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Paracorr settings and myopic observers.

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#1 earlyriser

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Posted 20 September 2019 - 06:35 AM

My understanding is that for a paracorr to provide its best correction of coma, it needs to be at an optimum position with respect to the mirror. When using a TeleVue Paracorr, proper positioning of the device is achieved by setting the tunable top to a position specific to an eyepiece, and focusing the telescope. If a different eyepiece is placed in the Paracorr that is not parfocal with the first eyepiece, focus should be regained by turning the tunable top rather than adjusting the focuser.

 

Enter the myopic observer who wears glasses, but removes them to look through the eyepiece. Normally, this is not a big deal. The observer can simply adjust the focuser inward to compensate for their nearsightedness. But with a Paracorr in place, I believe this will result in the Paracorr not being in it's optimum position. 

 

What made me think of this? 

 

I was out this morning testing out a new-to-me Paracorr I in an f/5 newtonian using the Double Cluster as a test subject. My observing eye requires a spherical correction of about -6.5 diopters, and a cylinder of .25 diopters. This is a pretty strong prescription, so when I'm not wearing corrective lenses, I need to rack the focuser in pretty far to compensate.

 

Anyway, I used a 22mm Nagler Type 4 to set the position of the Paracorr while wearing glasses, and began swapping between the 22mm and a 31mm Nagler Type 5. At some point, I decided to remove my glasses to make looking through the 31mm more comfortable. It was then I noticed the the coma correction seemed to suddenly be less than great. At first I thought maybe it was my astigmatism. However, when I tried resetting the Paracorr using the 22mm without wearing glasses, and then put my glasses back on, I realized something else was happening. It was then it occured to me that by racking in the focuser, I had upset the position of the Paracorr with respect to the mirror.

 

Can anyone confirm my discovery? Certainly, from now on I will plan on using corrective lenses while observing with my dobsonian.


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#2 junomike

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Posted 20 September 2019 - 07:04 AM

The relationship for the Coma Correction is between the eyepiece and Coma Corrector (or so I thought).

Removing your glasses will definitely require re-focusing but the distance between the Coma Corrector and Eyepiece should be the same.


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#3 TOMDEY

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Posted 20 September 2019 - 07:34 AM

Yes, earlyriser... you are entirely correct in your gestalt there! Except maybe your last sentence, that is >>>

 

Set/Tune the eyepiece at nominal distance to Paracorr and then, with glasses on, and looking thru the far focus part of the eyeglass lens, focus the telescope. Now you are free to remove your glasses, but only focusing eyepieces by adjusting the Tunable Top... not the telescope focus. I can derive and share all the math parts... but the empirical recipe still holds. And yes, -6.5D is indeed a lot... placing your personal ~far point~ a mere six inches in front of your eye! (The technically ideal solution would be to get eyeball lens replacement, Lasik or PRK to fix the weak link in the imaging chain. The only reason I mention this is because trimming myopic focus with glasses off by twisting the Tunable Top... results in non-nominal use of the eyepiece itself... by a lot.)  Tom


Edited by TOMDEY, 20 September 2019 - 07:36 AM.

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#4 earlyriser

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Posted 20 September 2019 - 07:50 AM

Yes, earlyriser... you are entirely correct in your gestalt there! Except maybe your last sentence, that is >>>

 

Set/Tune the eyepiece at nominal distance to Paracorr and then, with glasses on, and looking thru the far focus part of the eyeglass lens, focus the telescope. Now you are free to remove your glasses, but only focusing eyepieces by adjusting the Tunable Top... not the telescope focus. I can derive and share all the math parts... but the empirical recipe still holds. And yes, -6.5D is indeed a lot... placing your personal ~far point~ a mere six inches in front of your eye! (The technically ideal solution would be to get eyeball lens replacement, Lasik or PRK to fix the weak link in the imaging chain. The only reason I mention this is because trimming myopic focus with glasses off by twisting the Tunable Top... results in non-nominal use of the eyepiece itself... by a lot.)  Tom

My 22mm and 31mm Naglers are already bottomed out in the Paracorr, so I think I'm out of luck using the tunable top to compensate for my vision. I have considered Lasik, but have yet to pull the trigger. 



#5 IVM

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Posted 20 September 2019 - 08:38 AM

...

And yes, -6.5D is indeed a lot... [...] The only reason I mention this is because trimming myopic focus with glasses off by twisting the Tunable Top... results in non-nominal use of the eyepiece itself... by a lot.) 

 

Hi, Tom, Ivan here. Isn't this latter problem the same (and of the same magnitude) as when we are focusing for our poor eyes without the Paracorr in place? Thanks!

 

P.S. To be completely clear, I regard the Televue recommendations for where to set the tunable top as only a means to then find the correct position of the Paracorr from the primary using the focuser (with glasses on). Once that is done, I regard tuning with the tunable top as operating the focuser on a telescope that is now aplanatic, and can be focused for poor eyes if need be. So whatever problems we might be introducing in the performance of the eyepiece by focusing for poor eyes without the Paracorr remain but no new ones are added. Is this correct?


Edited by IVM, 20 September 2019 - 09:19 AM.


#6 Pierre Lemay

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Posted 20 September 2019 - 03:55 PM

My 22mm and 31mm Naglers are already bottomed out in the Paracorr, so I think I'm out of luck using the tunable top to compensate for my vision. I have considered Lasik, but have yet to pull the trigger. 

Earlyriser,

I have about the same Myopia and astigmatism as you. Before the SIPS first came out on the market TV were asking folks at Stellafane to test it in what looked like a 24 inch f/4. I remember Al Nagler asking me to go up and give my impressions. The first thing I noticed was that I could not reach focus when removing my glasses and told Al about this. I was racking in the eyepiece and at some point, before reaching focus, the bottom of the eypiece (an Ethos if I remember) would touch the top of the Paracorr lens group.

 

Last August at the OSP I observed extensively with both Mel Bartels' 25 inch f/2.6 and Ed Allan's 24 inch f/2.75. Mel's scope had a 2 inch Paracorr II and Ed was using the SIPS. I don't recall "hitting the bottom" in either scope while observing without my glasses. I wonder if the fast focal ratios combined with the Paracorr II lens group makes this less of a problem for us very myopic people?


Edited by Pierre Lemay, 20 September 2019 - 03:57 PM.

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#7 TOMDEY

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Posted 20 September 2019 - 04:30 PM

My 22mm and 31mm Naglers are already bottomed out in the Paracorr, so I think I'm out of luck using the tunable top to compensate for my vision. I have considered Lasik, but have yet to pull the trigger. 

Hah! Yes, indeed... I was obtusely alluding to that eventuality when I said I could provide the quantitative stuff and the Lasik reference... indeed the longer focal length eyepieces especially disfavor non-nominal vision, because they result in grossly non-nominal use of the eyepiece... Tom


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#8 TOMDEY

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Posted 20 September 2019 - 04:33 PM

Hi, Tom, Ivan here. Isn't this latter problem the same (and of the same magnitude) as when we are focusing for our poor eyes without the Paracorr in place? Thanks!

 

P.S. To be completely clear, I regard the Televue recommendations for where to set the tunable top as only a means to then find the correct position of the Paracorr from the primary using the focuser (with glasses on). Once that is done, I regard tuning with the tunable top as operating the focuser on a telescope that is now aplanatic, and can be focused for poor eyes if need be. So whatever problems we might be introducing in the performance of the eyepiece by focusing for poor eyes without the Paracorr remain but no new ones are added. Is this correct?

Yep! That's all true. And the longer the eyep focal length and the more the eye's power correction --- the more non-nominal is the use of the eyepiece. No free lunch.    Tom


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#9 earlyriser

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Posted 20 September 2019 - 04:42 PM

Earlyriser,

I have about the same Myopia and astigmatism as you. Before the SIPS first came out on the market TV were asking folks at Stellafane to test it in what looked like a 24 inch f/4. I remember Al Nagler asking me to go up and give my impressions. The first thing I noticed was that I could not reach focus when removing my glasses and told Al about this. I was racking in the eyepiece and at some point, before reaching focus, the bottom of the eypiece (an Ethos if I remember) would touch the top of the Paracorr lens group.

 

Last August at the OSP I observed extensively with both Mel Bartels' 25 inch f/2.6 and Ed Allan's 24 inch f/2.75. Mel's scope had a 2 inch Paracorr II and Ed was using the SIPS. I don't recall "hitting the bottom" in either scope while observing without my glasses. I wonder if the fast focal ratios combined with the Paracorr II lens group makes this less of a problem for us very myopic people?

I wonder how much adding an additional constraint that there must be sufficient space between the eyepiece facing element of the paracorr and the field lens of the eyepiece to allow for correction for myopic observers would impact the performance, size, weight, and cost of the paracorr? In my case, I'm thinking the answer is may be to trade the 31mm Nagler for an eyepiece with greater eye relief, such as the 35mm Panoptic. I haven't measured how much infocus I need to compensate for my nearsightedness, but I bet it's quite a bit.



#10 TOMDEY

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Posted 20 September 2019 - 09:02 PM

I wonder how much adding an additional constraint that there must be sufficient space between the eyepiece facing element of the paracorr and the field lens of the eyepiece to allow for correction for myopic observers would impact the performance, size, weight, and cost of the paracorr? In my case, I'm thinking the answer is may be to trade the 31mm Nagler for an eyepiece with greater eye relief, such as the 35mm Panoptic. I haven't measured how much infocus I need to compensate for my nearsightedness, but I bet it's quite a bit.

I can provide all of that stuff, if anyone is interested. I know I derived it a few months ago and may have shared it here someplace or not... don't recall. I can duplicate that, if anyone wants... It's rather interesting (easy to derive)...

 

But the upshot conclusion is still inexorable --- if your vision suffers significant near or far sighted --- then (sans glasses) you will wind up compromising the designed performance of the eyepiece by viewing (refocusing) ... because the telescope (objective + eyepiece) is (necessarily) no longer afocal. One (non-surgical!) way to avoid this problem would be to have/make something akin to the astigmatic Dioptrx... but one that introduces your prescription power, rather than astigmatism. That is what your eyeglasses do.  Tom


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#11 Starman1

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Posted 21 September 2019 - 12:15 AM

If you are mildly myopic, the obvious thing to do is to set the Paracorr top to the position that is optimum for the eyepiece when you are wearing glasses.

This will set the Paracorr in the light cone of the scope in the right position.

 

When you view without glasses, focus using the tunable top if you can and maybe a tiny touch on the fine focus knob if necessary.

What you're doing is setting the focal plane of the eyepiece at the correct point from the Paracorr's lens for your eye's determination of where the focal point in the eyepiece is.

You aren't changing the focal plane position of the scope, only bringing the eyepiece closer so you can focus it.

 

Alas, eyepieces like the 31mm Nagler, 21mm and 17mm Ethos, 17mm Nikon NAV-HW, and 22mm T4 Nagler will not be likely to be focusable this way because the tunable top will not move in far enough.

And this would especially be true if you are very myopic.

So, bring the eyepiece in as far as you can get with the tunable top, and use the focuser to focus.  You won't get optimum coma correction, but you'll still get a lot better correction than removing the Paracorr entirely.

You wouldn't have that possibility with the SIPS unless you were willing to move the SIPS lens away from the optimum position, and returning it to the correct position might not be doable on the spot.

 

Ultimately, you may want to view with glasses on and use long eye relief eyepieces.

This would also help when you want to walk around or read notes.

And you'll find just about all eyepieces will focus within the range of the tunable top of the Paracorr after that.

Some may require more out travel, but that is easy to accomplish with barrel extenders on the eyepieces.


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#12 earlyriser

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Posted 03 October 2019 - 05:06 AM

Just as another data point. I was experimenting this morning, and a correction of -6.5 diopters equates to rotating the tunable top almost exactly one mark inward on a Paracorr 1 (e.g., going from mark 2 to mark 1). I'm in the process of getting my ES82 30mm back from the guy I loaned it to so I can test it out. I believe it may allow me to correct my nearsightedness using the tunable top rather than by refocusing. 


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#13 Starman1

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Posted 03 October 2019 - 02:47 PM

Just as another data point. I was experimenting this morning, and a correction of -6.5 diopters equates to rotating the tunable top almost exactly one mark inward on a Paracorr 1 (e.g., going from mark 2 to mark 1). I'm in the process of getting my ES82 30mm back from the guy I loaned it to so I can test it out. I believe it may allow me to correct my nearsightedness using the tunable top rather than by refocusing. 

Let us know whether this results in better edge correction than putting the eyepiece in its nominal position and refocusing the scope.


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#14 MitchAlsup

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Posted 04 October 2019 - 10:13 AM

Let us know whether this results in better edge correction than putting the eyepiece in its nominal position and refocusing the scope.

It should because:

a) Paracorr is supposed to be at a single distance from the original focal plane

b) in/out is used to correct for spherical aberration of the eye.


Edited by MitchAlsup, 04 October 2019 - 10:14 AM.

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#15 phonehome

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Posted 04 October 2019 - 07:52 PM

Last August at the OSP I observed extensively with both Mel Bartels' 25 inch f/2.6 and Ed Allan's 24 inch f/2.75. Mel's scope had a 2 inch Paracorr II and Ed was using the SIPS. I don't recall "hitting the bottom" in either scope while observing without my glasses. I wonder if the fast focal ratios combined with the Paracorr II lens group makes this less of a problem for us very myopic people?

Pierre,

 

I believe you are correct regarding fast optics and myopic vision.  I've had many hundreds of folks observing through the 24"/f2.75 (Elvira) and as I recall not one has ever had a problem coming to focus regardless of eyepiece FL.  Fast optics = fast focus which means the adjustable length of the focuser covers a wider range  (and hence the use of a large fine focus knob to "slow down" the focus action).



#16 Sarkikos

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Posted 08 October 2019 - 08:39 AM

Ultimately, you may want to view with glasses on and use long eye relief eyepieces.

This would also help when you want to walk around or read notes.

And you'll find just about all eyepieces will focus within the range of the tunable top of the Paracorr after that.

Some may require more out travel, but that is easy to accomplish with barrel extenders on the eyepieces.

Yes, this is what I do.  Unless absolutely necessary, I keep my eyeglasses on at the telescope.  Myopic, presbyopic and astigmatic.  Yes, I keep my eyeglasses on.  Makes things much easier for me.

 

Mike


Edited by Sarkikos, 08 October 2019 - 08:45 AM.

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