20 replies to this topic

[Scott in NC]

Some of you may have read my prior thread discussing my newfound adventure into the world of DPAC testing. It’s been quite educational, and has markedly increased my understanding of just what makes these refractors of ours “tick.”  I’ve learned not only what makes our fine refractors so good, but what makes others less than desirable.  And no, I’m not going to revise my sig line “I’ve never met a refractor I didn’t like.”  Here I present the example of a refractor that has one of those “less than desirable” lenses.  Yet, I still like this refractor, as it’s probably taught me more about optics than all of my near perfect apochromats put together.

 

Here in my prior DPAC thread I presented the test results of this particular Orion 120mm f/8.3 doublet achromat:  https://www.cloudyni...nge/?p=12558236.  In summary, the optics appear to have significant overcorrection (perhaps 1/2 wave, although I haven’t measured it), and two large zones centrally and peripherally.

 

So I got to wondering whether maybe a prior owner had disassembled the optics and perhaps reassembled them incorrectly.  The scope was easy enough to disassemble.  The fixed dew shield was simply a press-fit one, and easily slid off.  Next there were three Phillips head screws attaching the lens cell to the tube, and those were easily unscrewed.  The lens retaining ring in front of the optics had two small slots for a spanner wrench. While I don’t own a spanner wrench that large, using the tip of a fine-point Phillips head screwdriver I was easily able to unthread that.  It was attached loosely enough that the tip of a paper clip would have easily worked as well.  Underneath the retaining ring was a rubber grommet, which easily fell right out.  I then carefully removed the lenses from the lens cell.

[Scott in NC]

Scope disassembled, showing all the parts:

 

[Scott in NC]

Optics (not sure which is the crown element and which is the flint):

 

 

 

[Scott in NC]

You’ll note that there are 3 thin spacers between the two elements.  The thinner element was oriented on the outside (skyward).  There were no markings showing how the lenses should be oriented rotationally.  I imagine it probably doesn’t matter, as I doubt a whole lot of thought was put into this when the scope was originally manufactured, but so I’d have a frame of reference as my starting point, I added some pencil marks.

 

The picture I posted above doesn’t clearly show this, but there is a small air gap, a little thinner than a sheet of paper.  Despite the air gap, the lenses are pretty firmly stuck together.  I guess I really don’t need to separate them at this point, as my first thought was to just flip the whole lens stack backwards, reassemble the scope, and then DPAC test it again.  But any thoughts on how these can be carefully separated would be welcome.

 

Time for breakfast, and I’ll be back later.

 

 

[gstrumol]

Optics (not sure which is the crown element and which is the flint):

 

6CC2ED7B-2D61-4027-98F0-5EF9B68B7AE9.jpeg

 

87EDF0F3-03D5-49B2-B018-90E3408B041C.jpeg

Crown is the front element, flint the back, no?

[clearwaterdave]

I have one of those scopes.,I haven't used it much and I don't recall anything "bad" about it.,I picked it up cheap and close by.,I didn't really NEED it.,It is a beast.,and I put a 2 speed focuser on it too that I had gotten for a different scope.,

  I will be following your progress.,Good luck.,

[Scott in NC]

Crown is the front element, flint the back, no?

I guess so.  The thinner element (which I’m assuming is the crown) has a convex outer surface, and the thicker element (which I’m assuming is the flint) has a concave inner surface, which sounds right to me.  And by “outer” I mean skyward, and by “inner” I mean towards the focal plane.

[Scott in NC]

Ah…some basic knowledge acquisition would have helped a little before showing my ignorance for all to see on CN.  

 

From Wikipedia:

 

 

 

[Scott in NC]

So it looks like my lens stack is indeed assembled correctly, and there’s no need to invert the optics and re-DPAC test.  I think the issue is with the way the lens was figured, and there’s nothing further than I can do to optimize it.  Flipping the lens stack would only make things worse.

 

So, it is what it is, but at least I’ve learned something.

[davidc135]

It's a good idea to separate the lenses which may be causing strain if they are stuck. There may be some gunk on the spacers preventing this or, if they are plastic, they may have perished. I'd try a few drops of alcohol or acetone plus soap and water with a rinse after separation.

 

It doesn't look like there are AR coatings.

 

David

Edited by davidc135, 13 March 2023 - 09:16 AM.

[Scott in NC]

Thanks, David.  I’ve reassembled the scope, and may save the rest of this project (incorporating your suggestion) for another day.  
 

Interestingly, despite the significant SA noted on DPAC testing (which of course by definition makes everything look twice as bad as it really is), I never really noticed anything amiss when using it for low power observation of DSOs.  I suspect that planetary observation would be where this aberration would be most apparent, but I don’t think I’ve ever looked at any planets with this particular scope before.  Does one really use an achromat for that kind of thing?  

[Scott in NC]

Here’s another look at this scope’s DPAC test results, showing spherical aberration (over-correction), and central and peripheral zones.

 

Inside focus:

 

 

 

At focus:

 

 

 

Outside focus:

 

[Rutilus]

It's a good idea to separate the lenses which may be causing strain if they are stuck. There may be some gunk on the spacers preventing this or, if they are plastic, they may have perished. I'd try a few drops of alcohol or acetone plus soap and water with a rinse after separation.

 

It doesn't look like there are AR coatings.

 

David

The Synta ones I've worked on over the years had spacers that were stuck on with a self-adhesive backing. 

I suspect that some of the adhesive has worked its way out to the outer edge of the spacer and is holding the

lenses together. I've had this happen more than once with these older lenses.

[davidc135]

Thanks, David.  I’ve reassembled the scope, and may save the rest of this project (incorporating your suggestion) for another day.  
 

Interestingly, despite the significant SA noted on DPAC testing (which of course by definition makes everything look twice as bad as it really is), I never really noticed anything amiss when using it for low power observation of DSOs.  I suspect that planetary observation would be where this aberration would be most apparent, but I don’t think I’ve ever looked at any planets with this particular scope before.  Does one really use an achromat for that kind of thing?  

Rutilus has made many high quality drawings of Mars using a 6'' f8 achromat posted in the Solar system forum. I expect his objective would test very well and much detail is observed in spite of the false colour. He may say more.

 

David

[Jeff B]

Scott, if the rear element, with the really flat surface is pointing at the eyepiece, the overall assembly is correct.  If the elements separate easily, try flipping the front element, making sure the centers of the elements are not touching.  If they don't, reassemble and retest.  I've had some achromats cross my test bench that were misassembled in some way with ugly DPAC results  similar to yours. DAVIDG has had exactly the same experiences. 

 

Jeff 

[russell23]

The sample I had years ago had major spherical over correction.

[Rutilus]

Rutilus has made many high quality drawings of Mars using a 6'' f8 achromat posted in the Solar system forum. I expect his objective would test very well and much detail is observed in spite of the false colour. He may say more.

 

David

The test done by the OP just goes to show what a complete lottery these cheaper made achromats are.

My own 120mm f/8.3 achromat sample that I bought some 15 years ago  has given me some excellent planetary, 

Lunar and double star views over the years.

Not long after buying I re-spaced the lens in my scope (more as an experiment). The photo below shows the

factory spacer ring that was taken out. 

I did side by side comparisons with my 4 inch Takahashi Apo, and the humble achromat actually did rather well,

much better than one would have expected. Below are some observations made of Mars some 13 years ago.

Four made with the Tak  with the other half made using the 120mm scope.

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[Jon Isaacs]

The test done by the OP just goes to show what a complete lottery these cheaper made achromats are.

My own 120mm f/8.3 achromat sample that I bought some 15 years ago  has given me some excellent planetary, 

Lunar and double star views over the years.

Not long after buying I re-spaced the lens in my scope (more as an experiment). The photo below shows the

factory spacer ring that was taken out. 

I did side by side comparisons with my 4 inch Takahashi Apo, and the humble achromat actually did rather well,

much better than one would have expected. Below are some observations made of Mars some 13 years ago.

Four made with the Tak  with the other half made using the 120mm scope.

Rutilus:

 

Your results are quite intriguing.  Did you use any filters?

 

I am interested in how you respaced your 120mm F/8.3?  Did you do it by trial and error?  Do you remember what the final spacing was?

 

As I mentioned earlier, I have an Bresser 120mm F/8.3. I was clearly manufactured by Synta/Skywatcher and I believe it is at least 20 years old.  I do most of my planetary and double star observing with my Dobs and I have some 80mm and 102 mm apo refractor but this scope has captured my eye and I am enjoying using it for double stars and general backyard stargazing.  I just upgraded the focuser to a Crayford.  It's a budget project scope.. 

 

Any help would be appreciated.

 

Jon

[CHASLX200]

I had one of these and sold it local and now have a chance to buy it back. I only used it a few times and had too many scopes and sold it. Mine seemed very good.  I will get some time in and push it hard on a nite of good seeing. My freaky sharp C102 will be hard to beat.

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Edited by CHASLX200, 02 April 2023 - 04:16 PM.

[RichA]

The test done by the OP just goes to show what a complete lottery these cheaper made achromats are.

My own 120mm f/8.3 achromat sample that I bought some 15 years ago  has given me some excellent planetary, 

Lunar and double star views over the years.

Not long after buying I re-spaced the lens in my scope (more as an experiment). The photo below shows the

factory spacer ring that was taken out. 

I did side by side comparisons with my 4 inch Takahashi Apo, and the humble achromat actually did rather well,

much better than one would have expected. Below are some observations made of Mars some 13 years ago.

Four made with the Tak  with the other half made using the 120mm scope.

The only really good 120mm f/8.3 I've seen was an Antares lens, obtained 20 years ago, purportedly made by Vixen.  The diffraction image was near text-book.  But then I've seen a Chinese 200mm achro off Ebay test out exceptionally, so like you said, it is definitely a gamble.

[Echolight]

The only really good 120mm f/8.3 I've seen was an Antares lens, obtained 20 years ago, purportedly made by Vixen.  The diffraction image was near text-book.  But then I've seen a Chinese 200mm achro off Ebay test out exceptionally, so like you said, it is definitely a gamble.

Well I guess I better get a 150 f5 instead then.