52 replies to this topic

[Scott in NC]

We've done a lot of apochromats and ED doublets lately, so I decided to shift gears a little and show you guys what a decently well-corrected achromat looks like on DPAC.  About a month or two ago I posted some green-light DPAC images of my Stellarvue SV80/9D, an 80mm f/9.4 doublet achromat that I've owned since 2004. This scope holds a special place in my heart, as it was the first refractor that I ever purchased, and started me down this road of refractor fanaticism.

 

Here's a picture of the set-up, just to show what the scope looks like. Since I last tested this scope, I've refined my photographic technique, and now use a recently acquired Canon 100mm f/2.8 macro lens, and taking the advice of one of my friends here I now use a black t-shirt as a backdrop to eliminate stray light and reflections.

 

[Scott in NC]

SV80/9D in green light

 

Inside focus:

 

 

 

At focus:

 

 

 

Outside focus:

 

[Scott in NC]

Pretty nice in green, huh?

[Scott in NC]

Now let's look at how colorful an achromat actually is (even a pretty well-corrected one with a relatively long focal ratio of f/9.4) compared to all the apochromats and ED doublets that we've seen lately.  White light really shows this scope's true colors (pun definitely intended!).

 

Inside focus:

 

 

 

Outside focus:

 

[Scott in NC]

One thing that these images illustrate really well is just how far apart the focal points for violet and yellow are.  
 

I also used an image processing program called GIMP that allows me to isolate the R, G, and B channels, but can't post those here as I haven't yet figured out how to save the images.  I hadn't used GIMP in several years and am having difficulty sorting out the most recent version, so if anyone knows how to do this, any help would be greatly appreciated!

[lee14]

Nothing like empirical evidence to separate fact from opinion. Well done!

 

Lee

[gstrumol]

One thing that these images illustrate really well is just how far apart the focal points for violet and yellow are.  
 

I also used an image processing program called GIMP that allows me to isolate the R, G, and B channels, but can't post those here as I haven't yet figured out how to save the images.  I hadn't used GIMP in several years and am having difficulty sorting out the most recent version, so if anyone knows how to do this, any help would be greatly appreciated!

Perhaps I could help, Scott.

 

I took your colorful image into GIMP and then clicked on the colors menu. Then slide down to levels. You'll get a pop up box as shown here:

 

(click to enlarge)

 

I then went to Channel: and selected Red. Go to 'output levels', grab the little white triangle to the right and drag it all the way to the left (effectively removing the red from the image), do the same for Green. This gives you the bule only channel, as you see.

 

Finally go to File and Export. There you can save the image in whatever format you like!

[Scott in NC]

Thanks!  That’s pretty much what I did, although used another method to separate out the individual channels. When I went to save those images using the File—>Export function, it just re-saved the original white-light multi-colored version.  Maybe I need to try your method of separating out the colors?  I’ll give it another try.

[gstrumol]

Thanks!  That’s pretty much what I did, although used another method to separate out the individual channels. When I went to save those images using the File—>Export function, it just re-saved the original white-light multi-colored version.  Maybe I need to try your method of separating out the colors?  I’ll give it another try.

That will work. To do another color just click on [base image] on the left and you'll be back to the colorful image you started with. Then repeat the process to get the red (by eliminating the blue and green) and export again.

 

Easy peasy!

[CHASLX200]

Looks ok to me. I would need to take it to 300-400x and see how a star image would look like to really know.

Edited by CHASLX200, 15 April 2023 - 01:29 PM.

[Scott in NC]

Yeah…we know. 

[Scott in NC]

Thanks, Gary—that worked!  Apparently the method I used was just a way of viewing the isolated channels but not a method of actually changing the image itself in a manner that can be saved.  

[gstrumol]

Thanks, Gary—that worked!  Apparently the method I used was just a way of viewing the isolated channels but not a method of actually changing the image itself in a manner that can be saved.  

Happy to help!

[Echolight]

Looks well figured at least. Even with the "chromatic blur", likely sharper than a butter knife.

[Scott in NC]

With Gary's help (thanks again!) I've learned how to save and post the individual color channels after extracting them with GIMP.

 

First red.

 

Inside focus:

 

 

 

Outside focus:

 

[Scott in NC]

Now green.

 

Inside focus:

 

 

 

Outside focus:

 

[Scott in NC]

And last blue.

 

Inside focus:

 

 

 

Outside focus:

 

[Scott in NC]

I've got to run to get some lunch right now, but will be back a little later with some commentary.

[Scott in NC]

Red and green are reasonably close together in focus, and blue is much farther out.  You can tell this because when those colors are isolated from the white-light image, the red/green banding looks pretty similar, but blue has a greater number of bands in the inside-focus image and a lesser number of bands in the outside-focus image, when compared to red and green.  This is so because the major determinant of how many bands you see in the Ronchi image is how close you are to focus.  The farther from focus, the greater the number of bands, and the closer to focus, the fewer the number of bands.

 

And the short version for those who'd rather not read all the gobbledegook above: Achromats have significantly different focal points for the various wavelengths of light compared to apochromats. Not that you all didn't already know that, but sometimes it's nice to see it illustrated.

[Scott in NC]

Spherical correction looks pretty good in all colors (especially green), considering this is an inexpensive achromat produced nearly 20 years ago. Interestingly, I see a minor edge abnormality which doesn't really show up in the images shot in green light, but does show up in the image shot in white light, with the green channel extracted via image processing.  See here:

 

Green light Ronchigram:

 

 

 

White light Ronchigram with green channel extracted:

 

 

 

Anyone have any ideas as to why this might be?

[scout]

Yeah…we know. 

Haha, no matter what the topic, Chas has to add how he uses physics-defying magnifications with a 0.1 exit pupil.

[CreatorsHand]

Spherical correction looks pretty good in all colors (especially green), considering this is an inexpensive achromat produced nearly 20 years ago.  Interestingly, I see a minor edge abnormality which doesn't really show up in the images shot in green light, but does show up in the image shot in white light, with the green channel extracted via image processing.  See here:

 

Green light Ronchigram:

 

IMG_8511.JPG

 

 

White light Ronchigram with green channel extracted:

 

IMG_8524green.JPG

 

 

Anyone have any ideas as to why this might be?

It looks to me like the minor edge abnormality is still there in the green LED Ronchigram, but the image is dimmer so it isn't as visible. You might try increasing the brightness of the green LED image; you may find the edge abnormality shows up better.

[CHASLX200]

Haha, no matter what the topic, Chas has to add how he uses physics-defying magnifications with a 0.1 exit pupil.

I always use crazy powers on any scope to see how far i can go. Sloppy optics won't cut it.

[Scott in NC]

Yeah, okay, but what I’m trying to show here is something that I can actually do on planet Earth, day or night, no matter what the weather. I’m glad that you have perfect skies most of the time, but I can’t hop on a plane with a scope and travel to your house every time I want to test out a scope.  You clearly don’t need to do DPAC testing, Chas, but I’m trying to let the other 99.9999999875% (that’s 1 minus (1 divided by 8 billion), multiplied by 100 to turn it into a percentage) of the world know an alternative way to evaluate a scope.

[CHASLX200]

Yeah, okay, but what I’m trying to show here is something that I can actually do on planet Earth, day or night, no matter what the weather. I’m glad that you have perfect skies most of the time, but I can’t hop on a plane with a scope and travel to your house every time I want to test out a scope. You clearly don’t need to do DPAC testing, Chas, but I’m trying to let the other 99.9999999875% (that’s 1 minus (1 divided by 8 billion), multiplied by 100 to turn it into a percentage) of the world know an alternative way to evaluate a scope.

I would love to sky test scopes that i can see a DPAC on and compare.