I bought a 6" f15 Jeagars lens and cell for $40 at a astro flea market a couple of years ago. I could not believe my luck. Just the night before I was telling a friend that one of my biggest astro-regrets was selling a scope I made with a 4" f15 Jeagars lens. Well over the past few years I have been building this scope and making it better and better (an additional $2K so far), but the scope does not seem to have quite the "snap" focus that my old 4" had. I wonder if it could be an alignment issue. Is there an easy test to tell?
aligning objective lens
Started by Jonathan Lingel, 12/31/2014 08:34PM
Posted 12/31/2014 08:34PM
Opening Post
Posted 12/31/2014 09:01PM
#1
Jonathan Lingel said:
I bought a 6" f15 Jeagars lens and cell for $40 at a astro flea market a couple of years ago. I could not believe my luck. Just the night before I was telling a friend that one of my biggest astro-regrets was selling a scope I made with a 4" f15 Jeagars lens. Well over the past few years I have been building this scope and making it better and better (an additional $2K so far), but the scope does not seem to have quite the "snap" focus that my old 4" had. I wonder if it could be an alignment issue. Is there an easy test to tell?
It could be alignment. The first thing I did with my 6" F/15 was to align the focuser with the center of the lens. I used my Glatter 2" laser, and a lens cap with a small hole on center. Aligning that made a big difference for me. With that long of a tube, the slightest tilt at the focuser makes a huge difference.
Then, in dim light, you can see the reflection of the laser bouncing back to the focuser to align the objective if need be.
Posted 12/31/2014 09:13PM
#2
What Jeffery said....laser is the best and most accurate way to test it....I just used one to rough align a mock up for an 8" f/16 I am building.....
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Stellarvue M7 Alt-Az, TSL7 Pier/Tripod
12.5" f/5 MidnighTelescopes DOB Swayze optics
Monroe GA
Posted 12/31/2014 09:53PM
#3
Lenses like the 6"F15 Jaegers are very insensitive to misalignment. Unless the cell is tilted by an obvious amount (several degrees), the lens really won't perform any better or worse.
Jaegers lenses, especially the 6"F15, had huge variations in accuracy of figure. Out of any 5 specimens, you could see a variation from some of the best to some of the absolute worst ever made by an optical shop.
I had a 6"F10 lens that had essentially perfect correction with no internal defects or striae in the glass. It produced a textbook perfect star image at high powers. One of my astronomy friends had a 6" F15 which produced nothing but a muddled fuzzy mess on any star. Either the glass was defective internally or the surfaces were so roughly polished that it was full of zonal defects. We could not put any kind of wavefront error on this lens, it was essentially all over the place.
The only way to know what you have is to put an eyepiece to the lens, aim it at a star, run the focus inside and outside and see what the extra-focal Fresnel disc looks like. get familiar with star testing and objective so you can recognize what defects might be present.
If you see that the lens might have huge spherical error, don't immediately despair. The previous owner may have taken it apart to clean it, and assembled the elements backwards.
There are 4 ways you can stack the two elements, but only one way is correct. That would be crown leading, with the shallower of the two convex crown surfaces toward the sky and the flint concave surface behind the steeper convex crown surface, also pointing toward the sky. There should be a small spacer of a few mm thickness between the two elements. Thickness of the spacer is not super critical, however it must be exactly the same thickness for the three.
Rolando
Jaegers lenses, especially the 6"F15, had huge variations in accuracy of figure. Out of any 5 specimens, you could see a variation from some of the best to some of the absolute worst ever made by an optical shop.
I had a 6"F10 lens that had essentially perfect correction with no internal defects or striae in the glass. It produced a textbook perfect star image at high powers. One of my astronomy friends had a 6" F15 which produced nothing but a muddled fuzzy mess on any star. Either the glass was defective internally or the surfaces were so roughly polished that it was full of zonal defects. We could not put any kind of wavefront error on this lens, it was essentially all over the place.
The only way to know what you have is to put an eyepiece to the lens, aim it at a star, run the focus inside and outside and see what the extra-focal Fresnel disc looks like. get familiar with star testing and objective so you can recognize what defects might be present.
If you see that the lens might have huge spherical error, don't immediately despair. The previous owner may have taken it apart to clean it, and assembled the elements backwards.
There are 4 ways you can stack the two elements, but only one way is correct. That would be crown leading, with the shallower of the two convex crown surfaces toward the sky and the flint concave surface behind the steeper convex crown surface, also pointing toward the sky. There should be a small spacer of a few mm thickness between the two elements. Thickness of the spacer is not super critical, however it must be exactly the same thickness for the three.
Rolando
Funding Member
