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Field Flatteners for Achromats, Visual Only

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#1 Sarkikos

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Posted 27 June 2019 - 03:56 PM

Anyone have any information on field flatteners for achromats, visual only?   No AP or NV or binoviewers, please.  Just an eyepiece in a diagonal.

 

Here are a couple data points from my own experience to start the ball rolling:

 

- The TS2FF works great for visual on the C80ED.  Just screw it onto the end of a 2" diagonal, insert the diagonal in the scope, put an eyepiece in the diagonal and start viewing.  Sharp stars at edge of field and center of field!  The image is getting close to something like a Petzval APO.  (I said close, people, not exactly.  No pouncing from the perfectionists.)  My eyes no longer accommodate for focus, so I am a good judge of a flat field vs field curvature.

 

- The TS2FF does not improve the image in my ST120, at least not so far.  I screwed the TS2FF onto the end of a Baader 2" Click-Lock mirror diagonal.  I had replaced the original focuser with a GSO Crayford (not the low-profile one).  

 

I have an ST120, 150 Omni XLT f/5, ST120 f/5 and Bresser AR-102xs 4.5 that need to have their fields flattened.  Anyone have experience with something that works?  And please keep explanations complete but simple.  If something works - or didn't work - explain exactly how you set it up.

 

Mike


Edited by Sarkikos, 27 June 2019 - 04:05 PM.

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#2 howardcano

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Posted 27 June 2019 - 04:12 PM

Since the focal lengths, and therefore the field curvature, of the C80ED and the ST120 are very similar, I'm guessing the big difference in the performance of the field flattener is the focal ratio difference between the two scopes.  Either your field flattener isn't happy at F/5, or the smaller depth of focus at F/5 accentuates whatever field curvature remains.  Have you tried stopping the ST120 down to 80mm?


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#3 Sarkikos

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Posted 27 June 2019 - 04:15 PM

No, I never stop down telescopes.  If I can't use the full aperture, I don't bother.  Maybe stopping down the ST120 would help explain why the TS2FF doesn't work.  But it still wouldn't work with full aperture, so I'm not going to bother.

 

Mike


Edited by Sarkikos, 27 June 2019 - 04:19 PM.

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#4 howardcano

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Posted 27 June 2019 - 04:16 PM

I understand, but doing so might at least answer why it's not working on the ST120.

 

By the way, I'm currently (and slowly!) assembling a scope using a 100mm F/5.5 triplet with a built-in Orion Variable Field Flattener 29567.  In preliminary tests, this flattener has shown promise with the ST120.


Edited by howardcano, 27 June 2019 - 04:17 PM.

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#5 Sarkikos

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Posted 27 June 2019 - 04:24 PM

How did you set up the Orion Variable Field Flattener 29567 in the ST120?  How was the image?  How flat was the field?

 

Mike



#6 sg6

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Posted 27 June 2019 - 04:31 PM

Assume the same as for an ED as both are doublets of crown and flint glass. Basically they are the same.

Likely to cost more for a flattener then the scope might be a hiccup.



#7 howardcano

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Posted 27 June 2019 - 04:49 PM

How did you set up the Orion Variable Field Flattener 29567 in the ST120?  How was the image?  How flat was the field?

 

Mike

I used a 2" extension tube on top of the field flattener as an eyepiece holder; the filter threads on the extension tube match one of the adapters that came with the flattener.  (The eyepiece can also be similarly connected directly to the flattener, though that's not very convenient.)  The flattener has a helical adjust for setting the spacing to the focal plane, much like on a Paracorr.

 

The preliminary tests were straight-through viewing during the day, using a distant house as the target.  I compared the best-focus positions at center and edge of field, measuring the focuser extension with a dial caliper. I was able to adjust the flattener for zero measurable best-focus shift.   But my eyes are not as sensitive to exact focus on a house in the daytime as on a star at night, hence my use of the word "preliminary".

 

The scope I'm assembling will use a Newtonian-style diagonal mirror in the main tube, with the helical portion of the field flattener serving as a focuser.  (It's a bit coarse, but usable at low magnifications.)  Think of a poor-man's cross between the Bresser AR102S "Comet Series" and a TV NP101.


Edited by howardcano, 27 June 2019 - 04:55 PM.

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#8 Sarkikos

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Posted 28 June 2019 - 07:34 AM

Assume the same as for an ED as both are doublets of crown and flint glass. Basically they are the same.

Likely to cost more for a flattener then the scope might be a hiccup.

The cost of a field flattener is not much compared to many eyepieces and other accessories.  If the FF can be used in more than one scope, the cost is partly mitigated.  

 

If the FF flattens the field in an ED, it makes the scope a near equivalent to a Petzval.  Of course, the FF isn't going to help the CA in the scope.  But a Petzval APO is going to cost quite a bit more than an ED with a TS2FF.  

 

Besides, last night I found out the TS2FF works great in my Bresser AR-102xs f/4.5.  Anyone know where they can get a a 102mm f/4.5 Petzval?  If so, how much would it cost?  lol.gif

 

Mike


Edited by Sarkikos, 28 June 2019 - 07:47 AM.

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#9 Sarkikos

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Posted 28 June 2019 - 07:46 AM

Yes, last night I took out the Bresser AR-102xs.  It's an f/4.5 achromat.  I screwed the TS2FF directly onto the end of a Baader Click-Stop 2" diagonal.  I put a 13 Ethos in the focuser.  I viewed the stars around Sadr in Cygnus.  The stars were sharp from center to edge of field.  It was difficult to tell if focusing separately for center of field and edge of field made any difference.  That's how well the field was flattened with the TS2FF.  And remember, my accommodation for focus is virtually nonexistent now.  

 

Removing the TS2FF and observing the same area in Cygnus with the 13 Ethos produced an obvious increase in apparent field curvature.

 

I also tried the experiment with a 32 Masuyama.  With that eyepiece it was difficult to tell if the TS2FF improved the view much at all.  But I think this is due to the outer field aberrations produced by the 32 Masuyama in fast scopes.  

 

I would have used the 21 Ethos but it was difficult to balance in the AR-102xs.

 

Mike


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#10 howardcano

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Posted 28 June 2019 - 07:57 AM

What would you estimate the spacing to be from flattener to focal plane with the flattener screwed into the Baader diagonal?


Edited by howardcano, 28 June 2019 - 07:57 AM.


#11 25585

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Posted 28 June 2019 - 07:57 AM

The cost of a field flattener is not much compared to many eyepieces and other accessories.  If the FF can be used in more than one scope, the cost is partly mitigated.  

 

If the FF flattens the field in an ED, it makes the scope a near equivalent to a Petzval.  Of course, the FF isn't going to help the CA in the scope.  But a Petzval APO is going to cost quite a bit more than an ED with a TS2FF.  

 

Besides, last night I found out the TS2FF works great in my Bresser AR-102xs f/4.5.  Anyone know where they can get a a 102mm f/4.5 Petzval?  If so, how much would it cost?  lol.gif

 

Mike

Pre-owned TV Genesis F5 



#12 Sarkikos

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Posted 28 June 2019 - 08:14 AM

I like to buy new if I can.

 

Mike



#13 howardcano

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Posted 28 June 2019 - 08:34 AM

I like to buy new if I can.

 

Mike

Same here.  I have been screwed too many times to risk that much money on a used telescope.  I'd love to buy a Televue Petzval, but I would need to wait for a used one to pop up at a good price and within reasonable driving distance.  That hasn't happened in decades of waiting.


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#14 Vla

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Posted 28 June 2019 - 10:08 AM

If it is only for visual use, it could be fixed with a pair of small equal-radius plano lenses inserted into eyepiece barrel. Here's illustration on a 150mm f/5 achromat. The only significant aberration such corrector induces is astigmatism of the opposite sign to that of achromat's Petzval, thus flattening the field. Final result is more than twice lower astigmatism and flat field.

Btw what may appear as field curvature is probably astigmatism. Full accommodation requires about 6 diopters (from infinity to about 8-9 inches), so most people would at least come close to it. But even when fully accommodated, the astigmatic blur at 1.5 degrees off axis (nearly 20mm) is about 10 times the Airy disc diameter. Since its angular size in the eyepiece is given by 4.6/P in arc minutes, where P is the eyepiece exit pupil diameter, with, say, about 4-5mm exit pupil the Airy disc is about 1 arc minute, and the blur about 10 - easily recognizable as a spot.

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#15 howardcano

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Posted 28 June 2019 - 11:42 AM

I always learn something new from you, Vla!  Thanks!


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#16 Sarkikos

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Posted 28 June 2019 - 04:30 PM

If it is only for visual use, it could be fixed with a pair of small equal-radius plano lenses inserted into eyepiece barrel. Here's illustration on a 150mm f/5 achromat. The only significant aberration such corrector induces is astigmatism of the opposite sign to that of achromat's Petzval, thus flattening the field. Final result is more than twice lower astigmatism and flat field.

Btw what may appear as field curvature is probably astigmatism. Full accommodation requires about 6 diopters (from infinity to about 8-9 inches), so most people would at least come close to it. But even when fully accommodated, the astigmatic blur at 1.5 degrees off axis (nearly 20mm) is about 10 times the Airy disc diameter. Since its angular size in the eyepiece is given by 4.6/P in arc minutes, where P is the eyepiece exit pupil diameter, with, say, about 4-5mm exit pupil the Airy disc is about 1 arc minute, and the blur about 10 - easily recognizable as a spot.

What I see in these fast achros is mostly field curvature, since when I focus the telescope separately for center of field or edge of field, I can reach comparatively sharp focus for each, but not both at the same time.   I can focus out field curvature.  I cannot focus out astigmatism. 

 

Besides possible astigmatism in the outer field produced by some telescopes, a specific eyepiece can add its own astigmatism or other outer field aberrations.  I see that in the 32 Masuyama, for instance when used in fast scopes.  

 

I know that as I've grown older, my eyes have lost most if not all of their ability to accommodate for focus.  I see this, for instance, in the Baader Zoom Mark III eyepiece which seemed virtually parfocal to my eyes years ago, but now I need to focus differently for short vs long focal length setting.  (This is even after doing the old trick of focusing initially for shortest focal length.)

 

The TS2FF does really seem to flatten the field for both the C80ED and the Bresser AR-102-xs.  I simply screw the FF onto the diagonal and see the improvement.  Of course, the CA in the Bresser is still there.  Not a good telescope for viewing Jupiter! grin.gif

 

Mike


Edited by Sarkikos, 28 June 2019 - 04:38 PM.


#17 Sarkikos

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Posted 28 June 2019 - 04:32 PM

I always learn something new from you, Vla!  Thanks!

 

So do I.  Or at least I try to understand and learn something.   If I don't always succeed, it's my own fault, not Vla's.

 

:grin:

Mike



#18 Sarkikos

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Posted 28 June 2019 - 04:35 PM

If it is only for visual use, it could be fixed with a pair of small equal-radius plano lenses inserted into eyepiece barrel. Here's illustration on a 150mm f/5 achromat. The only significant aberration such corrector induces is astigmatism of the opposite sign to that of achromat's Petzval, thus flattening the field. Final result is more than twice lower astigmatism and flat field.

 

It would be nice if the manufacturers of these fast achros - or maybe an after-market company - would provide a corrective lens assembly for their specific telescopes, to flatten the field … for visual.  It's not something I'd be able to figure out and put together.

 

Mike


Edited by Sarkikos, 28 June 2019 - 04:40 PM.

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#19 Sarkikos

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Posted 28 June 2019 - 06:12 PM

What would you estimate the spacing to be from flattener to focal plane with the flattener screwed into the Baader diagonal?

About 140mm.

 

Mike



#20 mrsjeff

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Posted 29 June 2019 - 01:57 AM

Yes, last night I took out the Bresser AR-102xs. It's an f/4.5 achromat. I screwed the TS2FF directly onto the end of a Baader Click-Stop 2" diagonal. I put a 13 Ethos in the focuser. I viewed the stars around Sadr in Cygnus. The stars were sharp from center to edge of field. It was difficult to tell if focusing separately for center of field and edge of field made any difference. That's how well the field was flattened with the TS2FF. And remember, my accommodation for focus is virtually nonexistent now.

Removing the TS2FF and observing the same area in Cygnus with the 13 Ethos produced an obvious increase in apparent field curvature.

I also tried the experiment with a 32 Masuyama. With that eyepiece it was difficult to tell if the TS2FF improved the view much at all. But I think this is due to the outer field aberrations produced by the 32 Masuyama in fast scopes.

I would have used the 21 Ethos but it was difficult to balance in the AR-102xs.

Mike


Ah, with this info you have me thinking about that Bresser AR-102xs again. I was already considering it even with some online reviews remarking how pronounced the FC was in that scope. But if the FC can be tamed with the FF, that makes this inexpensive scope even more appealing.

Thanks for the update!

Mary
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#21 Sarkikos

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Posted 29 June 2019 - 09:34 AM

Yep, the TS2FF tames the FC in the AR-102xs!  Very nice. :grin:

 

Mike


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#22 howardcano

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Posted 29 June 2019 - 11:14 AM

Yep, the TS2FF tames the FC in the AR-102xs!  Very nice. grin.gif

 

Mike

Do you have an estimate of the size of the fully-illuminated field with that combination?



#23 Sarkikos

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Posted 29 June 2019 - 12:45 PM

The entire field looked fully illuminated as far as I could tell.  No outer field darkening.  Beyond that, I have no idea.  What my eye can't see doesn't matter to me.  :grin:

 

Mike


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#24 Sarkikos

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Posted 28 July 2019 - 11:20 AM

Last night I took out my ST120, 10 Ethos, 8 Ethos, Baader 2" ClickLock Mirror Diagonal and TS2FF.  I removed the barrel of the diagonal, exposing the SCT threads.  I attached an SCT-48mm ring adapter to the diagonal, and then screwed on the TS2FF.

This setup produced a flat field in the ST120, through both the 8 Ethos and 10 Ethos.  The field stop of the 8 Ethos is 17.8mm below the shoulder.  The field stop of the 10 Ethos is 6.9mm below the shoulder.  (These are Tele Vue specs from their website.)  The 10.9mm difference between the field stop locations of the two eyepieces didn't appear to make any difference in the flatness of the field.

The total optical length w/o the diagonal barrel for the 8 Ethos is 112mm - 18mm = 94mm.  The total optical length w/o the diagonal barrel for the 10 Ethos is 112mm - 7mm = 105mm.

The total optical length with the diagonal barrel for the 8 Ethos is 138mm - 18mm = 120mm.  The total optical length with diagonal barrel in place for the 10 Ethos is 138mm - 7mm = 131mm..

The TS2FF w/o the diagonal barrel flattened the field nicely.  The TS2FF with diagonal barrel did not.

According to TS, the optimal spacing for a refractor with a 600mm focal length (like the ST120 or C80ED) is 113mm.  This is 19mm greater than the spacing w/o diagonal barrel for the 8 Ethos, 8mm greater than for the 10 Ethos.  Would inserting a 19mm or 8mm spacing ring in the setup make any visible difference in field flatness?  I doubt it. 

Keep in mind that the C80ED - which also has a 600mm focal length - showed a flat field with the TS2FF with the diagonal barrel left on.  Would the C80ED show a visibly flatter field with the diagonal barrel removed?  I doubt it.  

Use the specs that these companies give you as general guidelines.  But decide what you want to do based on what you actually see in the field.

Mike


Edited by Sarkikos, 28 July 2019 - 11:20 AM.

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#25 Sarkikos

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Posted 28 July 2019 - 11:24 AM

"What would you estimate the spacing to be from flattener to focal plane with the flattener screwed into the Baader diagonal?"

 

About 140mm.

 

Mike

 

Never rely on an estimate when you can get the specifications.

 

According to Baader, the optical length of the Baader 2" ClickLock Mirror Diagonal is 112mm.  I measure the length of the barrel to be 26mm.  112mm + 26mm = 138mm.  So, yeah, I was only 2mm off.  Not too bad.

 

:grin:

Mike




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