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Nagler 22 mm (Type 4) in APO: Would I see field curvature?

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#26 213Cobra

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Posted 24 July 2019 - 07:38 PM

Just another example... I bought a 480Fl 80mm refractor for wide field viewing thinking I was really upgrading from my 600FL 80mm. Oh what a mistake. I’m 48 y/o and my eyes can not adjust to the field curvature. To make matters worse I have astigmatism with exit pupils over 4mm.

I now use a 24mm/68 with that scope as my largest FOV. It’s still around 3 degrees and I can somehow accommodate the FC easier.

Good luck and best wishes

FC in an 80/480 is easily corrected by adding a TSFlat2. It does a virtually perfect job flattening for visual and quite good for normal size chips AP. It's what I use on my 80/480 LOMO. I also use one on my 80/600 LOMO, though it is less acutely needed there. This is a cheap and effective remedy. I am 15 years past cataract surgery so the issue has to be dealt with. The rest of my scopes are flat field quads + a ff corrected hyperbolic Newtonian astrograph. I haven't been bothered by FC since....well.....1988. Why does anyone else put up with it?

 

Phil


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#27 db2005

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Posted 25 July 2019 - 12:05 AM

I was under the impression that most field flatteners were only useable for AP use because they change the focus position drastically. But I looked up the product on-line and it seems to be advertised for visual use too. Very interesting... waytogo.gif. So.... if I add a field flattener, all the field curvature I see would be caused by the eyepiece alone?



#28 213Cobra

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Posted 25 July 2019 - 01:57 AM

I was under the impression that most field flatteners were only useable for AP use because they change the focus position drastically. But I looked up the product on-line and it seems to be advertised for visual use too. Very interesting... waytogo.gif. So.... if I add a field flattener, all the field curvature I see would be caused by the eyepiece alone?

Yes, and that's not going to be significant. Many field flatteners are AP-specific but not all. The TSFlat2 is flexible. The key is getting the working distance to the sensor right (in this case the eye-facing ocular lens). From the TS web site:

 

In principle this rule applies: the shorter the refractor´s focal length, the longer the working distance to the sensor has to be.

♦ focal length < 450 mm: 128 mm
♦ focal length 450-490 mm: 123 mm
♦ focal length 500-550 mm: 118 mm
♦ focal length 560-590 mm: 116 mm
♦ focal length 600-690 mm: 113 mm
♦ focal length 700-800 mm: 111 mm
♦ focal length ab 800 mm: 108 mm

 

For an 80/480 you can pretty much screw the TSFlat into the nosepiece of most 2" mirror diags and you'll be spot-on or close enough. At longer focal lengths, the TSFlat will *be* the nosepiece for the diag. And there are workarounds in between. For visual there is at least a 5% error factor. It works! I am amazed how many times I have to post this. It ought to be common knowledge. But generally, between the option to buy flat field quad telescopes or use TSFlat 2 or 2.5 with triplets and doublets, there is no reason for most refractor users to be putting up with telescope FC.

 

Eliminate that and get eyepieces that leverage flat fields, and you're done with FC forever. The two phenomena that baffle me most on astro forums are 1/ FC complaints, and 2/ undercut rants about eyepieces. Both of these are easily solved. And if people bought more quads or more TSFlats, the market would get the message. Flat field scopes solve pernicious observing problems, so however you get there -- buying a quad or using something like a TSFlat with a triplet or doublet -- makes your astro life much easier. I've known this for decades because I started buying astrographs for visual use over 30 years ago. I'd have thought more observers would have caught on. But no. Ban FC from your astro observing life. You have multiple ways to do it, and you won't regret removing FC as an observing concern.

 

Phil


Edited by 213Cobra, 25 July 2019 - 01:58 AM.

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#29 db2005

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Posted 25 July 2019 - 02:40 AM

Thanks for the advice waytogo.gif. I have been wondering about getting a flattener before, but I have just ordered the TSFLAT2 a moment ago on your advice.

 

I love to experiment with my scopes, and I think getting a flattener might be worth a shot. At least, it makes sense to me to spend a little $ on getting the maximum performance from $$$ eyepieces.


Edited by db2005, 25 July 2019 - 05:34 AM.


#30 Sarkikos

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Posted 25 July 2019 - 08:16 AM

It also depends on the experience of the observer, the expectations.. 

 

Jon

An inexperienced observer might not notice the FC, or if they notice it, they might not know how to determine that what they see is FC and not something else.

 

Mike


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#31 Sarkikos

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Posted 25 July 2019 - 08:24 AM

FC in an 80/480 is easily corrected by adding a TSFlat2. It does a virtually perfect job flattening for visual and quite good for normal size chips AP. It's what I use on my 80/480 LOMO. I also use one on my 80/600 LOMO, though it is less acutely needed there. This is a cheap and effective remedy. I am 15 years past cataract surgery so the issue has to be dealt with. The rest of my scopes are flat field quads + a ff corrected hyperbolic Newtonian astrograph. I haven't been bothered by FC since....well.....1988. Why does anyone else put up with it?

 

Phil

I've found that the TS2FF flattens the field very nicely in my C80ED and my Bresser AR-102xs.  All I need do is screw the TS2FF onto the 2" diagonal.   So far it does not work well in my ST120.    The problem is finding an FF that will work in a specific telescope, and determining the correct spacing to make it work.  

 

I am 63 and have lost most of my accommodation for focus.  If there is field curvature in the image, chances are I will notice it.

 

Mike


Edited by Sarkikos, 25 July 2019 - 08:30 AM.

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#32 213Cobra

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Posted 25 July 2019 - 08:49 AM

I've found that the TS2FF flattens the field very nicely in my C80ED and my Bresser AR-102xs.  All I need do is screw the TS2FF onto the 2" diagonal.   So far it does not work well in my ST120.    The problem is finding an FF that will work in a specific telescope, and determining the correct spacing to make it work.  

 

I am 63 and have lost most of my accommodation for focus.  If there is field curvature in the image, chances are I will notice it.

 

Mike

Per the distance specs I cited, if your ST120 is f/5, you would need 10mm LESS working distance, so you would simply have to replace the nosepiece of your diagonal with spacers + TSFlat 2 to reach the 113mm working distance accordingly, at which point you should see same benefits you see in your 80.

 

Phil


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#33 db2005

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Posted 25 July 2019 - 09:13 AM

Update:

 

Well... I have taken the plunge and ordered a Nagler 22 (Type 4). It's going to be my most expensive eyepiece ever, but I'm hoping it will be my best eyepiece too...

 

I believe the 19 mm eye relief should be sufficient for observing with my eyeglasses (as far as I can see about 16-17 mm of honest eye relief is adequate, which is why I usually look for eyepieces with 20 mm stated eye relief), but just in case it's not enough I can use my a Dioptrx corrector instead.

 

I really look forward to trying my first 82 degree eyepiece on the night skies in the fall jump.gif

 

Thanks for all the advice and insights, your help is much appreciated! bow.gif

 

Clear Skies,

 

Daniel


Edited by db2005, 25 July 2019 - 09:45 AM.

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#34 SteveG

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Posted 25 July 2019 - 01:42 PM

Update:

 

Well... I have taken the plunge and ordered a Nagler 22 (Type 4). It's going to be my most expensive eyepiece ever, but I'm hoping it will be my best eyepiece too...

 

I believe the 19 mm eye relief should be sufficient for observing with my eyeglasses (as far as I can see about 16-17 mm of honest eye relief is adequate, which is why I usually look for eyepieces with 20 mm stated eye relief), but just in case it's not enough I can use my a Dioptrx corrector instead.

 

I really look forward to trying my first 82 degree eyepiece on the night skies in the fall jump.gif

 

Thanks for all the advice and insights, your help is much appreciated! bow.gif

 

Clear Skies,

 

Daniel

You’re going to love the views! If the Instadjust bothers you, there’s a fix for it. Let us know either way.


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#35 rkelley8493

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Posted 25 July 2019 - 06:29 PM

You’re going to love the views! If the Instadjust bothers you, there’s a fix for it. Let us know either way.

What's the fix for the instadjust? It's looser on my 17 than the 22.. 



#36 Starman1

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Posted 25 July 2019 - 06:42 PM

http://www.televue.c...n=Advice&id=100

This chain of posts, starting here: https://www.cloudyni...just/?p=6409968

shows how the Radian Instadjust eyecup is made snugger.  You use the same technique on the Nagler T4s.


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#37 Sarkikos

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Posted 27 July 2019 - 09:53 AM

Per the distance specs I cited, if your ST120 is f/5, you would need 10mm LESS working distance, so you would simply have to replace the nosepiece of your diagonal with spacers + TSFlat 2 to reach the 113mm working distance accordingly, at which point you should see same benefits you see in your 80.

 

Phil

The funny thing is the C80ED and the ST120 have the same focal length, 600mm.  According to the TSFlat2 specs, a 600mm focal length should require 113mm spacing.  But the TSFlat2 flattened the field nicely in the C80ED, but did not in the ST120.  For both telescopes, I screwed the TSFlat2 onto the barrel end of the Baader 2" Click-Lock mirror diagonal.  For both telescopes I used the same Ethos eyepieces.  So what gives?  shrug.gif

 

Mike


Edited by Sarkikos, 27 July 2019 - 09:54 AM.


#38 213Cobra

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Posted 27 July 2019 - 01:15 PM

The funny thing is the C80ED and the ST120 have the same focal length, 600mm.  According to the TSFlat2 specs, a 600mm focal length should require 113mm spacing.  But the TSFlat2 flattened the field nicely in the C80ED, but did not in the ST120.  For both telescopes, I screwed the TSFlat2 onto the barrel end of the Baader 2" Click-Lock mirror diagonal.  For both telescopes I used the same Ethos eyepieces.  So what gives?  shrug.gif

 

Mike

I don't have direct experience with either scope, but TSFlat2 is designed to work with f ratios from f/4 - f/9 and in my experience as long as you can get the working distance right (or close to it for visual) it works essentially equally well in that range. Without seeing your results directly, I have to wonder whether what you're seeing in the ST120 is something remaining after FC has been flattened out.

 

Phil



#39 Sarkikos

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Posted 27 July 2019 - 02:57 PM

No, what I'm seeing in the ST120 is definitely field curvature.  If I focus the telescope for a star toward edge of field, a star in the center of field will be out-of-focus.  If I focus for a star at center of field, a star toward edge of field will be out-of-focus.  This is how you test for field curvature. 

 

The TSFlat2 screwed onto the end of a Baader 2" Click-Stop mirror diagonal will flatten the field in the C80ED, but will not flatten the field in the ST120.  Both telescopes have a 600mm focal length.  The C80ED is f/7.5.  The ST120 is f/5.

 

What I see in the field trumps what TS says should happen.

 

Mike


Edited by Sarkikos, 27 July 2019 - 02:58 PM.


#40 Sarkikos

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Posted 27 July 2019 - 02:59 PM

What's the fix for the instadjust? It's looser on my 17 than the 22.. 

 

For me it was to sell my Radians.  No more instadjust.  I hate the instadjust the way many hate the undercuts. grin.gif

 

Mike


Edited by Sarkikos, 27 July 2019 - 03:01 PM.


#41 Starman1

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Posted 27 July 2019 - 03:41 PM

I just adjusted mine until they were tight enough it required a deliberate oof! to change their positions.



#42 213Cobra

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Posted 27 July 2019 - 04:29 PM

No, what I'm seeing in the ST120 is definitely field curvature.  If I focus the telescope for a star toward edge of field, a star in the center of field will be out-of-focus.  If I focus for a star at center of field, a star toward edge of field will be out-of-focus.  This is how you test for field curvature. 

 

The TSFlat2 screwed onto the end of a Baader 2" Click-Stop mirror diagonal will flatten the field in the C80ED, but will not flatten the field in the ST120.  Both telescopes have a 600mm focal length.  The C80ED is f/7.5.  The ST120 is f/5.

 

What I see in the field trumps what TS says should happen.

 

Mike

I get it. I'm saying that I've seen it be effective below f/5, no problem. So I don't have an explanation at the moment. -Phil



#43 Sarkikos

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Posted 27 July 2019 - 04:59 PM

My AR-102xs is f/4.5.  The TSFlat2 flattened the field for that telescope very nicely.  Sharp stars edge to edge in Ethos when the TSFlat2 is screwed onto the end of the Baader Click-Lock.  

 

Last night I also tried the same method with a TSFlat2 on my AT72EDII.  The field was flattened. 

 

So far the only telescope I've tried the TSFlat2 in that hasn't worked to flatten the field was the ST120.  Too bad, because I really wanted a flat field in a 120mm f/5.

 

Mike


Edited by Sarkikos, 27 July 2019 - 04:59 PM.


#44 SteveG

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Posted 28 July 2019 - 05:03 PM

I just adjusted mine until they were tight enough it required a deliberate oof! to change their positions.

I took mine a step further, and put black electrical tape on the spring, about half way around the barrel. Mine is now locked in the fully down position, right where I want it. It will take a pretty good tug to get it apart again.



#45 GeneT

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Posted 28 July 2019 - 05:48 PM

The 22 Nagler is an excellent eyepiece. Until I bought my 21 Ethos, it was my favorite wide angle eyepiece. The 22 Nagler has 82 AFOV compared to your Pentax 20 which has 70. That is a nice bump up. I did not find field curvature to be an issue in my 12.5 inch, F5 Dob. The 22 Nagler provided bright, sharp views. Its footprint is quite small and weight of 24 oz is quite reasonable compared to 36 oz for the Ethos 21. Of course, the weight of a 22 Nagler is about twice the weight of your 20 Pentax which is about 13 oz. However, the extra 12 degrees of AFOV for the 22 Nagler, compared to the Pentax is considerable, and in my opinion, should be seriously considered.   


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