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Is a fast flat-field refractor "worth it" for visual use?

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#1 bokemon

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Posted 26 November 2022 - 08:46 PM

Hello folks, 

Browsing the telescopes at TS again.  I have a 71mm F5.9 and 115EDT F7 that I use for visual.  NOT for imaging, since I use newtonians on an EQ mount for that.

I was thinking what if I get a get a "fast" and flat field refractor around 90-100mm to fill in the gap and be a more portable setup.  The 115EDT is really long and unwieldy, and since it is front-heavy, the eyepiece end sticks out a lot and changes height for different altitudes. Right now I am using a Stellarvue alt-az mount and Amici diagonal, and very happy to just push the scope around with correct-image.

The advantage of this type of scope is that it is physically shorter, plus I can just drop in a camera to do EAA / plate solving without worrying about field flatteners and lens spacings.  In this instance, I am referring to the 4+ lens scopes with the flattener embedded further up the tube, so that I can insert diagonals and barlows and etc and just set the focus.  NOT referring to the kind where the flattener is added retroactively and must be a certain distance from the eyepiece or camera.

 

My questions are if there is any drawback to getting this type of scope.

1) The resolution should be determined by the aperture anyway, so what's the difference between F7 and a particular eyepiece vs F5.6 and a slightly shorter length eyepiece?

2) Will the flat field "help" some of the eyepieces be more in-focus near the edges?  Or are most eyepieces already designed with field curvature in mind?  For example, I have a Baader 8-24x zoom and Paradigm 15mm and both have a little bit of field curvature.

 

Thanks



#2 Sacred Heart

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Posted 26 November 2022 - 08:59 PM

If it were me, I'd put that question to Mr Nagler at Televue.

 

According to Astronomy Tools, not much difference between a AT115EDT with a type 4 22MM Nagler and a Borg 107 F3.9 with a Nagler type 4 12MM.

 

Joe

 

http://astronomy.too.../field_of_view/



#3 bobhen

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Posted 27 November 2022 - 09:28 AM

 

My questions are if there is any drawback to getting this type of scope.

1) The resolution should be determined by the aperture anyway, so what's the difference between F7 and a particular eyepiece vs F5.6 and a slightly shorter length eyepiece?

2) Will the flat field "help" some of the eyepieces be more in-focus near the edges?  Or are most eyepieces already designed with field curvature in mind?  For example, I have a Baader 8-24x zoom and Paradigm 15mm and both have a little bit of field curvature.

 

Thanks

1. No difference, if the eyepieces are of good quality like, for example, TV Delites.
2.  Put it this way, a flat field refractor will not "add" to eyepiece edge aberrations. But a flat field will also not correct for eyepieces that do have edge aberrations.

 

The drawback to getting a scope with a wide flat field like a TV 101 or TV 127 is that to take advantage of that wide, flat field for visual, deep sky observing, you need a nice dark sky. If you have mild or heavy light pollution you might not or will not be able to take advantage of what a low power, wide, flat field refractor has to offer. So, you will be spending extra money on something you cannot take full advantage of.

 

Something else top consider: In the summary of his review of a TV 101 Rodger Vine stated:  A Takahashi FC-100D (or TSA-102, TMB 100-8 etc) (Takahashi FC 100dz) might well be just a tiny fraction sharper on planets at high power.

 

And for "low power", wide field, deep sky observing you don't really need an apo for that specific task. A fast achromat like the Sky Watcher 120 F5 coupled with a field flattener accessory will deliver the goods for a fraction of the cost of a TV 101.

 

Bob



#4 ewave

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Posted 27 November 2022 - 09:35 AM

A fast quad refractor like the TV 101 is the only thing I can think of at the moment, I'm sure there are other 4" fast quads out there.

Even so, the TV 101 isn't exactly considered a short OTA.  You will still need flat field eyepieces, but at least there are other manufacturers

that make FF eyepieces besides Televue nowadays.



#5 ewave

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Posted 27 November 2022 - 09:47 AM

As a matter of fact, I was thinking something similar for my current 4" Tak DZ doublet. 

Since my eyes are sensitive to field curvature, I might try the APM 30mm UFF eyepiece.

It goes a little wider than my 27mm TV Pan, but still provide a rich field in focus. But I wouldn't attempt

to go much wider than this on my F8 Tak as I think the FC would suffer the laws of diminishing returns.

The 4" Tak DZ weighs around 10 lbs but is rather long operating at F8.



#6 Space_Race_T.J.

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Posted 27 November 2022 - 12:38 PM

Askar FRA500 90mm f/5.6 Quintuplet Petzval at $1,999 is another option in this category.

 

https://agenaastro.c...aph-fra500.html



#7 RyanAstroMan

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Posted 27 November 2022 - 03:57 PM

As Bobhen mentioned, a ST120, with an upgraded 2 inch focuser, the TSFLAT2 field flatener to replace the diagonal nose and an APM 30mm UFF is a super combo for sky sweeping and is my go to RFT (rich field telescope). It does scratch the itch for wanting a TV 101.

#8 tony_spina

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Posted 27 November 2022 - 11:34 PM

Agree the ST120 with upgraded 2" focuser,  TSFLAT2,  and APM 30mm is a wonderful sweeper. Add a UHC filter and it gets even better 



#9 Eddgie

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Posted 28 November 2022 - 10:05 AM

Hello folks, 

Browsing the telescopes at TS again.  I have a 71mm F5.9 and 115EDT F7 that I use for visual.  NOT for imaging, since I use newtonians on an EQ mount for that.

I was thinking what if I get a get a "fast" and flat field refractor around 90-100mm to fill in the gap and be a more portable setup.  The 115EDT is really long and unwieldy, and since it is front-heavy, the eyepiece end sticks out a lot and changes height for different altitudes. Right now I am using a Stellarvue alt-az mount and Amici diagonal, and very happy to just push the scope around with correct-image.

The advantage of this type of scope is that it is physically shorter, plus I can just drop in a camera to do EAA / plate solving without worrying about field flatteners and lens spacings.  In this instance, I am referring to the 4+ lens scopes with the flattener embedded further up the tube, so that I can insert diagonals and barlows and etc and just set the focus.  NOT referring to the kind where the flattener is added retroactively and must be a certain distance from the eyepiece or camera.

 

My questions are if there is any drawback to getting this type of scope.

1) The resolution should be determined by the aperture anyway, so what's the difference between F7 and a particular eyepiece vs F5.6 and a slightly shorter length eyepiece?

2) Will the flat field "help" some of the eyepieces be more in-focus near the edges?  Or are most eyepieces already designed with field curvature in mind?  For example, I have a Baader 8-24x zoom and Paradigm 15mm and both have a little bit of field curvature.

 

Thanks

First Radius of Curvature (RC) in a doublet or triplet is totally a function of focal length. It does not matter what aperture. If you are going with a doublet or triplet, the longer the focal length, the flatter the field.  The RC is about 1/3rd of the focal length, so a doublet or triplet with 1000mm focal length will have an RC of about -330mm.  By comparison, a reflector has an RC that is about the same as the focal length, so a reflector with a focal length of 1000mm will have an RC of about 1000m.

 

As mentioned, you can also add a field flattener, but these are typically designed to work with imagers and a specific spacing is required for that flattening to be most effective.  If you use a diagonal behind it, don't expect the same pinpoint stars you would get with a camera sensor.

 

Telescopes like the TV 101 will have much flatter fields but of course this comes at a cost.  To get that field to be flat, the TV 101 essentially has a built in field flattener that is optimized for the focal plane to be much more distant than is typical with field flatteners designed for imaging. 

 

The easiest way to deal with field curvature is to use longer focal length, narrower field eyepieces of the same apparent field. The longer the focal length, the less visual accommodation is needed to keep the field looking sharp.  For example, rather than a 31mm Nagler, a 41mm Panoptic has lower power but still offers a wide field of view (in this case, even wider than the 31mm Nagler) will show stars sharper at the edge of the field.

 

A 27mm Panoptic has a 30.5mm field stop vs the 27.4 field stop of the 20mm Nagler and once again, due to the shorter focal length, even though it has a bigger true field, the 27mm Pan will have less power and stars will appear sharper at the edge.

 

In these cases, it is not that the eyepiece is better, it is that anywhere at the outside of the field, the defocused blur is smaller and your eye has an easier time accommodating any defocus.

 

Young observers can accommodate 2 diopters of defocus but seniors will struggle to accommodate .5 diopters of focus.  Depending on your age, the use of a longer focal lenght eyepiece with about the same size true field will generally produce sharper stars at the edge. 

 

As powers go up, you can use wider field eyepieces but at the lower power end of the spectrum, where you are seeing well away from the center of the field. 

 

A flat field eyepiece is not always the best choice.  They are best with telescopes with low field curvature, such as Newtonians, because in those scopes, the field is much less curved than with refractors. 

 

Here is a graphic that explains why field curvature in an eyepiece can actually be good! Scroll down about half way on the page and you will see how eyepieces with field curvature can actually make a telescope that has field curvature produce a field that is sharp all the way across.

 

https://www.telescop... toward the eye.

 

So, in a telescope with a curved field, the best eyepiece would be one that has a curve that is as close to the curve of the telescope's field as possible. 


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#10 Jon Isaacs

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Posted 28 November 2022 - 12:51 PM

 

So, in a telescope with a curved field, the best eyepiece would be one that has a curve that is as close to the curve of the telescope's field as possible. 

 

Of course that is unrealistic. That would require eyepieces with differing amounts of field curvature to match the many difference telescopes. It's much more practical to correct the field curvature and use quality eyepieces with a flat field. 

 

I like fast refractors with a flat field.. some random thoughts:

 

- The low power, wide field views a scope like the NP-101 provides require dark skies. If I didn't spend a lot of time under dark skies, the sharp-sharp views right to the edge wouldn't be worth it.. low power wide field is not too interesting under light polluted skies and the light pollution tends to hide the aberrations anyway.

 

- Field curvature is an issue with low power, wide field eyepieces but disappears quite quickly with shorter focal length eyepieces. The radius of curvature of a 400 mm focal length refractor is about 5 inches. That's a 10 inch diameter, slightly larger than a basketball.

 

If you look at a 2 inch diameter section of the surface of a basket, it's obviously curved, if you look at a 1/2 inch diameter section, it seems flat. The 2 inch section has 16 times more curvature.

 

- Fast, flat field refractors are very demanding when it comes to eyepieces. This is because they're fast and so perfect across the field that eyepieces that are not sharp-sharp across the field in a fast scope, it's right there in your face. And this applies to all eyepieces, not just the low power wide field eyepieces.

 

Splitting close doubles with the NP-101 requires short focal length eyepieces. An eyepiece like the 4 mm TMB planetary is pretty good at F/7 but in the NP-101 at F/5.4, it quickly degrades off axis. The 3.5 mm Nagler is sharp to the edge. The 4 mm TMB improves dramatically if Barlowed with a 1.5 x or 2x Barlow.

 

- The TSFLAT2 is a game changer. Field flatteners have been around a long time but in general they can't be used visually. There's not enough back focus to be used in front of a 2 inch diagonal, too much to be used behind it and most flatteners are reducers that eatup focuser travel.

 

The TSFLAT2 has enough back focus that it can replace the nosepiece in a 2 inch diagonal and suddenly you have a pretty good imitation of an NP-101. For November, I brought .my Astro-Tech AT-80LE to the high desert it's an 80 mm F/6 FPL-53 doublet and with the TSFLAT2, it's really an effective wide, flat field refractor and rides nicely on my favorite mount, the Vixen Portamount. 

 

With the 41 mm Panoptic, it provides a 5.5° field at 12x. With the 28 mm UWA, it provides a 4.9° field at 17x. 

 

- The TSFLAT2 also works with achromats. To my eye, the better color correction of an apo/Ed does provide slightly sharper views with eyepieces like these but it's subtle.

 

So, no doubt about it, the TSFLAT2 is a game changer. It's similar to a Paracorr. One corrector with a little fiddling solves visual field curvature issues for refractors from 400 mm-700 mm focal length.

 

Jon


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#11 CQDDEMGY

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Posted 28 November 2022 - 10:19 PM

Is the TSFLAT2 removed from the diagonal to use high power eyepieces



#12 tony_spina

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Posted 28 November 2022 - 11:17 PM

Is the TSFLAT2 removed from the diagonal to use high power eyepieces

No need to do so. 


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#13 bokemon

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Posted 29 November 2022 - 01:03 AM

 

- The low power, wide field views a scope like the NP-101 provides require dark skies. If I didn't spend a lot of time under dark skies, the sharp-sharp views right to the edge wouldn't be worth it.. low power wide field is not too interesting under light polluted skies and the light pollution tends to hide the aberrations anyway.

 

 

- Fast, flat field refractors are very demanding when it comes to eyepieces. This is because they're fast and so perfect across the field that eyepieces that are not sharp-sharp across the field in a fast scope, it's right there in your face. And this applies to all eyepieces, not just the low power wide field eyepieces.

 

Splitting close doubles with the NP-101 requires short focal length eyepieces. An eyepiece like the 4 mm TMB planetary is pretty good at F/7 but in the NP-101 at F/5.4, it quickly degrades off axis. The 3.5 mm Nagler is sharp to the edge. The 4 mm TMB improves dramatically if Barlowed with a 1.5 x or 2x Barlow.

 

- The TSFLAT2 is a game changer. 

Thanks for the reply.  There's a couple of things I'd appreciate more elaboration on:

1) requires dark skies - I mean, what doesn't benefit from dark skies?  From my Bortle 7/8 driveway with the nearby streetlights, pointing at random things with the 30mm Ultra Flat and a 420mm focal length scope was more fun than the 805mm scope because it showed "more spots per area".

2) Regarding a fast flat scope being more demanding on eyepieces:  The "flat" part should make it less demanding?  Or you mean the slightly lower F number?  But if F 5.x is so "demanding", then there's no way these eyepieces would work in a F 4 newtonian.  (Which I also want to try reviving for visual use)

3) Regarding the TSFLAT2 - yes, I've had one for more than a year, but I think mine is defective.  It makes the stars more bloated and ADDS chromatic aberration.  When I asked TS about it, they said that "maybe it was tilted".  I put it away for a while to do imaging with my reflectors.  But recently I took it out again, and it showed the same symptoms in my other triplet refractor too.  Anyway, I would rather have the flattener element embedded further up the tube where I don't have to worry about spacing issues.


Edited by bokemon, 29 November 2022 - 01:05 AM.


#14 tony_spina

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Posted 29 November 2022 - 01:14 AM

My first TSFLAT2 was also defective.  Astigmatism.  TS replaced it with a working unit 




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