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Orion ST80 is back

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#51 CHASLX200

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Posted 15 November 2020 - 07:01 PM

It wouldn't be cheap, it doesn't make sense to built it as an achromat, might as well make it an ED.

 

I'm using the TSFLAT2 with my Astro-Tech AT-80 LE, an 80 mm F/6 FPL-53 doublet. Spacing was tricky. I had to borrow a diagonal as replace the nosepiece with the flattener but the views are amazing. 5.5 degrees with the 41 mm Panoptic, 5.0 degrees with the 31 Nagler and 4.3 degrees at 23x with the 21mm Ethos. It's not as perfect as the NP-101 but it's close.

 

The fields are flat and sharp. I haven't tried my ST-80 yet..  the TSFLAT2 is about $200 shipped from Germany.  I got mine shipped from Prescott Az via Astromat and Jeff Morgan.

 

Jon

I find F/7 gives me a wide enough FOV and FC is not a problem.  I loved my FS78 as a sweeper with 2" eyepieces. I know it was not a wide field RFT but did pretty good. I could not live with my ST80  so i just used it as a finder and it made a great finder.


Edited by CHASLX200, 15 November 2020 - 07:02 PM.

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#52 MortonH

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Posted 15 November 2020 - 07:47 PM

I higher priced and better built flat field ST80 would be great with a 21mm Ethos and i would for sure buy one as a super sweeper as long as there was no FC.

 

Field curvature is based on the focal length so pretty much any refractor with 400mm focal length will have it.  My 66ED f/6 is the same although the smaller aperture (compared to an 80mm) means the stars are dimmer and therefore FC is slightly less obvious).

 

I find 480mm is about the shortest focal length I can tolerate for FC (a Pronto with 24mm Panoptic is pretty good).


Edited by MortonH, 15 November 2020 - 07:50 PM.


#53 Sarkikos

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Posted 16 November 2020 - 07:02 AM

I did the same thing last nite with a 45 degree diagonal and a 90mm F/10.  Stars looked like a double star at high power so i removed the diagonal and looked straight thru and i had a normal looking star image.

 

I would like a ST80 is there was some way to remove FC. We need a FC eater like a Paracorr eats coma.

TSFLAT2 removes the FC.

 

Mike



#54 Jon Isaacs

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Posted 16 November 2020 - 09:41 AM

Field curvature is based on the focal length so pretty much any refractor with 400mm focal length will have it.  My 66ED f/6 is the same although the smaller aperture (compared to an 80mm) means the stars are dimmer and therefore FC is slightly less obvious).

 

I find 480mm is about the shortest focal length I can tolerate for FC (a Pronto with 24mm Panoptic is pretty good).

 

I use my ST-80 with the 41 mm Panoptic (6.6°) and the 31 mm Nagler (6.0°.) 

 

There's plenty of field curvature but I just live with it.. It comes with the territory.

 

Until now. I'll see how I like the TSFLAT2, hopefully it's a game changer. 

 

Jon


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#55 Ihtegla Sar

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Posted 16 November 2020 - 09:47 AM

TSFLAT2 removes the FC.

Mike

Really? Cause I read this post to say the opposite, at least for visual use with an 80mm f/5 refractor. Have you tested your TSFLAT2 on an ST-80? I am eagerly awaiting Jon's test results.

Edited by Ihtegla Sar, 16 November 2020 - 09:48 AM.


#56 Sarkikos

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Posted 16 November 2020 - 10:52 AM

Really? Cause I read this post to say the opposite, at least for visual use with an 80mm f/5 refractor. Have you tested your TSFLAT2 on an ST-80? I am eagerly awaiting Jon's test results.

https://www.cloudyni...only/?p=9801978

 

Posted 29 November 2019:

A couple nights ago I took out my ST80, which has a 2" Crayford focuser.  A Baader 2" ClickLock Mirror Diagonal was in the focuser.  I put a 10mm Ethos in the diagonal.  Yes, there was noticeable field curvature.  If I focused on a star at the center of field, and then moved that star to edge of field, the star was no longer sharply in focus.

I tried it again, but this time I screwed the TSFLAT2 onto the barrel of the diagonal.  Now the stars were sharply in focus from edge of field to center of field.  My test objects were the Pleiades, Hyades, Double Cluster and the Mellote 20 open cluster around Mirfak in Perseus.

Mike

 

Mike


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#57 Ihtegla Sar

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Posted 16 November 2020 - 11:14 AM

https://www.cloudyni...only/?p=9801978

Posted 29 November 2019:

Mike

Hmm... Piotr said the TSFLAT2 just made things worse in an 80mm f/5 with the TSFLAT2 attached directly to the diagonal in lieu of a nosepiece. His post was the main reason I haven't bought one yet. But you are saying for the ST-80 the spacing requires the TSFLAT to be attached to the barrel of the diagonal, not directly to the body.

Based on the strength of your report, I went ahead and ordered one. Teleskop Services has them on a 22% discount today. I can't pass up a good sale. https://www.teleskop...connection.html

Edited by Ihtegla Sar, 16 November 2020 - 11:22 AM.

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#58 Sarkikos

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Posted 16 November 2020 - 12:09 PM

Which 80mm f/5 does Piotr have?  Would that make a difference?  Could he have mistaken 

 

I know from my experience that my ST80 + 2" Crayford focuser +  Baader 2" ClickLock Mirror Diagonal (incl. barrel) + TSFLAT2 + 10mm Ethos = flat field.  My eyes have virtually no accommodation for focus.  If there is field curvature in the system, I will see it. 

 

Mike


Edited by Sarkikos, 16 November 2020 - 12:18 PM.


#59 Ihtegla Sar

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Posted 16 November 2020 - 12:31 PM

Which 80mm f/5 does Piotr have? Would that make a difference? Could he have mistaken

I know from my experience that my ST80 + 2" Crayford focuser + Baader 2" ClickLock Mirror Diagonal (incl. barrel) + TSFLAT2 + 10mm Ethos = flat field. My eyes have virtually no accommodation for focus. If there is field curvature in the system, I will see it.

Mike


What makes an ST-80 unique is the six degree field when used with a low power eyepiece. Field curvature tends to be more noticeable at low powers.

A 10mm Ethos isn't exactly a low power eyepiece. The field of view in an ST-80 with a 10mm Ethos is only 2.5 degrees.

Piotr was using a 31 mm Nagler and a 22mm Ethos. That could explain the difference.

Have you used your ST-80 with the TSFLAT and a low power eyepiece?
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#60 Sarkikos

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Posted 16 November 2020 - 12:57 PM

Isn't the apparent field of view also a factor in appearance of field curvature?  The Ethos 10 is a 100 degree eyepiece.  If the telescope is focused at center of field, FC will be more pronounced the closer we look toward the edge of field.  The wider the apparent field of view, the more obvious the FC will be.

 

I don't recall all of the eyepieces I used in the ST80.  That was a year ago when I tested it with the TSFLAT2.

 

But notice in my field report from last year that when I put a 10mm Ethos in the diagonal without the TSFLAT2, "there was noticeable field curvature.  If I focused on a star at the center of field, and then moved that star to edge of field, the star was no longer sharply in focus."   But when I screwed on the TSFLAT2, the field was flat.  Quod erat demonstrandum ... at least for me, based on my experience, which is what I act on.

 

Mike


Edited by Sarkikos, 16 November 2020 - 01:05 PM.


#61 Ihtegla Sar

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Posted 16 November 2020 - 02:17 PM

Isn't the apparent field of view also a factor in appearance of field curvature?  The Ethos 10 is a 100 degree eyepiece.  If the telescope is focused at center of field, FC will be more pronounced the closer we look toward the edge of field.  The wider the apparent field of view, the more obvious the FC will be.

 

I don't recall all of the eyepieces I used in the ST80.  That was a year ago when I tested it with the TSFLAT2.

 

But notice in my field report from last year that when I put a 10mm Ethos in the diagonal without the TSFLAT2, "there was noticeable field curvature.  If I focused on a star at the center of field, and then moved that star to edge of field, the star was no longer sharply in focus."   But when I screwed on the TSFLAT2, the field was flat.  Quod erat demonstrandum ... at least for me, based on my experience, which is what I act on.

 

Mike

In my experience true field makes a bigger difference than apparent field in the appearance of field curvature.  I never notice the field curvature with my ES 9mm 100 but do notice it with my Pentax 40mm and my Vixen 42mm which have a narrower apparent field but a much wider true field due to low power. 



#62 CHASLX200

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Posted 16 November 2020 - 07:40 PM

TSFLAT2 removes the FC.

 

Mike

Well i may give it a try. Now i gotta wonder if the ST80 will fit my Orion 90mm Achro's rings.  I have the Alt - AZ mount.  I can somewhat live with FC if i am viewing a object near the center of the FOV. It is when i am sweeping that FC drives me nuts.  Just looks like the sky is bending as i sweep.


Edited by CHASLX200, 16 November 2020 - 07:43 PM.


#63 MortonH

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Posted 16 November 2020 - 10:26 PM

Which 80mm f/5 does Piotr have?  Would that make a difference?  Could he have mistaken 

 

I know from my experience that my ST80 + 2" Crayford focuser +  Baader 2" ClickLock Mirror Diagonal (incl. barrel) + TSFLAT2 + 10mm Ethos = flat field.  My eyes have virtually no accommodation for focus.  If there is field curvature in the system, I will see it. 

 

Mike

 

I'm curious what mount you would be using with this combination as it must be extremely back heavy.


Edited by MortonH, 16 November 2020 - 10:28 PM.


#64 Sarkikos

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Posted 17 November 2020 - 07:21 AM

I'm curious what mount you would be using with this combination as it must be extremely back heavy.

I mount the ST80 on a Bogen 055XB tripod with either a MicroStar Deluxe or a 501HDV head.  Yes, the ST80 is very back heavy if you put a very heavy eyepiece in the diagonal.  This is why I tend to avoid the heaviest eyepieces in these small scopes.  Do you really need to put a Terminagler or a 41 Pan in an ST80?   But at least a partial remedy is to mount a weight at the front of the dovetail and/or use a long dovetail.

 

Actually, the counterbalance spring in the 501HDV goes a long way to compensate for a small, back-heavy scope setup.

 

Mike


Edited by Sarkikos, 17 November 2020 - 07:23 AM.

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#65 jcj380

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Posted 17 November 2020 - 08:38 AM

Now i gotta wonder if the ST80 will fit my Orion 90mm Achro's rings.  

Probably will need some foam tape or something inside the rings.



#66 Jon Isaacs

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Posted 17 November 2020 - 09:00 AM

Field curvature increases with the square of the off-axis distance.  I calculate the curvature at the edge of the 31 mm Nagler in an ST-80 to be 1.6 mm. By comparison, the 10 mm Ethos is about 0.29mm.  That's a factor of 5.5.

 

The visibility also depends on the magnification so for a constant AFoV, the diameter of the defocused blur is linear with increased eyepiece focal length but the area of the blur which is what I think we tend to see is still the square.  

 

My wife is bringing my ST-80 with her when she comes out tonight so I should be able to give the ST-80 plus the TSFLAT2 a good workout with the 41mm and 35 mm Panoptics plus the 31 mm Nagler, 21mm, 13mm, and 8 mm Ethos. 

 

So far, these eyepieces are performing quite nicely with the TSFLAT2 in my 80mm F/6.

 

Jon


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#67 Ihtegla Sar

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Posted 17 November 2020 - 02:16 PM

Field curvature increases with the square of the off-axis distance.  I calculate the curvature at the edge of the 31 mm Nagler in an ST-80 to be 1.6 mm. By comparison, the 10 mm Ethos is about 0.29mm.  That's a factor of 5.5.

 

The visibility also depends on the magnification so for a constant AFoV, the diameter of the defocused blur is linear with increased eyepiece focal length but the area of the blur which is what I think we tend to see is still the square.  

 

My wife is bringing my ST-80 with her when she comes out tonight so I should be able to give the ST-80 plus the TSFLAT2 a good workout with the 41mm and 35 mm Panoptics plus the 31 mm Nagler, 21mm, 13mm, and 8 mm Ethos. 

 

So far, these eyepieces are performing quite nicely with the TSFLAT2 in my 80mm F/6.

 

Jon

Thanks for the calculations on field curvature.  I probably should have waited for your report on how the TSFLAT2 works with the ST-80, but I saw the sale yesterday and went ahead and ordered one.  Should be here in a few days.  I also got an adaptor and some spacers from Agena so I should be ready for testing myself if we ever get clear skies again.  I will be testing with an Adventure Scope and a 30mm ES 82, 40mm Pentax XW and a 42mm Vixen LVW. 

 

But, I am looking forward to your report on the TSFLATT2 with your low power eyepieces.  Wish I had your weather.  This time of year, I am lucky if I can get a few hours of only partly cloudy skies for observing.  



#68 Sarkikos

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Posted 19 November 2020 - 08:30 AM

Last night I took out my ST80.  The weather was clear but cold, in the low 30's F.  I usually mount it on a Bogen 055XB tripod with either a MicroStar Deluxe or a 501HDV head.  But this time I used a Stellervue M2 on Manfrotto 058B.  This setup gave me much better stability and balance control with heavy eyepieces.  My selection of eyepieces was 41 Pan, 31 Nagler and 21 Ethos.  I had the TSFLAT2 screwed onto the barrel of a Baader 2" BBHS diagonal.  

 

M45, the Pleiades, was the main object for testing the TSFLAT2 with these eyepieces.  The 41 Pan and 31 Nagler both showed flat fields.  There was some astigmatism, but I think this was from my eyes, not the optical system.  I could see astigmatism with my progressive eyeglasses on or off, but much worse with them off.  My prescription is two years old, so it has probably changed.  The 21 Ethos, with the widest apparent field of view, showed the most astigmatism.

 

I took the three eyepieces back into my house and screwed a Dioptrx onto each one.  I looked at the Pleiades again through each eyepiece with Dioptrx - without my eyeglasses, of course.  For each, I dialed in the best correction with the Dioptrx.  Now the stars were pretty sharp from center to edge in the 41 Pan and the 31 Nagler.  However, the 21 Ethos still showed some aberration at edge of field when the telescope was focused at center, and vice versa.  It looked to me like a combination of FC and astigmatism.

 

One take away from this, for my own purposes, is that I will probably leave a Dioptrx on the 41 Pan, 31 Nager and 21 Ethos.  In the past I haven't liked using Dioptrx because with Dioptrx I need to flip my eyeglasses on to see the sky naked eye and read charts, and then flip them off again to view through the eyepiece.  This makes star hopping a PITA.  But at these wide fields and low powers, finding the object shouldn't really be a big problem.

 

Mike 


Edited by Sarkikos, 19 November 2020 - 03:03 PM.

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#69 sickfish

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Posted 19 November 2020 - 08:51 AM

I love mine, never letting it go.

I like dropping the Pan 35 in it and wondering around.

Not good on planets but I have other scopes for them.


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#70 tony_spina

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Posted 19 November 2020 - 09:05 AM

Last night I took out my ST80.  The weather was clear but cold, in the low 30's F.  I usually mount it on a Bogen 055XB tripod with either a MicroStar Deluxe or a 501HDV head.  But this time I used a Stellervue M2 on Manfrotto 058B.  This setup gave me much better stability and balance control with heavy eyepieces.  My selection of eyepieces was 41 Pan, 31 Nagler and 21 Ethos.  I had the TSFLAT2 screwed onto the barrel of a Baader 2" BBHS diagonal.  

 

M45, the Pleiades, was the main object for testing the TSFLAT2 with these eyepieces.  The 41 Pan and 31 Nagler both showed flat fields.  There was some astigmatism, but I think this was from my eyes, not the optical system.  I could see some astigmatism with my progressive eyeglasses on or off, but much worse with them off.  My prescription is two years old, so it has probably changed.  The 21 Ethos, with the widest apparent field of view, showed the most astigmatism.

 

I took the three eyepieces back into my house and screwed a Dioptrx onto each one.  I looked at the Pleiades again through each eyepiece with Dioptrx - without my eyeglasses, of course.  For each, I dialed in the best correction with the Dioptrx.  Now the stars were pretty sharp from center to edge in the 41 Pan and the 31 Nagler.  However, the 21 Ethos still showed some aberration at edge of field when the telescope was focused at center, and vice versa.  It looked to me like a combination of FC and astigmatism.

 

One take away from this, for my own purposes, is that I will probably leave a Dioptrx on the 41 Pan, 31 Nager and 21 Ethos.  In the past I haven't liked using Dioptrx because with Dioptrxy I need to flip my eyeglasses on to see the sky naked eye and read charts, and then flip them on again to view through the eyepiece.  This makes star hopping a PITA.  But at these wide fields and low powers, finding the object shouldn't really be a big problem.

 

Mike 

Mike if you removed the TSFLAT2 was the astigmatism better, worse,  or the same with these eyepieces and the ST80? 



#71 CharlesC

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Posted 19 November 2020 - 09:09 AM

Long live the ST80 rockon.gif

Long live the TRS80 applause.gif


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#72 Sarkikos

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Posted 19 November 2020 - 09:20 AM

Mike if you removed the TSFLAT2 was the astigmatism better, worse,  or the same with these eyepieces and the ST80? 

I'll have to check another time.  The night was cold, I don't like the cold, so I was pushing it to be out as long as I was.   

 

But it is significant that I did not see obvious astigmatism with 41 Pan or 31 Nagler with Dioptrx and TSFLAT2 in the stack, even though these eyepieces have longer focal lengths and wider field stops than the 21 Ethos.  I'm thinking that the wider AFOV of the 21 Ethos might be an important factor (100 degrees in the Ethos vs 82 in the Nagler and 68 in the Pan).  This even though the field stop of the 21 Ethos is somewhat narrower at 36.2mm, while the 31 Nagler is 42mm and the 41 Pan is 46mm. 

 

Or maybe the 21 Ethos requires a different spacing for the TSFLAT2 than the 31 Nager or 68 Pan?  That would depend on the field stop position of these eyepieces.  The field stop for the 21 Ethos and 31 Nager are both 0.38" above the shoulder of the eyepiece.  The field stop for the 41 Pan is 0.27" below the shoulder.  shrug.gif  https://www.televue....page.asp?id=214

 

Mike


Edited by Sarkikos, 19 November 2020 - 10:05 AM.


#73 Sarkikos

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Posted 19 November 2020 - 09:22 AM

Long live the TRS80 applause.gif

Hmmm poke.gif ... the Trash80 was a computer, not a telescope. https://en.wikipedia.org/wiki/TRS-80

 

grin.gif

Mike


Edited by Sarkikos, 19 November 2020 - 09:58 AM.


#74 CharlesC

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Posted 19 November 2020 - 11:04 AM

Hmmm poke.gif ... the Trash80 was a computer, not a telescope. https://en.wikipedia.org/wiki/TRS-80

 

grin.gif

Mike

I used to work for Tandy and visited the Fort Worth "ivory towers" a few times.


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#75 Ihtegla Sar

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Posted 19 November 2020 - 11:51 AM

Last night I took out my ST80.  The weather was clear but cold, in the low 30's F.  I usually mount it on a Bogen 055XB tripod with either a MicroStar Deluxe or a 501HDV head.  But this time I used a Stellervue M2 on Manfrotto 058B.  This setup gave me much better stability and balance control with heavy eyepieces.  My selection of eyepieces was 41 Pan, 31 Nagler and 21 Ethos.  I had the TSFLAT2 screwed onto the barrel of a Baader 2" BBHS diagonal.  

 

M45, the Pleiades, was the main object for testing the TSFLAT2 with these eyepieces.  The 41 Pan and 31 Nagler both showed flat fields.  There was some astigmatism, but I think this was from my eyes, not the optical system.  I could see astigmatism with my progressive eyeglasses on or off, but much worse with them off.  My prescription is two years old, so it has probably changed.  The 21 Ethos, with the widest apparent field of view, showed the most astigmatism.

 

I took the three eyepieces back into my house and screwed a Dioptrx onto each one.  I looked at the Pleiades again through each eyepiece with Dioptrx - without my eyeglasses, of course.  For each, I dialed in the best correction with the Dioptrx.  Now the stars were pretty sharp from center to edge in the 41 Pan and the 31 Nagler.  However, the 21 Ethos still showed some aberration at edge of field when the telescope was focused at center, and vice versa.  It looked to me like a combination of FC and astigmatism.

 

One take away from this, for my own purposes, is that I will probably leave a Dioptrx on the 41 Pan, 31 Nager and 21 Ethos.  In the past I haven't liked using Dioptrx because with Dioptrxy I need to flip my eyeglasses on to see the sky naked eye and read charts, and then flip them off again to view through the eyepiece.  This makes star hopping a PITA.  But at these wide fields and low powers, finding the object shouldn't really be a big problem.

 

Mike 

Thanks for your report!  My TSFLATT2 should be here next week and I am looking forward to trying it out on my ST-80 and my FC100DF.  I recently purchased one of those 8000m down suits on half price sale so long hours observing in sub freezing temperatures is no longer uncomfortable.  Was out for six hours last week in 28 degree weather and it was more comfortable than summer observing and my gear acclimatized to the temperature much better, not that the ST-80 would have any issues acclimatizing.  But I will be ready to put it through its paces with the TSFLATT2, cold weather notwithstanding.  Now if only the clouds would clear . . .

 

Even without the TSFLATT2, I too have noticed the edges cleaning up quite a but with my glasses on, but the center field isn't nearly as good viewing though my glasses, particularly for subtle contrast features on planets, but really just about everything looks about 50% better without my glasses.  So I am constantly flipping them on to read the charts, look through the Telrad, scan the horizon with a red light for cougar, etc. then flipping them off for observing.  I keep a librarian cord on my glasses when observing so I can flip them on and off without  having to worry about dropping them or misplacing them.  I'd never be able to find them if I misplaced them since my eyesight is so bad.  I could give Mr. Magoo a run for his money.


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