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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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#17 earlyriser

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Posted 18 October 2019 - 09:12 AM

I'm still comparing the ES82 30mm and Nagler 31mm with no corrective lenses, with contacts, and with glasses, but I do have a couple of observations:

 

(1) Using glasses with the ES82 is not a viable solution due to the short ER. I see a larger portion of the sky using the 22mm Nagler. 

 

(2) Using the Nagler 31mm without corrective lenses adds a significant amount of distortion in the outer 50% of the FOV. It's better than (1), but it's not very satisfying knowing what the Nagler can do in (3).

 

(3) I can use the Nagler 31mm with glasses if I press in a bit. This gives the best views of any combination of eyepiece/corrective lenses - the stars are quite tight across the FOV.

 

If I could increase the effective ER of the 31mm Nagler by even 1 mm, I think I'd be in tall cotton. I'm going to try removing the rubber eyeguard and adding a thin piece of felt to protect my glasses, and see what if any improvement I get. 



#18 earlyriser

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Posted 18 October 2019 - 12:38 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. 

As a follow-up to this post, the amount of in-focus I need to correct for -6.5 myopia appears to be dependent on the eyepiece. For example, a 10mm Delos only needs a fraction of the in-focus the the 30mm ES needs using the tunable top. Not sure why, but I presume it is related to the focal length of the eyepiece, with longer eyepieces needing more in-focus.



#19 a__l

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Posted 20 October 2019 - 02:01 AM

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).

Many hundreds observing only in E21 or E17? Since there are no problems with other TV eyepieces (I think that you are not using the N31).

In order to make sure that you have the best focus point, you need to go through it and go back. This can be a problem. It seems that some of the many hundreds did not. For different reasons.


Edited by a__l, 20 October 2019 - 02:23 AM.


#20 earlyriser

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Posted 20 October 2019 - 06:09 AM

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?

Could be. I believe the paracorr II has more in-focus, and I think shorter f-ratios make focusing more sensitive. On the other hand, if you had stopped down the scopes to f/4, it's hard for me to see how it would have increased the amount of in-focus necessary to correct for you myopia. But, I really don't know. It's a good datapoint in any case. 


Edited by earlyriser, 20 October 2019 - 06:16 AM.


#21 Mike Lockwood

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Posted 21 October 2019 - 09:29 PM

Many hundreds observing only in E21 or E17? Since there are no problems with other TV eyepieces (I think that you are not using the N31).

In order to make sure that you have the best focus point, you need to go through it and go back. This can be a problem. It seems that some of the many hundreds did not.

I have used Ed's 24" f/2.75 telescope quite extensively, with the Nagler 31mm and other eyepieces including the 21mm Ethos, and have never had any trouble focusing or going through focus, nor have I heard of anyone else having trouble.  The only thing that I have seen is observers enjoying what they see through the eyepiece.

 

Could be. I believe the paracorr II has more in-focus, and I think shorter f-ratios make focusing more sensitive. On the other hand, if you had stopped down the scopes to f/4, it's hard for me to see how it would have increased the amount of in-focus necessary to correct for you myopia. But, I really don't know. It's a good datapoint in any case. 

The P2 and SIPS have been around for over 10 years now.  They have a solid track record of performance and work well for most users.

 

The P2 has a wider range of adjustability than the original Paracorr, as I understand it.  Also, I believe at some point (quite a while ago) the SIPS was modified to give a larger range of focus to address the problem of those with myopia.

 

F/# does not affect relative focusing positions of eyepieces.  The focal plane location is set to be the same if the aid that is provided with the SIPS is used properly.  After the intial setup, the relative focusing location is the same no matter the f/#.  An eyepiece comes to focus when the focal plane reaches a particular plane inside the eyepiece.  So, if you set up the SIPS in an f/2.75 telescope AND an f/5 telescope, eyepiece X should come to focus with the same in-travel for both f/#s.

 

Let me say that again - set up correctly, for example, if a 31mm Nagler requires X mm of in-travel compared to some other eyepiece, this in-travel is the same regardless of f/#.  This is why the Paracorr (1 or 2) settings (you know, A through H) only vary with eyepiece, and are not dependent at all on f/#.

 

If a person's eyes differ significantly from the norm, this is not an issue that can, or should be, solved with a corrector such as the Paracorr.  In these cases, the person with vision problems should LEAVE THEIR GLASSES ON WHEN OBSERVING and choose eyepieces that have sufficient eye relief to allow this to work.  In this age of trying to please/accommodate everyone, the laws of optics do not always allow it.


Edited by Mike Lockwood, 21 October 2019 - 09:33 PM.

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#22 phonehome

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Posted 21 October 2019 - 10:10 PM

Many hundreds observing only in E21 or E17? Since there are no problems with other TV eyepieces (I think that you are not using the N31).

In order to make sure that you have the best focus point, you need to go through it and go back. This can be a problem. It seems that some of the many hundreds did not. For different reasons.

I don't know where this is coming from but NO, most if not nearly all of these fine folks observing with Elvira (24"/f2.75+SIPS) used a 31mm (Nagler Type 5) at some point and without focusing issues.  Many wore glasses and some removed them for observing.  I did not query anyone about their corrective prescription.  Should that be a requirement?

 

All my eyepieces including the entire Delos line and Ethos come to focus - period.  As to moving/passing back & forth through focus to find the "edges" this is a common technique I thought just about everyone uses.  Of course with faster scopes it helps to have an over-sized and weighty fine-focus knob sumo.gif

 

IF a scope is designed and built correctly then except for it's magnification+reduced FOV the SIPS corrector becomes "transparent " to the user/observer [pun intended].  I prefer it for several reasons - here is a few:  it's always in the best possible position relative to the primary for focusing, it's a very solid mechanical configuration, it does not have to be removed for collimation and minus the tunable top it's a simple focus procedure that everyone understands.

 

Now when a Night Vision device is employed (A-focal method) I recommend to everyone to leave their glasses on since the NV device has a diopter adjustment and I prefer to keep the NV ocular mechanical wear to a minimum.  But that's entirely another story...


Edited by phonehome, 22 October 2019 - 12:51 PM.

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#23 earlyriser

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Posted 25 October 2019 - 07:09 PM

I've had some more time to compare the 31mm Nagler and 30mm ES82 with and without corrective lenses using the Paracorr type I in an f/5 (f/5.75 with paracorr) 10 inch Newtonian. There were too many variables and subjectivity for this to really be a scientific test. For example, I think my focuser might have been pulled a little out of square by the weight of the eyepieces, and I had some problems consistently producing the same views as I swapped out eyepieces, but here goes. 

 

With glasses, the ES82 was essentially unusable due to a lack of effective eye relief. In contrast, I could see the entire field in the Nagler if I pressed in. The combination of glasses and the Nagler gave the best view overall of all combinations. Stars were tight nearly to the edge. This set the standard.

 

With contacts, both the ES82 and Nagler gave better views than without any corrective lenses at all, but views were not as good as with glasses. Even though I only have 0.25 cylinder correction in my dominant eye, this astigmatism was the dominant aberration. Both eyepieces are really quite good, and if you are lucky enough to not have any astigmatism, either will provide good service.

 

Without corrective lenses, I could compensate for my nearsightedness on the ES82 using the tunable top of the Paracorr so that the Paracorr remained optimally positioned with respect to the primary. With the Nagler, I had to use the focuser to compensate. This resulted in the Nagler showing noticeably more coma than the ES82. With both eyepieces, my astigmatism was also noticeable. In addition, it seemed like stars were not as tight on center as when I was wearing contact lenses. The observing session with contacts and without corrective lenses was about 36 hours apart, so it's hard to quantify this. Maybe it is my imagination, but the views without contacts just didn't seem as sharp as they did with contacts. I wonder if having non-optimal spacing between the eyepiece and the Paracorr (as with the ES82), or between the Paracorr and the mirror (as with the Nagler) introduces some spherical aberrations on center. I can't say for sure.

 

Anyway, I grew tired of testing last night, and decided to just do some observing with the ES82 and Nagler. Then, something wonderful happened. As I wandered through Cygnus, I stopped noticing the flaws in my optics. All I saw were stars. The bottom line is both eyepieces deliver good enough views that if I'm not looking for problems, I don't see any. I've experienced something similar listening to music. When you start listening to the music instead of the equipment, the equipment disappears. 

 

At the end of the day, because I can use my glasses with the Nagler (albeit with some discomfort), but not with the ES82, the Nagler wins. 


Edited by earlyriser, 25 October 2019 - 07:11 PM.

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#24 25585

25585

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Posted 28 October 2019 - 07:46 AM

 

 

If a person's eyes differ significantly from the norm, this is not an issue that can, or should be, solved with a corrector such as the Paracorr.  In these cases, the person with vision problems should LEAVE THEIR GLASSES ON WHEN OBSERVING and choose eyepieces that have sufficient eye relief to allow this to work.  In this age of trying to please/accommodate everyone, the laws of optics do not always allow it.

And thankfully there are these days several top quality makers of models with good eye relief and viewing comfort, so wearing glasses no longer restricts quality and expanse of view, as was in the past.   


Edited by 25585, 28 October 2019 - 07:47 AM.


#25 Ihtegla Sar

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Posted 21 November 2019 - 08:15 AM

I am wondering if this problem could be solved with an Explore Scientific HRCC in lieu of a Paracorr? Yes, it requires your telescope to have a lot of in focus, but if your telescope can handle the HRCC (mine can), the HRCC doesn't seem to have a lack of in focus with any eyepiece I have tried with or without glasses. I don't have any naglers or erfles but I do have an ES 30mm 82 (similar to the N31) and the 17mm 92 (similar to the E17) and they both come to focus fine with and without my glasses using the HRCC tunable top in my f/5 Skywatcher. I'm about as nearsighted as you can get at -10.5.
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