20 replies to this topic

[Paul Schroeder]

I have a question for the Questar optical experts here.

 

I’m currently using two older Questars (one from 1971, one from 1974), both of which I believe have their original optics, which should mean that they both have the secondary spot on the outside of the corrector (R1), vs. the inside of the corrector (R2) — as is the case with more recent Questars.

 

The 1974 Questar exhibits some halo around the brightest objects (particularly Jupiter), where that halo is more pronounced at lower magnifications (such as using a 24mm Brandon with no barlow).  With the 24mm Brandon, the halo itself is roughly a dozen Jupiters in diameter.  As magnification increases, the halo becomes less obvious; using a 16m Brandon and the internal barlow, there is very little halo visible.

 

Set up side by side, on the same night and swapping the same Brandon back and forth, the 1971 Q shows no halo.  Over the years I’ve used a number of older Questars, and my rough estimate is about one-quarter of the older Qs exhibit this halo.

 

The halo doesn’t seem to significantly impact the quality of the Jupiter image (if anything it’s sharper in the one with the halo), but it is a bit distracting.  Also, when viewing M42 in the 1974 Q, stars in the Trapezium are extremely sharp, with no halos.

 

I should add that the 1974 has standard coatings, and they look to be in great shape, with no obvious haziness or deterioration to the corrector coatings.  The painted spot over the secondary is in excellent condition as well.  From what I can see, the 1974 and 1971 Qs are very much in comparable condition, when it comes to coatings.

 

I’ve gone through the various comments and suggestions in the thread below, but none of them seem to capture what I see in this scope:

 

https://www.cloudyni...1#entry13073705

 

Any suggestions for what else I can do to diagnose the issue, and possibly correct it?

 

Many thanks!

 

Paul

[ckaine45069]

I do not believe the 74 would be on the Outside of the Corrector-my 73 in on the inside.

[Paul Schroeder]

Thanks for the thoughts!

 

I spent a little more time on this tonight, trying to figure out why a 1974 Questar exhibits a halo around a bright object (such as Jupiter), yet a very similar 1971 Questar does not.

 

- Based on one of the suggestions in the thread I linked above, I tried and was able to focus on the fainter image coming off the interior (R2) surface of the corrector, on both the 1971 Q and the 1974 Q.  I had to focus out counterclockwise a significant amount, but this second image did come to focus, where my target was a neighbor’s outdoor lights about 200 yards away.

 

- I think this confirms that both of these Qs have their secondary spots on the outside of the corrector.  However, when I did the above test, the two scopes performed the same (the fainter versions of the image reflected off of R2 were quite similar), yet in focus the 1971 has no halo, but the 1974 does.

 

- I also took a closer look at the correctors on the two scopes.  The 1971 has BB coatings, but they're in excellent shape; the 1974 has standard coatings, also in excellent shape.  The exteriors of both correctors are very clean (I’m a bit of neat freak), and in examining the interior surfaces, both are quite clean (but not as clean as the exterior).  If anything the 1971 is a little bit dirtier, or has more what look to be cleaning marks — yet the 1971 has no halo.

 

- Finally, I was able to confirm that the problem is not in the prism diagonal of the 1974, by viewing through its axial port (the non-Brandon eyepieces from the 1971 directly thread onto the port).  When viewed straight though, without any diagonal in the light path, the halo in the 1974 remained the same.

 

So, I remain stumped — if anyone has any further suggestions on what I can try please feel free to pass them along!

 

Thanks,

 

Paul

[Gregory Gross]

As a result of your rigorous testing, it sounds like you've conclusively determined that both your scopes have an R1 secondary spot. Questar put into production scopes with secondary spots on the R2 surface of the corrector in 1978. See this discussion for details. (I confess that in prior posts here, here, and here, I mistakenly noted that 1979 was the changeover year.)

 

It also sounds like one variable -- broadband vs. standard coatings -- is at play here. I am by no means an optics expert, but I would hazard a guess that the 1971 scope with the broadband coatings has more effective anti-reflection coatings applied to the inside surface of the corrector lens than the standard magnesium-fluoride anti-reflection coatings of the newer 1974 scope.

 

Based on what Questar wrote when it introduced broadband coatings in 1967, the term broadband referred to the dielectric coating applied to the primary mirror. I assume the same coating was used for the secondary spot. In addition, scopes with broadband mirror coatings also had a VLR (very low reflection) coating which was applied to both sides of the corrector lens. Questar claimed these two improvements improved light grasp by 22%. See this discussion for details.

 

As far as action to take, I would actually suggest that you leave your 1974 scope alone. If your optics are clean (which it sounds like they are), the halo you see around very bright objects may be the result of the nature of the coatings and secondary spot placement, two things you can't easily change or address.

[Gregory Gross]

This may be obvious, but I should add that having more effective anti-reflection coatings is especially critical for Questars with R1 optics because of the problems associated with the secondary spot being on the less optimal outside surface of the corrector lens. The halo you see is unfocused light being reflected off the unmirrored surface of the corrector lens. Although better anti-reflection coatings can help mitigate this, the best solution was to move the secondary spot to the R2 or inside surface of the corrector. Doing so altogether eliminated problems with light being reflected off the unmirrored inside surface of the corrector lens.

 

Braymer's placement of the secondary spot on the R1 surface was a compromise prompted by intellectual property concerns at the time when he first designed the Questar in the 1940s and 50s.

[Gregory Gross]

The engineer in me likes to dwell on the pessimistic side of things. Bottom line, though, is that, if using the scope gives you pleasure, then that's by far the most important thing regardless of where the secondary spot is sitting.

[DAVIDG]

 Having designed and made optics for 40+ years there is another possibility beside the AR coating, that is the refractive index of the glass used to make the correctors. The original correctors were stated to be made from Bausch and Lomb BSC-2 Bausch and Lomb some time in the 70's wasn't a major supplier of optical glass and Schott was coming up to be the preferred supplier.

United Lens was one and still is one of the major supplier on molded optical glass blanks and they would most likely have been the suppliers of the banks to Cumberland and UL would most likely would have used BK7 or some other Schott glass for the molded  Mak correctors. 

 

 So Cumberland could have switched  to Schott glass like BK-7 between 1971 and 1974. They are similar in refractive index but not exactly the same. That would require that the corrector to  be redesign with different radii. A different radius on R2 could cause the ghost image to be present. It is common issue in designing  objective lenses to choose radii that while giving good correction also don't introduce ghost images.

 

                  - Dave  

[Paul Schroeder]

Dave and Gregory, thanks for sharing your thoughts, they're very helpful.  It's sounding like there might not be a lot I can do mitigate this ghost image.

 

I hope to get the scope out again soon to give it a test on a somewhat wider range of objects.

 

Paul

[GR1973]

I have a question for the Questar optical experts here.

I’m currently using two older Questars (one from 1971, one from 1974), both of which I believe have their original optics, which should mean that they both have the secondary spot on the outside of the corrector (R1), vs. the inside of the corrector (R2) — as is the case with more recent Questars.

The 1974 Questar exhibits some halo around the brightest objects (particularly Jupiter), where that halo is more pronounced at lower magnifications (such as using a 24mm Brandon with no barlow). With the 24mm Brandon, the halo itself is roughly a dozen Jupiters in diameter. As magnification increases, the halo becomes less obvious; using a 16m Brandon and the internal barlow, there is very little halo visible.

Set up side by side, on the same night and swapping the same Brandon back and forth, the 1971 Q shows no halo. Over the years I’ve used a number of older Questars, and my rough estimate is about one-quarter of the older Qs exhibit this halo.

The halo doesn’t seem to significantly impact the quality of the Jupiter image (if anything it’s sharper in the one with the halo), but it is a bit distracting. Also, when viewing M42 in the 1974 Q, stars in the Trapezium are extremely sharp, with no halos.

I should add that the 1974 has standard coatings, and they look to be in great shape, with no obvious haziness or deterioration to the corrector coatings. The painted spot over the secondary is in excellent condition as well. From what I can see, the 1974 and 1971 Qs are very much in comparable condition, when it comes to coatings.

I’ve gone through the various comments and suggestions in the thread below, but none of them seem to capture what I see in this scope:

https://www.cloudyni...1#entry13073705

Any suggestions for what else I can do to diagnose the issue, and possibly correct it?

Many thanks!

Paul

Is it a halo with defined outer edge ring or just diffuse glare or around Jupiter?

[Paul Schroeder]

It is very clearly a halo, a ring that is distinct from Jupiter, not a diffuse glare around Jupiter.  In focus, the images of Jupiter itself are quite sharp and contrasty, there's just a halo out there, separate from the planet.

 

Paul

[DAVIDG]

Paul,

   Do both  scopes have the same focal length ? Is the length of the tube the same and does it look like the diameter of the baffle tube on both scopes are the same ?  When you look up the baffle do you see any differences like an aperture stop ? 

   I know that when the design changed to put  the spot on R2 that the tube was increased in length so the baffling should have changed as well. So if the designed changed  because of the refractive index of the corrector the baffling should have changed as well. That might not have happened if they had an inventory of mechanical parts. 

 

             - Dave 

[Gregory Gross]

When I observe the Moon with my 1962 Questar with R1 optics, I see a uniform area of haze surrounding the Moon's disk. That area has a blurry but still distinct outer edge. I have always assumed that that is the unfocused light reflecting off the unmirrored inside surface of the corrector lens. But the halo or ring that Paul is describing sounds different than what I'm seeing in my '62 R1 Questar.

 

Dave, I know you have extensive experience in optics, and I'm glad you chimed in on this thread. Are you suggesting that the baffle tube might not be sufficiently wide or long enough? Is it slicing into the cone of light off the secondary mirror?

[Paul Schroeder]

Hi Dave --

 

As best I can tell the optical tubes are the same length, at about 8 1/2" (see photo below).  I also inspected the baffle tubes from the corrector end and through the axial port, and while a more discriminating person might detect a difference, they looked identical to me.

 

I recall discussing this issue with Jim Reichert at Questar maybe 15 years ago, as it related to a 1969 Questar?  IIRC he was pretty sure that he knew what the issue was -- if only I could recall what he said . . . .

 

Paul

 

 

Attached Thumbnails

• 

[DAVIDG]

 On the R1 Questars the MgF2 coating was applied to only to the rear surface of the corrector. This is because the light goes in and out of  that surface. At least that is what I have seen of the Questars I have examined and believe it states that in their old literature.

 My own 1961 Questar was like this until I sent the corrector back to Cumberland to recoat it since the MgF2 was getting hazy in my scope. Cumberland recoated both the inner and outer surface of the corrector and replace the secondary spot. 

   Does the Questar that doesn't shows the halo look to be coated on the outer surface of the corrector ?  I'm also wondering if the size of the secondary spot is same in both scopes ?  If the one that shows the halo has a larger spot that could be why since it is picking up off axis light.   On the Meade ETX 90 the baffle they use on the secondary spot actually stops down the spot size. 

  I just looked at my 1961 Q and the end of the baffle tube is nicely blacken along with the inside but in the middle of the outside of the baffle is a shiny ring. Is that ring shiny on both your scopes ? 

 

                - Dave 

[Paul Schroeder]

Dave, thanks for putting the time and thought into this --

 

In response to your questions:

 

 - If you look at the photo below (specifically the reflections on the corrector), my take is that it indicates only the scope on the right has coatings on the exterior of the corrector.  This is indeed the 1971 which does not show the halo.

 

 - When I look at the painted spots on the correctors, and attempt to measure them, they appear to be the same size

 

 - Both of these Questars have the shiny ring on the exterior of the baffle tube, as you describe

 

 - In addition, when viewing the inside of the baffle tube, from the axial port, in both Qs there's a section that relatively speaking is shinier than the rest of the baffle tube.  As best I can tell, the two scopes are equivalent in this regards.

 

I had the 1974 out again last night, on Jupiter.  When Jupiter was perfectly centered in a 24mm Brandon so too was the halo; the halo itself extended pretty much to the edge of the field, and was a few Jupiter diameters thick (it's not a thin ring, but rather a broader one).  What it's most reminiscent of is very defocused star test, which I think supports that it might be a ghost image off of R2.  

 

When Jupiter moved off center, the halo became skewed (no longer round), much as a defocused Q star test becomes skewed when moved off center.

 

I would be surprised if the scope originally shipped exhibiting this kind of image, which to me at least suggests that something has changed, where the leading theory is that  the anti-reflection coatings on the inside of the corrector have become less effective.  However, as best I can tell, there's no general haze around Jupiter, which might suggest the coatings haven't completely failed.

 

Not sure if there's a lot that can be done to improve this scope.  I do think cleaning the inside of the corrector might prove helpful, as removing 50+ years of whatever has accumulated won't likely hurt.  Beyond that, I don't see a lot of options, unless Cumberland reappears and can re-coat this.

 

 

Attached Thumbnails

• 

[DAVIDG]

 Hi Paul,

    When it comes to the secondary what  would matter is  the actual size of the spot vs the painted spot. In a dark room if you shine a bright flashlight in the axial point you should be able to see the outline the aluminumized spot through the black paint.

   Just to clarify, the 1971 Q is the one with the star map/dew shield and it does not show the halo but the duplex version is the one from 1974  and is the one that does or do I have that backwards ?

 

                   - Dave 

[Paul Schroeder]

Hi Dave -- 

 

I tried your "bright light through the axial port" test, and as best I could tell the aluminized spots looked to be the same size.

 

And you're right, the 1971 standard (with the star map dew shield, on the right in the above photo) does not show any halo, while the 1974 duplex does.

 

Thanks again,

 

Paul

[DAVIDG]

 Paul,

   Clearly there is some difference in the two scope especially since one is  a Duplex. I thought that I read that the Duplex had a larger range of focus since it could be used for bird watching. If that is true then to achieve that the primary would need a larger range of travel then the standard model. That may have caused the primary to be in different position to achieve focus at infinity and that would require the baffling  to be different. We had a long distance microscope Questar in one of our labs and it  had a longer barrel  to increase the spacing between the primary and corrector to allow  it to achieve focus closer up. 

   Do you have the photographic stop down ring ? If so try that on the Duplex and see if the halo goes away. If it does then that points to the baffling not be ideal in the Duplex.

 

                    - Dave 

[DAVIDG]

 Paul,

    I just noticed the your two Q's have different eyepiece adapters. Are they the same height and is ID of barrel were they screw into the body the same ? If the ID on the  star map Q is smaller then it is acting as aperture stop and that could be blocking the halo.

 

                  - Dave 

[Paul Schroeder]

Hi Dave --

 

More good questions!

 

Relative to the eyepiece adapters, this is something I'd previously considered.  If you look at the photo below, you can see that the diameter of the threaded secion is equivalent, although there is some slight height difference (the one on the right is from the 1971 standard, which has no halo).  I think I've already tried the following, but tonight I'll swap eyepiece + adapter between the two scopes to see if that has an impact.

 

I do have the Questar camera coupler and a few other rings, but I don't see how any of them could be used to stop down the aperture of the eyepiece or eyepiece adapter.  I also have the close focus ring (see second photo below), which I could try to see if it makes any difference.

 

Paul

 

 

 

 

Attached Thumbnails

• 
• 

[Paul Schroeder]

Hi Dave --

 

More good questions!

 

Relative to the eyepiece adapters, this is something I'd previously considered.  If you look at the photo below, you can see that the diameter of the threaded secion is equivalent, although there is some slight height difference (the one on the right is from the 1971 standard, which has no halo).  I think I've already tried the following, but tonight I'll swap eyepiece + adapter between the two scopes to see if that has an impact.

 

I do have the Questar camera coupler and a few other rings, but I don't see how any of them could be used to stop down the aperture of the eyepiece or eyepiece adapter.  I also have the close focus ring (see second photo below), which I could try to see if it makes any difference.

 

Paul

Further to my note above, I tried swapping the eyepiece + adapter from the 1971 with the eyepiece + adapter from the 1974, but it made no difference.  The 1974 continued to have a halo around bright light sources.

 

I also tried using the close focus ring pictured above.  That too made no difference.

 

I remain stumped . . . thanks again for your help!

 

Paul