35 replies to this topic

[deSitter]

"There is some astigmatism. The defocused star is slightly oval close to focus). It is not enough to turn the stars into "plus signs" in focus. In focus, it produces an airy disc with a bright, core with dim diffraction rings. I figure that any scope can do that can deliver a bright airy disc, it is more limited by diffraction than anything else. The diffraction rings are stronger on one side, like slight coma."

 

This is characteristic of miscollimation. My SkyChief's main problem is the mating of the cell to the tube, which is so loose it can be pulled off. I had to bulk up the threads with blue tape.

 

-drl

[ericb760]

I have tested the old sky chief a bit more.

STAR TEST

I used an artificial star and did a star test. It is brighter than most real stars, and brings out more fuzz. My star testing is basic. I can't tell 1/6 wave from 1/7. I have three categories:

1. There's something seriously wrong here...
2. Decent. Don't worry. Enjoy the views.
3. Fantastic! Be happy!

Keep in mind that a star test is VERY sensitive. There is some signs of spherical aberration. The defocused star is a bit more fuzzy on one side of focus than the other. There is some astigmatism. The defocused star is slightly oval close to focus). It is not enough to turn the stars into "plus signs" in focus. In focus, it produces an airy disc with a bright, core with dim diffraction rings. I figure that any scope can do that can deliver a bright airy disc, it is more limited by diffraction than anything else. The diffraction rings are stronger on one side, like slight coma. Something you don't expect to see in a refractor. I guess the lens is a bit decentered because i opened it. It is something I will try to correc.

All in all, the verdict is "decent". I have seen worse, but also better. Maybe even a decent+ if I can center the optics better.


UNDER THE STARS

I took a quick look at Jupiter last night through a hole in the clouds.
The moons look pinpoint. The cloud bands are readily visible. The FOV of the original eyepieces is really narrow. I know there is false color in achromats, but I didn't really notice much.

The most annoying defec is reflections due to scratches inside the diagonal.

The mount is bad. It's very light, which is in itself a good thing. I think the wooden tripod is a bit better than light camera tripod. The alt-az head The most annoying thing is that it is front heavy, and that the thumb screws that fixes it in Alt becomes loose when the scope is moved. Not a good design.


The focuser is pretty ok. Tight at sturdy.

CONCLUSION
I remember everyone was knocking ok these scopes 20 years ago. There were comments like "Is that a Bushnell, or did someone cr@p on my tripod?". It was a bit off putting, and I was convinced that the scope was useless. I don't think it's a Takahashi by any means, but it's decent. Little CA. Fairly sharp. Mostly glass, steel and wood. It's a real telescope with real optics, and certainly way better than the modern cheap frac that Ed Ting recently reviewed.

WHAT DOES THE FUTURE HOLD.
It has spent 35 years in the attic. I want to bring it to life again. I have bought a Vixen bar and a 0.96" - 1.25" adapter, and I will do some imaging one day. Jupiter maybe. :-)

I have a Bushnell Sky Chief Jr. I believe it to be 60/700. I, too, tried an 0.965" to 1.25" adapter but it would not come to focus with it. I ended up buying a 0.965" to 1.25" hybrid diagonal from Scopestuff and it does work. Mine has amazingly crisp views of the moon and planets. I was lucky enough to have a set of rare 60mm rings that I use to attach it to either an AZ3 mount or my C4.5's Polaris.

[Erlend L]

Finally, I got around to take an image through my 40 year old Sky Chief refractor. I piggybacked it on a C14 and used a ASI178mm filter at visual R, G and B filters. A 0.96 - 1.25" adapter from TEMU was used to attach the camera. (The C14 only served as a mount).
.
I'm happy with it, all things considered.

Edited by Erlend L, 14 February 2025 - 02:20 AM.

[deSitter]

Finally, I got around to take an image through my 40 year old Sky Chief refractor. I piggybacked it on a C14 and used a ASI178mm filter at visual R, G and B filters. A 0.96 - 1.25" adapter from TEMU was used to attach the camera. (The C14 only served as a mount).
.
I'm happy with it, all things considered.

That is a LOT of CA for a 60mm f/15 refractor, and I would suspect this lens is assembled wrong. My 60mm f/15 refractors - a Bushnell, an Astro-Optical, and a Kenko are completely colorless.

 

-drl

[Erlend L]

That is a LOT of CA for a 60mm f/15 refractor, and I would suspect this lens is assembled wrong. My 60mm f/15 refractors - a Bushnell, an Astro-Optical, and a Kenko are completely colorless.

-drl

First of all, it's F/11. It has never been completely colorless, but there is little CA visually. I think the problem is that a camera is quite sensitive to violet and UV light. There is always a blue halo in pictures taken with an achromat. Also, the picture is taken with visual R, G and B filters so that the colors are quite a bit off.

[deSitter]

First of all, it's F/11. It has never been completely colorless, but there is little CA visually. I think the problem is that a camera is quite sensitive to violet and UV light. There is always a blue halo in pictures taken with an achromat. Also, the picture is taken with visual R, G and B filters so that the colors are quite a bit off.

f/11 is still slow enough to be essentially colorless. 11/2,4 = right at 5 (4.6) which is the Conrady standard for a colorless achromat. You can check without taking anything apart by examining the reflections from the front. The right-side-up reflection should be larger than the upside-down one. Without disassembling the lens there will be some confusion about the reflection from the rear surface of the back (concave) lens, which is very shallow and so will give a large upside-down image. However this will be much displaced relative to the front reflections if viewed from an angle.

 

-drl

[Erlend L]

f/11 is still slow enough to be essentially colorless. 11/2,4 = right at 5 (4.6) which is the Conrady standard for a colorless achromat. You can check without taking anything apart by examining the reflections from the front. The right-side-up reflection should be larger than the upside-down one. Without disassembling the lens there will be some confusion about the reflection from the rear surface of the back (concave) lens, which is very shallow and so will give a large upside-down image. However this will be much displaced relative to the front reflections if viewed from an angle.

 

-drl

I am pretty sure that neither the lens assembly, nor any lens element is flipped. The only thing a person with some knowledge could get wrong is  to flip the front element. I did, and de CA and SA were out of this world until I fixed it. Now, the star test is decent, and the CA is not noticable on Jupiter visually. It is a little CA on the limb of th moon, but that is to be expected IMO.

 

There is one thing though: There is no air space . I don't know if  there ever was any spacers between the lens elements. In other words, I don't know if this is a true Frauenhofer or a more simple design. The only drawback of achromats without air space is that they are not corrected for coma.  A simple design would make sense because coma would be minscule at that focal ratio. But I am not sure. If anyone else have disassembled the lens cell I would like to know. 

 

I really don't think it is possible to judge CA by this image. You would have to look through the scope. I used a visual A80 blue filter which lets through UV light. The chromatic blur from UV is huge in any achromat. I also refocued between the different color channels, which seemed like a good idea at the time, but it could easily have made things worse. The focuser was a hassle  in the freezing temperature. The 40 y.o grease turns into glue. 

[Kasmos]

I am pretty sure that neither the lens assembly, nor any lens element is flipped. The only thing a person with some knowledge could get wrong is  to flip the front element. I did, and de CA and SA were out of this world until I fixed it. Now, the star test is decent, and the CA is not noticable on Jupiter visually. It is a little CA on the limb of th moon, but that is to be expected IMO.

 

There is one thing though: There is no air space . I don't know if  there ever was any spacers between the lens elements. In other words, I don't know if this is a true Frauenhofer or a more simple design. The only drawback of achromats without air space is that they are not corrected for coma.  A simple design would make sense because coma would be minscule at that focal ratio. But I am not sure. If anyone else have disassembled the lens cell I would like to know. 

 

I really don't think it is possible to judge CA by this image. You would have to look through the scope. I used a visual A80 blue filter which lets through UV light. The chromatic blur from UV is huge in any achromat. I also refocued between the different color channels, which seemed like a good idea at the time, but it could easily have made things worse. The focuser was a hassle  in the freezing temperature. The 40 y.o grease turns into glue. 

Towas like that usually have a ring spacer between the elements. The older ones had a metal ring but yours looks new enough to have had the semi- transparent plastic ones and some had a second one behind the flint. One of mine (a 60/700mm) was sharp but had bothersome CA (yellow and purple fringing) so I replaced it with 3 thinner foil spacers and it vastly improved it.

Edited by Kasmos, 16 February 2025 - 04:13 AM.

[Erlend L]

Towas like that usually have a ring spacer between the elements. The older ones had a metal ring but yours looks new enough to have had the semi- transparent plastic ones and some had a second one behind the flint. One of mine (a 60/700mm) was sharp but had bothersome CA (yellow and purple fringing) so I replaced it with 3 thinner foil spacers and it vastly improved it.

My scope is a Bushnell. There are two thin transparent rings, but I figured they should go between the metal and the glass to protect the glass. I can try to put one or both between the lens elements and see what happens.

[Kasmos]

My scope is a Bushnell. There are two thin transparent rings, but I figured they should go between the metal and the glass to protect the glass. I can try to put one or both between the lens elements and see what happens.

Yes, I know but it's made by Towa.

 

My Penncrest (Towa) has 2 plastic ring spacers and like many others with 2 that have been seen in the Classics forum, they should be arranged like this:

 

1 between the elements and the other between the back of the flint and cell.

[Erlend L]

Yes, I know but it's made by Towa.

My Penncrest (Towa) has 2 plastic ring spacers and like many others with 2 that have been seen in the Classics forum, they should be arranged like this:

1 between the elements and the other between the back of the flint and cell.

Thanks a lot to you all! I guess the lens cell is still not put together correctly after all. Maybe the star test will go from decent to great? I will keep you all updated :-)