Cosmos
Posted 16 February 2024 - 04:11 AM
This chart makes a nice visual.
A lot of telescope math is not linear. It doesn't scale up or down. I wonder if a long refractor focal length might throw some CA outside of the eyepiece, thus reducing the effect. Just a thought that I haven't tried wrapping my head around that much. A 6" F15 rates very nicely in the chart.
Edited by apfever, 16 February 2024 - 10:39 AM.
Messenger
Posted 16 February 2024 - 11:59 AM
I've visited Yerkes during the day but haven't had the privilege of looking through the 40 inch...here's a thread from 2008 that discusses the view through it though. https://www.cloudyni...inch-refractor/
David
I wonder if using just a green filter that blocks red and blue would give sharp and contrasty planetary views on Yerkes? Has anyone here viewed Jupiter or Mars through Yerkes?
Cosmos
Posted 16 February 2024 - 01:23 PM
The above link is what I was wondering about dispersal of some of the CA.
What is slightly related and what got me going is the non linear aspect of a longer Newtonian tube. A given diameter mirror does not need a larger diameter tube the longer the focal ratio is. Just the opposite. The fully illuminated field of a mirror approaches a small convenient size for the upper end of the tube which is the diameter of the tube minus the diameter of the primary. This is why a large long light bucket only needs an upper tube assembly that is 2" larger diameter than the primary for a 2" fully illuminated focal plane. The rest of the light is expanded so much from the long focal length that it can't get into the focuser. I figured that somehow the CA of a crazy long focal length refractor is dispersed out of the eyepiece capability. All the telescopes we deal with on a personal basis probably put more concentrated CA into the eyepiece.
Just a mind thought, I haven't computed any theory.
Still in Old School
Posted 17 February 2024 - 07:45 AM
This chart makes a nice visual.
A lot of telescope math is not linear. It doesn't scale up or down. I wonder if a long refractor focal length might throw some CA outside of the eyepiece, thus reducing the effect. Just a thought that I haven't tried wrapping my head around that much. A 6" F15 rates very nicely in the chart.
Yellow is too much CA for me. But it is amenable to filtration with a Baader Semi-APO filter. That thing is great.
-drl
Fly Me to the Moon
Posted 20 February 2024 - 05:03 PM
I leaned pretty heavily on my friend with a lathe and he made some amazing adapters so I could piece the 3" Unitron finderscope together. Before this was a 75/1200 Unitron lens in a crude plastic pipe cap on an 83mm ID tube (not Unitron standard) with a plumbing parts Jaegers focuser attached with rotting wood. I bought a Unitron Dew Shield and focuser here on CN and my friend's adapters bring it all together. A front adapter that is internal threaded for the lens cell, external threads for the dew shield and then perfect fit on the tube with threaded M4 holes lined up to the existing holes on the tube. On the focuser end, it needed a thin adapter with a small shoulder with holes cut to match the existing focuser threaded holes. Now it's a beautifully functional piece and once sanded down and powder coated, will be visually stunning as well. The king of finderscopes for this Jaegers 6" f/15!
Mine had the same Unitron 75mm focuser on it as yours. The mount on mine was a heavily modified Meade DS-16 with a Byers 11.3" Starmaster RA drive powered by a big stepper drive with a Byers controller and the original 12v DC tangent arm DEC drive plumbed into the Byers controller. I still have that mount in my collection.
Skylab
Posted 02 March 2024 - 11:04 AM
Here's a photo showing the new dew shield on the scope, I like it! Much better functionally and personally I like it more esthetically too.
ISS
Posted 02 March 2024 - 06:51 PM
Skylab
Posted 02 March 2024 - 09:36 PM
Wow. That is such a gorgeous scope! I have Jaegers 6" f/10 and f/15 objectives set aside as retirement projects. I am sooo looking forward to turning these into finished scopes.
I had a lot of fun rebuilding and constructing this one, and the views are WELL worth it! VERY sharp.
Viking 1
Posted 02 March 2024 - 10:29 PM
Skylab
Posted 02 March 2024 - 10:33 PM
What a fabulous job, both by you and your collaborators along the way. The photo you posted a couple of posts back looks like it's straight out of a 70's issue of S&T. It was a lot of fun reading about the project and so happy it came out a success. Happy observing!
But now I'm wondering, did you ever do the DPAC testing, resolve any issues with the missing spacers? Or is that TBD some day?
Yeah, still would like to do it. Star tests look good and planets are quite detailed so I'm not in a rush to do that part yet.
Viking 1
Posted 03 March 2024 - 07:22 AM
Someone here once said something along the lines, if you want to become disappointed with a telescope you like, DPAC it. If it puts up good images, then why not just sit back and enjoy it! Thanks again for a great story.
ISS
Posted 03 March 2024 - 08:58 AM
Not to de-rail, so briefly (for a Southerner): I didn't build my first DPAC Rig until after years of Old Scope collecting - had maybe a half-dozen refractors at that point. I was relieved then pleased to see that those lenses I rated Best through star & sky testing were, indeed, The Best. So, testing can be a real confidence booster, too. (Best of The Best at that time: Swift 838 & Tak FC-50 - proving that 50mm fracs Rule!)
On Topic: Would love to gawk at Mars in that sweet Jaegers 6" F15. Seriously. Never saw any traditional refractor that large at star parties in B'ham, Modesto, or Memphis. CA, schmee-A! What can you see? That's the bottom line. Heck! Tough to imagine even steering that LONG tube, but... the Views!
Edited by Bomber Bob, 03 March 2024 - 09:12 AM.
Skylab
Posted 03 March 2024 - 11:11 AM
On Topic: Would love to gawk at Mars in that sweet Jaegers 6" F15. Seriously. Never saw any traditional refractor that large at star parties in B'ham, Modesto, or Memphis. CA, schmee-A! What can you see? That's the bottom line. Heck! Tough to imagine even steering that LONG tube, but... the Views!
Can't wait for Mars to return! The views are very nice, and the CA so minor as to deem it inconsequential. Here's a video showing how easy it is to steer that long tube. I'm still planning to add some handles or a captain's wheel of some sort in the future.
https://youtube.com/...xpFEoEv-pdHQD4o
ISS
Posted 03 March 2024 - 11:14 AM
Can't wait for Mars to return! The views are very nice, and the CA so minor as to deem it inconsequential. Here's a video showing how easy it is to steer that long tube. I'm still planning to add some handles or a captain's wheel of some sort in the future.
If it is very good Mars will be a thrill in that scope when close.
Nihon Seiko
Posted 03 March 2024 - 12:43 PM
Beautiful scope. I am sure you are loving every second of ownership and use. 
Anachronistic
Posted 09 May 2024 - 07:57 PM
Great job!!!
And the DPAC.....? 
Skylab
Posted 09 May 2024 - 09:23 PM
Great job!!!
And the DPAC.....?
I know, I know! Ok, I'll see what I can do.
Apollo
Posted 10 May 2024 - 02:29 AM
Fly Me to the Moon
Posted 10 May 2024 - 03:31 PM
Someone here once said something along the lines, if you want to become disappointed with a telescope you like, DPAC it. If it puts up good images, then why not just sit back and enjoy it! Thanks again for a great story.
DPAC laid bare what I saw in out of focus images with a good few optics I’ve had. My 6” F10 Jaegers is pretty good, but an F15 I had was not great. I have a SS Wollensack 127mm 1200fl that has one of the best DPAC tests of my optics.
Gemini
Posted 24 October 2024 - 06:31 AM
F15 is the shortest focal length that won't vignette in a 2-inch focuser.
A curious statement.
What does it mean? A 150/F7 doesn't function at full aperture?
Let Me Think
Posted 24 October 2024 - 07:39 PM
A curious statement.
What does it mean? A 150/F7 doesn't function at full aperture?
It depends on the eyepiece, the longer more correct statement would be f/15 is the minimum focal length that won't vignette a 1.5" illuminated field. Or I should have said: "Won't vignette a 2" eyepiece", like a 35 Pan for example. When you draw out the optical path, especially with a diagonal, you'd be surprised what a fast scope cuts off!
Gemini
Posted 25 October 2024 - 02:47 AM
That's very interesting. I didn't know it was possible to obtain a 38-39mm area of full illumination, even at F15.
I will have a look down my slower scopes. I'd imagine there will be plenty of off-axis vignetting.
If your building your own scope like yourself, I can see how it could be a different story.
Thanks for replying.
Leah.
Skylab
Posted 25 October 2024 - 07:34 AM
You really need to ray trace the whole scope including the drawtube and diagonal stops at the exact focal distance to know for sure. I don't think it's as big of a deal though. I would rather have better stray light control and lose 0.1 magnitude of light grasp as a compromise.
Gemini
Posted 25 October 2024 - 10:46 AM
You really need to ray trace the whole scope including the drawtube and diagonal stops at the exact focal distance to know for sure. I don't think it's as big of a deal though. I would rather have better stray light control and lose 0.1 magnitude of light grasp as a compromise.
I agree, however we are talking off-axis vignetting here. A collimation cap allows you to see any on-axis hard vignetting. At least that's how I understand it.
But Sean makes a good point. A slower scope is going to have a bigger area of maximum illumination.
jim_mcgill - May 14 2025 12:24 AM
Vendor and Group Announcements
rschellie - May 13 2025 11:21 PM
Beginners Forum (No Astrophotography)
Lewis75 - May 13 2025 10:23 PM
Electronically Assisted Astronomy (No Post-Processing)
 Cloudy Nights LLC Cloudy Nights Sponsor: Astronomics |
