available.

Eric


weihaowang
 

That looks very exciting.  For the flattener, I think F6 to F7 is reasonable.  It will be awesome if it can cover a 44x33mm sensor with ~4 micron pixels.  Actually, I hope the reducer can also do the same.


Chris White
 
Edited

On Mon, Aug 1, 2022 at 11:11 PM, Roland Christen wrote:
I have not yet settled on a flattener F-ratio. Something between F6 and F7 for larger format coverage and better resolution for small objects. depends somewhat on what users are looking for.
 
Roland
Personally, I'd opt for f7 or longer.  There just isnt enough difference between f6 and f5 for me to want both focal lengths.  Something that would be close to seeing limited for average skies would be compelling.  (For the sampling of modern CMOS cameras at 3.76um pixel size)


midmoastro
 

Looking forward to those images :-)
Todd