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Comparing Telescopes - exit pupil or magnification ?

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#1 vkhastro1

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Posted 02 September 2020 - 09:07 AM

How would you compare Telescopes ?

 

For example: a 120mm f/5 achromat with focal length of 600mm

 

versus 

102mm f/7 ED with focal length of 714mm

 

An APM 30mm UFF, TV 35mm Panoptic and Vixen 42mm LVW would yield different magnifications and exit pupils (EP) in both scopes:

 

120mm f/5 : 30mm UFF = 20X + EP = 6mm

102mm f/7 : 35mm Pan = 20.4X + EP = 5mm

The magnifications are basically the same but EPS are different

 

120mm f/5 : 30mm UFF = 20X + EP = 6mm

102mm f/7 : 42mm LVW = 17X + EP = 6mm

exit pupils are identical but magnifications are different

Since the focal lengths and F/ratios are different it is impossible to have both the magnifications and exit pupils be identical in both scopes.

 

What would be the best options for a comparison? magnication, exit pupil or both?

 


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#2 KBHornblower

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Posted 02 September 2020 - 09:19 AM

I don't think there is an objective answer about best options.  It depends on your needs and desires.


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#3 Jon Isaacs

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Posted 02 September 2020 - 09:25 AM

It depends on the object as well as many other things..

 

I generally compare scopes based on the best view of the object each provides.

 

Jon


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#4 Tyson M

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Posted 02 September 2020 - 09:35 AM

My instincts say exit pupil on faint dso and lunar viewing

 

Magnification for planets.


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#5 TOMDEY

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Posted 02 September 2020 - 09:37 AM

It's like looking into the silverware drawer for a utensil. What is best for the job (ice cream, spaghetti, steak, soup, serving...) is context-dependent. Aperture provides total light, focal length + eyepiece provides magnification, exit pupil provides scene brightness. And all relate to each other.    Tom


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#6 ButterFly

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Posted 02 September 2020 - 10:11 AM

All three - you forgot the true field of view.

 

Some general goals of a set of eyepieces used in a particular telescope include: a nice big exit pupil with a big field for the dim stuff; a nice high power for the small bright stuff (a zoom fits the bill nicely here); and, reasonable steps between the extremes that aren't unnecessarily redundant.

 

For most sets of eyepieces, the middle ground works in any scope.  For a particular scope, you may lose some of the set at the higher or lower ends.

 

 

It's like looking into the silverware drawer for a utensil. What is best for the job (ice cream, spaghetti, steak, soup, serving...) is context-dependent. Aperture provides total light, focal length + eyepiece provides magnification, exit pupil provides scene brightness. And all relate to each other.    Tom

Very apt.  Try eating steak with only a kniife.  Spaghetti can be had with only a fork, but it's less messy paired with a spoon.



#7 Hesiod

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Posted 02 September 2020 - 10:25 AM

I test at both the same magnification if are interested in judging their relative "quality" and are close in sizes; at the same pupil if the sizes differ a lot

#8 JGass

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Posted 02 September 2020 - 10:59 AM

I find that modest differences in magnification are not usually a big deal.  The eye and brain compensate somewhat. Image brightness, contrast, and clarity are more important to me. 

 

Maximum detail is revealed at about 1mm exit pupil, but 2mm is closer to optimal for visual work.



#9 rhetfield

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Posted 02 September 2020 - 11:07 AM

I would say both.  Higher magnification will always show more detail if the exit pupil is consistent.  However, if the exit pupil is at an extreme, the quality of the view will be deteriorated.   The telescope that is able to show the optimal magnification/FOV at the optimum exit pupil will always outperform one that doesn't



#10 rhetfield

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Posted 02 September 2020 - 11:29 AM

Try eating steak with only a kniife.  Spaghetti can be had with only a fork, but it's less messy paired with a spoon.

gramps.gif Both steak and spaghetti can be eaten with your fingersbelushi.gif


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#11 ButterFly

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Posted 02 September 2020 - 11:43 AM

gramps.gif Both steak and spaghetti can be eaten with your fingersbelushi.gif

That's naked eye astronomy.

 

A bib is still advisable for drool worthy views.


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#12 Echolight

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Posted 02 September 2020 - 12:06 PM

I have learned from this forum, thanks SeattleScott, exit pupil for nebula filters.

 

But I ordered an HDC 20mm 100° specifically for open clusters in my C8 under light poluted skies.

Works out to have what I have read is the minimum optimum exit pupil for medium sized DSO, 2mm. And the maximum optimum magnification for same, 100x. And the widest AFoV in that category.

 

But I also got a 40mm 72° for max exit pupil and max fov in most scopes. Which I hope to obtain a filter or two to use with in the near future.

 

Now if I was buying for my yet to be unobtained

ED80, I'd have went with a 30 UFF for a decent fov and exit pupil without dipping into binocular territory with the magnification. And a 17.5 Morpheus for for  general purpose scanning and viewing. Mostly similar specs to a ST120.

 

Depending on how well I like using a 100 degree eyepiece, I might pick the 30uff and 17.5 morpheus up anyway for their notorious ease of viewing.


Edited by Echolight, 02 September 2020 - 12:07 PM.


#13 gnowellsct

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Posted 02 September 2020 - 01:04 PM

How would you compare Telescopes ?

 

Since the focal lengths and F/ratios are different it is impossible to have both the magnifications and exit pupils be identical in both scopes.

 

What would be the best options for a comparison? magnication, exit pupil or both?

That is true.  

 

Stating magnification typically lets people know: did you have a tiny view of Saturn at 40x or did you have a humongo view at 300x?  This is useful particularly when referring to seeing conditions. 

 

Exit pupil is the much more useful and much less understood metric.  For one thing, realize that exit pupil in mm at the eyeball is just one way of stating magnification, it's just that we customarily look at magnification as the total sum, rather than the function of the parts.

 

On any telescope 0.5x per mm of aperture = 2 mm exit pupil

On any telescope 1x per mm of aperture = 1mm exit pupil.

On any telescope 2x per mm of aperture =0.5 mm exit pupil.

 

If you know the scope's focal ratio you know the oculars needed to get both.  And by using ratios you can interpolate everything else (magnification, approximate ocular size required).   There is an enormous amount of information packed into this way of thinking.

 

If you have an 80mm f/7 and are at 0.5x per mm of aperture we know you are at 40x and a 2mm exit pupil and using a 14 mm ocular.  If you have 2x per mm of aperture you are at 160x and using a 3.5 mm ocular.  And if you are at 80x you are using a 7 mm ocular.  The ocular at the focal length of the telescope always yields the magnification of the objective (or primary mirror) in mm.  In an 8" (200 mm) f/10 SCT a 10 mm ocular yields 200x.

 

This is why we are so stupid to keep using imperial measurement and talking about things like "50x per inch".  Inches are not useful in this game.

 

ANYHOW, we also know

 

That the zone of optimal viewing, relative to the organic structure of the eye, is 1mm to 2 mm.

That high powers, relative to what the eye will accommodate, basically start at 1mm exit pupil and below.

That there is seldom any utility (in terms of information in the view)  to pushing magnification below 0.5 mm exit pupil, though all of us do it.

That as a rule color saturation starts to fade at 0.5 mm and below (as in 0.3 mm exit pupil etc.) This is also called "dimming out". 

=> How dim do you like it?  That is one of the determinants of how far to push magnification.  (along with floaters, see below)

That if you want to convert exit pupil to magnification per mm (and from there to the total magnification of the scope) you just take the reciprocal of the exit pupil.  0.5mm exit pupil = 1/0.5 = 2x per mm.  0.3 mm exit pupil = 1/0.3 = 3.3x per mm.   Because that is substantially more than 2x per mm, we know the magnification is in all likelihood being pushed past what yields a meaningful improvement in the view--"empty magnification."

A lesser known feature of exit pupil is the "astigmatism boundary."  About 30 to 60% of the population has astigmatism.  Astigmatism creates viewing defects as magnification decreases.  Where the exact boundary is depends on the individual, but let us say that people with astigmatism who don't use corrective lenses will probably begin to have problems as exit pupil increases beyond 4 to 5 mm.

Another organic eyeball structural feature is on the opposite end.  Everyone has what are called "floaters" which are junk that comes between you and what you want to see.  It is illuminated very well at exit pupils of 0.5 mm and below.  And I have heard people say they prefer to stay 0.8 mm.

=>The general rule for floaters is, the better the telescope you can afford, the older you are, and as a result, less able to appreciate your precision optic with your decrepit eyes.  This also applies to astigmatism.

 

If you have certain magnification goals in mind as a regular basis then you should pay attention to magnification per mm.  If you're going over 2x per mm then you should probably be thinking about a bigger telescope.  

 

There is also what may be called the general magnification ceiling prevailing over most places most times, 200x to 300x being where magnification caps except in extraordinarily stead places like on top of Hawaii, in the Andes, or in Florida.  You find out what it is for your area by trying it out or talking to other amateur astronomers.

 

Once you know how tight an exit pupil you can tolerate, what the local seeing cap is, and whether you are bothered by low powered astigmatism, you are in a position to consider your optimal focal ratio and aperture.  

 

so in the case of these scopes:

 

a 120mm f/5 achromat with focal length of 600mm versus

102mm f/7 ED with focal length of 714mm

 

I say well do I want to use the refractor for low power viewing or high power viewing.  Because to get to 2x per mm in the 120mm f/5 I will need a 2.5 mm ocular.  Those tend not to be pleasant to look through for long times.  In addition, it is an achromat, and that would bother me, so I would automatically cut in half the magnification I expect to get from it:  1x per mm or less.  On the other end, in the low magnification zone, I would consider my personal low exit pupil comfort zone to be about 5 to 6 mm.  That would suggest a 25 to 30 mm eyepiece.  Beyond that it is likely to be unusable to me without either a dioptrx or wearing contacts, etc.  

 

I like my 40 mm XW.  This scope could not use it.

Because it is a large fast achromat this scope will also be losing about half the useful high power magnification on planets.  

With such limitations at the bottom and top of the exit pupil scales, I would not buy this scope.  The 102 mm f/7 is the obvious choice.

 

A more interesting case is:

130 mm f/7 ED vs 102 mm f/7 ED.

 

I like magnifications in the mid 200s.  That's a big plus for the 130 mm.  To get into the mid 200s with the 102 mm I would either have to use eyepieces below 3.5 mm or take say a barlow and put it on a 5 mm.  Those combinations do not appeal.

 

On the other end it is pretty clear that the 102 mm is in a good place at the 5 mm exit pupil for wide fields.  That would be one fifth the 1mm exit pupil that gives 1x magnification per mm, we can see from the relationship that 0.2x per mm will give a 5 mm exit pupil and 102*0.2=20x magnification.  That is by definition a wide field view, if we need a 7mm ocular for 1x per mm then we will need a 35mm ocular or, for some of us, a 40mm such as the Pan 41 or XW40.

 

On the 130 mm f/7 telescope the 7 mm gives us 130x at 1mm exit pupil and we can work from that to get the 5 mm exit pupil ratios 130/5=~25x magnification, and 5*7 = 35 mm ocular zone again (hence the famous Pan 35).   

 

The catch is the *lowest* magnification is now 25x instead of 20x, so the field of view MUST decrease.  Hence one critical element of having a wide field is not to push the aperture too big...in the usual focal ratios.  

 

On the other hand our "sacrifice" of a 20x wide field view has "purchased" for us easier access to high magnifications.  A 1mm exit pupil "buys" us 130x, not 100x.    

 

The quest for magnification at one end of the viewing experience and wide fields at the other is what pushes some people to buy different aperture refractors, or to buy a small refractor and some other aperture telescope (not necessarily a refractor) to use with it.

 

To wit:

 

20190425_180731-1_resized.jpg

 

Well it's all just arithmetic folks no fancy calculus here.  But I fear the MEGO factor in this answer is too great (My Eyes Glaze Over).


Edited by gnowellsct, 02 September 2020 - 01:11 PM.

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#14 Xyrus

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Posted 02 September 2020 - 09:28 PM

My instincts say exit pupil on faint dso and lunar viewing

 

Magnification for planets.

Yes.

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#15 Jon Isaacs

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Posted 03 September 2020 - 07:51 AM

How would you compare Telescopes ?

 

For example: a 120mm f/5 achromat with focal length of 600mm

 

versus 

102mm f/7 ED with focal length of 714mm

 

An APM 30mm UFF, TV 35mm Panoptic and Vixen 42mm LVW would yield different magnifications and exit pupils (EP) in both scopes:

 

120mm f/5 : 30mm UFF = 20X + EP = 6mm

102mm f/7 : 35mm Pan = 20.4X + EP = 5mm

The magnifications are basically the same but EPS are different

 

120mm f/5 : 30mm UFF = 20X + EP = 6mm

102mm f/7 : 42mm LVW = 17X + EP = 6mm

exit pupils are identical but magnifications are different

Since the focal lengths and F/ratios are different it is impossible to have both the magnifications and exit pupils be identical in both scopes.

 

What would be the best options for a comparison? magnication, exit pupil or both?

The reason that it is impossible to have the same magnification and exit pupil is that one scope is an 102mm and one scope is a 120mm.

 

Exit pupil = Aperture / Magnification  =>  Aperture = Exit pupil x Magnification.  

 

As Butterfly said, you also need to include the TFoV.    

 

Are you going to test these telescopes side by side or compare them on paper?

 

Jon


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#16 Jeff B

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Posted 03 September 2020 - 08:05 AM

I do both as best I can as I find both very useful to me. 

 

Trouble is, with very different scopes, it's hard to match magnifications and exit pupils and I typically have to use eyepieces that are of differing designs as well, which is just one more potential variable.  Every once in a while though, I get lucky as when I compared a TEC 140 FL with my 140ED, and my CFF 160 F6.5 to my home-brew 152 F6.5 achromat.  

 

Jeff


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#17 StarAlert

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Posted 03 September 2020 - 09:22 AM

If you’re asking how one would make a “fair” comparison between two scopes that have different apertures, it simply can’t be done. I do capital markets research and I face the same problem when I want to make a comparison between large firms and small firms. In order to make a fair (i.e., equivalent) comparison I need to scale the “performance” variable by size. For example, Net Income/total assets = Return on Assets. If I don’t do this, the large firm would most certainly beat the small firm every time, simply because it’s bigger, not because it’s better or more efficient.  

 

Larger aperture scopes enjoy the same advantage, and unfortunately (or perhaps, fortunately), we can’t scale the performance (i.e., magnification, exit pupil, etc.) by the size advantage in order to make a “fair” comparison. It will always have an advantage. This is why “Aperture Rules”. 


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#18 gnowellsct

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Posted 03 September 2020 - 12:40 PM

 This is why “Aperture Rules”. 

...except for wide fields.  Aperture tends to scale up focal length, and shortening focal ratio to keep focal length in check leads to many optical difficulties that can either increase costs or compromise views.

 

GN


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#19 gnowellsct

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Posted 03 September 2020 - 01:33 PM

All three - you forgot the true field of view.

 

Some general goals of a set of eyepieces used in a particular telescope include: a nice big exit pupil with a big field for the dim stuff; a nice high power for the small bright stuff (a zoom fits the bill nicely here); and, reasonable steps between the extremes that aren't unnecessarily redundant.

 

For most sets of eyepieces, the middle ground works in any scope.  For a particular scope, you may lose some of the set at the higher or lower ends.

 

 

Very apt.  Try eating steak with only a kniife.  Spaghetti can be had with only a fork, but it's less messy paired with a spoon.

Actually magnification helps to see dim galaxies, and there are many many dsos that are invisible in my 40mm which are pretty easy to see with a more aggressive ocular/magnification.  Greg N


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#20 Sketcher

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Posted 03 September 2020 - 02:10 PM

Exit-pupil is not a property of the telescope -- until an eyepiece is added, but eyepieces of different focal-lengths will provide different exit-pupils with the telescope.  Therefore, one cannot use exit-pupils.

 

Magnification is not a property of the telescope -- until an eyepiece is added, but eyepieces of different focal-lengths will provide different magnifications with the telescope.  Therefore, one cannot use magnifications.

 

I'm beginning to feel like the Sicilian, from The Princess Bride smile.gif

 

I don't think "comparing" means what you think it means (shades of Inigo Montoya) smile.gif .  Besides, we're probably more interested in "contrasting."  

 

I suggest sticking with those things that are properties of the telescopes -- aperture, focal-length, weight, length, tube diameter, dew-shield length to diameter ratio, OTA color, focuser type and size, materials used in the construction of the various component parts, etc.

 

Then there are quality differences:  color-correction, strehl ratios, percentage of incident light that's transmitted through the objectives, optical coating type and quality, Does the glass add color to the images?

 

If you're going to compare (or contrast) the telescopes, then compare (or contrast) the telescopes.  This cannot be accomplished with exit-pupils nor can it be accomplished with magnifications.


Edited by Sketcher, 03 September 2020 - 02:14 PM.


#21 ButterFly

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Posted 03 September 2020 - 04:09 PM

Actually magnification helps to see dim galaxies, and there are many many dsos that are invisible in my 40mm which are pretty easy to see with a more aggressive ocular/magnification.  Greg N

Totally agree.  Contrast sensitivity (very personal) depends very much on the size of the object's features and how much brighter they are than the background.  At a very dark site, both M51's spirals and the tidal bridge are very obvious at the same time in an 8mm Ethos (~230x) on the 15".  Any more or less and I start losing contrast of some features over others.  "Invisible" may be an overstatement unless the skies are just too milky and the object too small.



#22 RichA

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Posted 03 September 2020 - 05:50 PM

How would you compare Telescopes ?

 

For example: a 120mm f/5 achromat with focal length of 600mm

 

versus 

102mm f/7 ED with focal length of 714mm

 

An APM 30mm UFF, TV 35mm Panoptic and Vixen 42mm LVW would yield different magnifications and exit pupils (EP) in both scopes:

 

120mm f/5 : 30mm UFF = 20X + EP = 6mm

102mm f/7 : 35mm Pan = 20.4X + EP = 5mm

The magnifications are basically the same but EPS are different

 

120mm f/5 : 30mm UFF = 20X + EP = 6mm

102mm f/7 : 42mm LVW = 17X + EP = 6mm

exit pupils are identical but magnifications are different

Since the focal lengths and F/ratios are different it is impossible to have both the magnifications and exit pupils be identical in both scopes.

 

What would be the best options for a comparison? magnication, exit pupil or both?

It's not possible to have both exit pupil and mag. the same in both scopes because of aperture.  In-terms of ability to show details, no way a 120mm f5 achromat will ever match a 102mm f/7 apo on a planet, unless the optics of the apo were much worse. A 120mm f5 achro has severe colour error and as such is really only suited to deepsky objects at lower powers.  Min I would  go with a 120mm achromat is f8.  However, on deepsky objects, the 120mm will have a light-gathering edge on the smaller apo but the better acuity of the apo will probably result in a more pleasing image.



#23 vkhastro1

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Posted 03 September 2020 - 06:02 PM

The reason that it is impossible to have the same magnification and exit pupil is that one scope is an 102mm and one scope is a 120mm.

 

Exit pupil = Aperture / Magnification  =>  Aperture = Exit pupil x Magnification.  

 

As Butterfly said, you also need to include the TFoV.    

 

Are you going to test these telescopes side by side or compare them on paper?

 

Jon

I plan on doing a direct comparison.

I managed to pick up a basically brand new (used of course) Astro-Tech 102ED f/7 OTA from another Canadian amateur.

Cosmetically it is an amazing scope. Waiting for first light.

 

I already made some upgrades to the AT 102ED.

Kept the nice metal objective (put away for safe keeping) and replaced with a retail material Lenccoat  “hoodie”

Replaced the original Vixen dovetail plate (rings attached with 2 metric M6 bolts) with the heavy duty William Optics Vixen dovetail (each ring is now attached with 2 M6 bolts) plus added a large safety bolt at the top end of the dovetail to prevent possible slippage out the mount’s dovetail clamp

Removed 2XM4 screws on the left side of the focuser and added a slotted finderscope dovetail to accept a 8x50 RACI Finder.

 

The 120mm f/5 has a 38% light grasp advantage but much worse field curvature. The AT 102ED f/7 should have the better optics and definitely the better focuser. Should be an interesting comparison. Not sure if I will kept the 120mm f/5 after the shootout because I optically prefer my Skywatcher 130PDS f/5 reflector over the 120mm achromat.


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#24 gnowellsct

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Posted 03 September 2020 - 07:22 PM

Totally agree.  Contrast sensitivity (very personal) depends very much on the size of the object's features and how much brighter they are than the background.  At a very dark site, both M51's spirals and the tidal bridge are very obvious at the same time in an 8mm Ethos (~230x) on the 15".  Any more or less and I start losing contrast of some features over others.  "Invisible" may be an overstatement unless the skies are just too milky and the object too small.

NGC 6207 last time I tried was invisible in my fov at 16x but visible at 80x.  It is Mag 11.4 and only three arc minutes long and is hard to see in a 4.5 degree field of view.  I"m sure someone has done it at a dark enough site but in a 270 arc minute field of view (4.5 degrees) it is about 1/90th of the total field.  It doesn't leap out, to me I could not see it.  Maybe someone will say they can do it 16x and I'll give it another try.    I think you'd need first class transparency.

 

At 80x not so much of a problem. Not exactly bright though in 80 or 90 mm.  

 

Interestingly one year I was observing NCG 5053 a 9 mag 10 arc min globular near M53 and we decided to back it down, from 14" to 4", then from 4" to 3" scope, then from there to a 50 mm right angle finder.  It was still visible!  Kinda blew my mind.  But 2 mags brighter and 3x the size of NGC 6207.  

 

Greg N



#25 vkhastro1

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Posted 03 September 2020 - 08:38 PM

Just from a short comparison (under full moon conditions) evaluation of the AT 102 ED.

Lasted 20 minutes after twilight ended and before the clouds rolled in.

 

The scope is a winner !

fantastic focuser - smooth inside/outside focus with no image shift

APM 30mm UFF was perfectly flat to the edge - 23.8x FOV 2.73° Surprisingly dark background considering the full moon interference. Double Cluster was perfectly framed - beautiful pinpoint stars.

used the same eyepiece and located NGC 7789 with averted vision with just a hint of a few stragglers 

M13 was nicely resolved with an 11mm DeLite and easily took a 5mm DeLite (143X) with snap focus 

 

Next clear night the shootout vs the Orion 120mm f/5.


Edited by vkhastro1, 04 September 2020 - 10:47 AM.

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