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maximum TFOV with Apex 90

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#1 Nicole Sharp

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Posted 05 December 2018 - 06:31 PM

What is the largest possible true field of view visible with an Orion Apex 90 (1250/90, baffle-tube diameter of 17 mm)?

 

With a baffle-tube diameter of 17 mm, what 1.25" eyepieces might have vignetting, other than 40-mm Plossls?

 

Would something like a 52-degree 32-mm Plossl, a 62-degree 26-mm Explore Scientific, or a 68-degree 24-mm Explore Scientific eyepiece work?


Edited by Nicole Sharp, 05 December 2018 - 06:45 PM.


#2 jallbery

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Posted 05 December 2018 - 07:21 PM

The vignetting comes from the telescope, not the eyepieces.

 

True field is a property of the telescope's focal length and the eyepiece field stop diameter.  Given that all the eyepieces you list have pretty much the same field stop diameter (27mm), they will all show pretty much the same true field.  

 

So everyone of these eyepieces will show you the same field with the same amount of vignetting (unless the eyepiece itself creates some) and they all will work.  I've never used the Mak in question, but based on similar scopes, I would expect there to be useful illumination all the way to the fieldstop.    On my B&L 4000 (a 4" SCT at F/12, so pretty similar to a 90mm Mak), I used to use a 40mm plossl as my "finder" eyepiece, and I never found the vignetting distracting or particularly noticeable, for that matter.  

 

Photographically, I'd expect the vignetting to be more obvious.

 

The baffle tube is a restriction but it is out of focus.   Falloff will start before you even get to the 17mm point.   Telescope designers have to balance full illumination of the field with obstruction size.   As a result, most obstructed scopes will show some degree of falloff.

 

Also see https://www.cloudyni...stro-questions/

I think they are talking about the classic c90, not the current model (which I believe is optically identical to the Apex 90), but the vignetting will be similar.


Edited by jallbery, 05 December 2018 - 07:28 PM.


#3 Nicole Sharp

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Posted 05 December 2018 - 07:24 PM

The vignetting comes from the telescope, not the eyepieces.

 

True field is a property of the telescope's focal length and the eyepiece field stop diameter.  Given that all the eyepieces you list have pretty much the same field stop diameter (27mm), they will all show pretty much the same true field.  

 

So everyone of these eyepieces will show you the same field with the same amount of vignetting and they all will work.  I've never used the Mak in question, but based on similar scopes, I would expect there to be useful illumination all the way to the fieldstop.    On my B&L 4000 (a 4" SCT at F/12, so pretty similar to a 90mm Mak), I used to use a 40mm plossl as my "finder" eyepiece, and I never found the vignetting distracting or particularly noticeable, for that matter.  

 

Photographically, I'd expect the vignetting to be more obvious.

Perhaps I should have added that I would like to use the OTA for afocal astrophotography with a smartphone or a point-and-shoot camera.  From what I understand about afocal imaging, I need a wide apparent field of view, and also good eyerelief (and both are good qualities for visual use as well).

 

What is a field stop?



#4 Nicole Sharp

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Posted 05 December 2018 - 07:30 PM

Found this:

 

http://www.televue.c..._page.asp?id=79

 

Not sure I understand how that all works together though.  If the maximum field stop for a 1.25" eyepiece is 27 mm, does that mean that eyepieces with a focal length longer than 27 mm will be vignetted?



#5 Nicole Sharp

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Posted 05 December 2018 - 07:40 PM

The vignetting comes from the telescope, not the eyepieces.

 

True field is a property of the telescope's focal length and the eyepiece field stop diameter.  Given that all the eyepieces you list have pretty much the same field stop diameter (27mm), they will all show pretty much the same true field.  

 

So everyone of these eyepieces will show you the same field with the same amount of vignetting (unless the eyepiece itself creates some) and they all will work.  I've never used the Mak in question, but based on similar scopes, I would expect there to be useful illumination all the way to the fieldstop.    On my B&L 4000 (a 4" SCT at F/12, so pretty similar to a 90mm Mak), I used to use a 40mm plossl as my "finder" eyepiece, and I never found the vignetting distracting or particularly noticeable, for that matter.  

 

Photographically, I'd expect the vignetting to be more obvious.

 

The baffle tube is a restriction but it is out of focus.   Falloff will start before you even get to the 17mm point.   Telescope designers have to balance full illumination of the field with obstruction size.   As a result, most obstructed scopes will show some degree of falloff.

 

Also see https://www.cloudyni...stro-questions/

I think they are talking about the classic c90, not the current model (which I believe is optically identical to the Apex 90), but the vignetting will be similar.

Actually, the C90 has a 15-mm baffle tube, and it also weighs about 1 pound more, so I think it has a different configuration.  Both OTAs are made by Synta though, so they should otherwise be pretty similar, in addition to the Sky-Watcher Skymax 90.  They are all 1250/90 at least.


Edited by Nicole Sharp, 05 December 2018 - 07:40 PM.


#6 jallbery

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Posted 05 December 2018 - 07:45 PM

Perhaps I should have added that I would like to use the OTA for afocal astrophotography with a smartphone or a point-and-shoot camera.  From what I understand about afocal imaging, I need a wide apparent field of view, and also good eyerelief (and both are good qualities for visual use as well).

 

What is a field stop?

The field stop is a ring or donut-like disk in the eyepiece that is positioned at the focal plane of the eyepiece and moved by the focuser so that it is also positioned at the telescope's focal plane (the telescope's image circle).  It defines the edge of the field of view.

 

TFOV = <fieldstop diameter>*57.3/<telescope focal length>

 

See http://www.skymtn.co...r/eyepiece.html  for some pictures and more explanation.

 

I think AFOV is less important than eye relief, but I don't do afocal imaging, so I could be wrong.



#7 Nicole Sharp

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Posted 05 December 2018 - 08:00 PM

The field stop is a ring or donut-like disk in the eyepiece that is positioned at the focal plane of the eyepiece and moved by the focuser so that it is also positioned at the telescope's focal plane (the telescope's image circle).  It defines the edge of the field of view.

 

TFOV = <fieldstop diameter>*57.3/<telescope focal length>

 

See http://www.skymtn.co...r/eyepiece.html  for some pictures and more explanation.

 

I think AFOV is less important than eye relief, but I don't do afocal imaging, so I could be wrong.

The eyerelief determines how close the camera sensor has to be to the eyepiece to achieve focus.  That depends on the camera design, and the distance between the camera lens and the camera sensor.  For a smartphone, it should be able to get pretty close I think (a typical smartphone is not as thick as the distance between eyeglasses and a human eye), but the lens built into the camera may also have a minimum focus distance.  Whether the field of view is vignetted also depends on the focal length of the camera lens.  Smartphone cameras have somewhat wide lenses I think, so a wider apparent field of view in the eyepiece is needed.  So really both are important to have.


Edited by Nicole Sharp, 05 December 2018 - 08:02 PM.


#8 Nicole Sharp

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Posted 05 December 2018 - 08:11 PM

The field stop is a ring or donut-like disk in the eyepiece that is positioned at the focal plane of the eyepiece and moved by the focuser so that it is also positioned at the telescope's focal plane (the telescope's image circle).  It defines the edge of the field of view.

 

TFOV = <fieldstop diameter>*57.3/<telescope focal length>

 

See http://www.skymtn.co...r/eyepiece.html  for some pictures and more explanation.

 

I think AFOV is less important than eye relief, but I don't do afocal imaging, so I could be wrong.

Still very confused.  Where does the number 57.3 come from?  I saw that listed on Televue.com as well.

 

I thought that the true field of view was the apparent field of view divided by the magnification in the eyepiece??  With the magnification in the eyepiece being the focal length of the telescope divided by the focal length of the eyepiece.  How is that different from the field-stop formula using the mysterious number 57.3?  And many manufacturers do not list field stop in specifications?  Do I need to send emails to all the eyepiece manufacturers to ask what the field stops of their eyepieces are?


Edited by Nicole Sharp, 05 December 2018 - 08:13 PM.


#9 Nicole Sharp

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Posted 05 December 2018 - 08:16 PM

So if the field stop diameter is larger than the diameter of the baffle tube, the eyepiece will be vignetted?  With a 17-mm baffle tube, how do I know what eyepieces have field stops smaller than 17 mm?


Edited by Nicole Sharp, 05 December 2018 - 08:16 PM.


#10 Nicole Sharp

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Posted 05 December 2018 - 08:31 PM

Found this: https://www.oreilly....04/ch04s15.html

 

So the mysterious number 57.3 is actually 360/(2*pi).

 

But according to that, I actually do need to contact each eyepiece manufacturer to ask what the field stop is if it is not listed on their website?

 

How does that relate to vignetting though?  Just that the field stop needs to be smaller than the baffle tube?



#11 S.Boerner

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Posted 05 December 2018 - 08:33 PM

53.7 is  360/2*pi



#12 Nicole Sharp

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Posted 05 December 2018 - 08:37 PM

Looks like someone has already taken the time to measure some eyepieces:

 

https://www.cloudyni...lx-field-stops/

 

So if the field stop has to be smaller than the baffle tube, then my original assessment that the focal length of the eyepiece must also be smaller than the diameter of the baffle tube in order to not be vignetted still seems to be true, if the field stop is approximately equal to the focal length.

 

A 17-mm baffle tube then would mean that any eyepiece with a focal length longer than 17 mm may be vignetted?



#13 Nicole Sharp

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Posted 05 December 2018 - 08:51 PM

This seems to confirm my fear:

 

https://www.cloudyni...es-on-c90-help/

 

But a 12-mm X-Cel (60-deg AFOV) is still 0.6 deg TFOV and an 18-mm X-Cel (60-deg AFOV) is 0.8 deg TFOV (vignetted by 0.9X), so that isn't too terrible maybe.  I can at least do full-disc Solar/Lunar imaging with it.  But could be better off with a small apo instead perhaps.  A 360/60 apo and a 1250/90 Mak could make a nice side-by-side combo though, basically using the apo as a finderscope.


Edited by Nicole Sharp, 05 December 2018 - 09:05 PM.


#14 vtornado

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Posted 05 December 2018 - 09:26 PM

Don Pensack has a spread sheet on this site that shows all the specs for hundreds  of eyepieces.

https://www.cloudyni...e/#entry8487683


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#15 Nicole Sharp

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Posted 05 December 2018 - 09:41 PM

Don Pensack has a spread sheet on this site that shows all the specs for hundreds  of eyepieces.

https://www.cloudyni...e/#entry8487683

But is it true that if the field stop is larger than the baffle tube, the view will be vignetted?

 

Looks like 18 mm is the longest focal length with a field stop smaller than 17 mm.


Edited by Nicole Sharp, 05 December 2018 - 09:43 PM.


#16 Nicole Sharp

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Posted 05 December 2018 - 10:15 PM

I don't know about afocal photography and light fall off due to vignetting.

I know once beyond the illuminated field the light falls off.  I know a camera is more sensitive to the fall off than your eye.

So even though the fully illuminated field is only 17mm, one needs to know when the light fall off will be detected.

 

I have seen you asking a lot of questions about lunar and solar imaging.

I have taken pics of the sun using a white light solar filter.

If I remember correctly  900mm fl  just about fills up an APS-C sensor with the sun.

Since the sun and moon are about the same angular diameter moon should look good at 900mm too.

I use a 900mm refractor. 

 

I have just picked up a c5.  1250mm focal length.  I assume it may clip the image a bit.

I may look for a focal reducer to get the right image scale.  

 

good luck in your search

VT.

You make a good point.  Most smartphone cameras use 1/3-inch (8-mm) sensors, which are about the same size as the maximum possible dilation of a human pupil.  But most adult humans older than age 20 can't dilate their pupils to 8 mm (maximum pupil size decreases with age).  So a smartphone camera has maybe the same pupil size as a 20-year-old human, but otherwise is collecting more light than most adult human eyes can.  The integrated lens in front of the camera sensor though widens the area that light can be collected from, which is where a wide apparent field of view in the eyepiece is helpful.  But a human eye is still basically an afocal camera with a sensor size equal to the pupil diameter and an effective lens focal length equal to the distance between the front and the back of the eyeball (about 22 mm).  The rules should be the same, with the only difference being between the dimensions of the digital camera versus the biological camera (but a human eye is connected to an advanced quantum computer which can do some tricks that a digital camera can't).  Not sure if I have all that right, but that is what I have been able to figure out so far.

 

But the question then isn't necessarily whether the field of view is vignetted at all, but rather whether the field of view is experiencing vignetting within the region that is perceptible to the camera sensor.  That depends on the camera then (digital or biological).  A large-eyed squid or an afocal DSLR could notice the vignetting though.

 

Theoretically, the field of view perceived by a smartphone camera could be reduced by adding a small telephoto lens, but that would probably increase the minimum focus distance too far back from the eyepiece, and I don't think would be necessary or would produce very good results.


Edited by Nicole Sharp, 05 December 2018 - 10:29 PM.


#17 jallbery

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Posted 06 December 2018 - 06:06 AM

Found this: https://www.oreilly....04/ch04s15.html

 

So the mysterious number 57.3 is actually 360/(2*pi).

 

But according to that, I actually do need to contact each eyepiece manufacturer to ask what the field stop is if it is not listed on their website?

 

How does that relate to vignetting though?  Just that the field stop needs to be smaller than the baffle tube?

>>So the mysterious number 57.3 is actually 360/(2*pi).

More importantly, it converts radians to degrees.

 

The math behind this uses some geometry and trigonometry and takes advantage of the small angle approximation (that for small angles, the sine of an angle and its measure in radians are approximately the same), which delivers the answer in radians.

 

>>But according to that, I actually do need to contact each eyepiece manufacturer to ask what the field stop is if it is not listed on their website?

Well, you could just measure the thing yourself (provided that you don't have an eyepiece that essentially starts with a barlow element and has the fieldstop between elements of the eyepiece).  Or you can estimate it by computing the field of view with the less accurate AFOV divided by magnification formula, and then work out the implied fieldstop (which assumes an accurate AFOV and zero linear distortion).  Don Pensack's spreadsheet will do it for you, I believe.  Also Agena Astro's website lists fieldstops for many eyepieces.

 

>>How does that relate to vignetting though?  Just that the field stop needs to be smaller than the baffle tube?

Again, the telescope, not the eyepiece is the source of the vignetting-- or at least the type of vignetting you are worrying about. (Eyepieces may add a small amount of vignetting, but it usually fairly insignificant and has nothing to do with baffle tube diameters).

 

The telescope produces an image circle.  The eyepiece provides a circular window to that image circle.  The  fieldstop is what creates the edge of this window, and blocks light that the eyepiece is not designed to deal with.   If the telescope vignettes before the size of the fieldstop, this vignetting will be present in the field viewed by the eyepiece.

 

You seem to think that once you hit the diameter of the baffle that the image goes dark almost immediately. That simply isn't the case.  You also seem to think that the field is fully and evenly illuminated right up to the point you hit the baffle tube diameter, and that simply is not the case either.  Unlike the fieldstop, the baffle tube is not at the focal plane.  The falloff it creates is not hard and quick.  And the secondary may be a source of vignetting as well.

 

And I've told you this, or implied it a number of times.  For example:

 

 

The baffle tube is a restriction but it is out of focus.   Falloff will start before you even get to the 17mm point.   Telescope designers have to balance full illumination of the field with obstruction size.   As a result, most obstructed scopes will show some degree of falloff.

I also told you in the 5SE thread

 

Yes, a C5 with F/6.3 R/C will vignette with a crop sensor camera, but it's not like it is dark in the corners.  You may also want to crop the photos square.  Either way, you may still appreciate the extra space from a compositional standpoint.  And visually, for night time astronomy, I was surprised how well the combo works.  Yes there is detectable falloff at the field edges with a max-field  1.25" eyepiece, but there is useful illumination to the edge.   In the day time, I could see how the 5.6mm exit pupil would cause problems for people (your day-time pupil can't accommodate the exit pupil, so it effectively stops down the scope, making the secondary even harder to ignore), but I find the combo usable at night with even my 35mm Ultima.   Different people have different sensitivities to the shadow of the secondary, though.

The Ultima has a 29mm field stop.   The R/C turns that into a virtual 46mm+ field stop.  On a C8 (37mm baffle tube) I don't generally visually notice the falloff at all.  And as I note above, I even find the combo useful on the C5, and was surprised how well it works.  Human beings don't detect increases in brightness linearly, and our brains are powerful image processors, so we are relatively insensitive  to falloff.

 

Some time ago, I took a photo someone had taken using a full frame camera with a 0.7X reducer on a C8 edge.  You need the reducer to make the falloff evident.  Then I faked the vignetting in the frame to create an image circle.  I also super imposed the fields of a 26mm, 32mm, and 38mm SWA when used with the 0.63X R/C.  Here is that image:

C8 with RC And 2in SWAs
 
the above picture reasonably depicts what I see visually.  The 26mm  is usable with the R/C.  It has a 32.2mm fieldstop, which the R/C turns into a roughly 51mm vitual fieldstop.  However, in the 32mm doesn't really give you much more in useable field (things are pretty much dark at its field stop) and the 38mm (an effective 71mm field stop with the R/C) has a significant dark circle around the edges of its field.

 

And note that on the original full frame image, at 0.7X reduction, the corners are obviously dark, but a little bit of cropping produces a totally useable image.

 

And here is an approximation of the same SWAs when used at F/10 (no reducer/corrector)
C8 And 2in SWAs

Note the 32mm SWA's image circle (the orange one).  That's an eyepiece with a 40mm fieldstop on a telescope with a 37-38mm baffle.  And the 38mm has a 45.5mm fieldstop (similar to a 40mm 68 and just slightly smaller than a 41mm 68 or 56mm plossl). Any other eyepiece with approximately the same fieldstop will show approximately this same field with the same amount of vignetting.

 

Of course, all that is for a C8, but the concept is similar for smaller CATs (just on a smaller scale).  I've heard a number of 8" SCT owners talk about the baffle size and say that you shouldn't use an eyepiece with a fieldstop bigger than 37mm, or maybe 40mm.   I'd rather have a larger field, even if it means it is a tad dimmer at the edge.   The edge also has coma and field curvature.  It's already not perfect.  But I'd rather have the option of a 1.2-1.3 degree field rather than a 1 degree and the false security that doing so eliminates any and all vignetting. Even a C8 doesn't produce a fully and even illuminated field at the 17mm mark in its image circle, let alone the 46mm mark.  I believe its designers were trying to strike a balance between obstruction size and the ability to fully illuminate a 35mm film camera frame (a little over 43mm on the diagonal).  I think they did a good job.

 

Anyway...  Yes, if you use an eyepiece with a fieldstop bigger than 17mm  on a Apex 90, the image you see will have some vignetting, but the vignetting is intrinsic to the telescope and yes, it is caused (at least in part) by the restriction of the baffle tube.  But that doesn't mean that the image produced by  isn't useful.  And again, even with an eyepiece with a 17mm fieldstop, image circle will not be illuminated absolutely evenly, and will be slightly dimmer at the edges, but that won't be obvious at all.  As you get wider, the vignetting may be more noticeable in photographs than in the eyepiece, but again, you may find the compromise acceptable.  Or perhaps not.   But some of this stuff you just have to try and see...

 

You seemed to dismiss the C5 on fears of vignetting, but now seem to be considering a scope with and even smaller baffle tube and a similar focal length.  I don't get that at all.

 

Finally, I don't see how that C90 page you linked to confirms your fears at all.  Quite the opposite.  I saw a bunch of people who use eyepieces like a 32mm plossls, 25mm 60-degee and 24mm 68 degree eyepieces on C90s and are happy with the results.   Note that the OP was only unhappy with the 24mm 68 (27mm fieldstop)  terrestrially because of its linear distortion, and was perfectly happy with it astronomically.


Edited by jallbery, 07 December 2018 - 12:38 AM.

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#18 jallbery

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Posted 06 December 2018 - 06:23 AM

 So a smartphone camera has maybe the same pupil size as a 20-year-old human

The sensor is not analogous to the pupil...  it is analogous to the retina.  The human retina is considerably larger than 8mm.

 

The pupil is the opening in the human eye's iris.  In a camera, the analogous part to the pupil would be the opening in the iris diaphragm of the camera lens.



#19 jallbery

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Posted 06 December 2018 - 11:23 AM

I thought that the true field of view was the apparent field of view divided by the magnification in the eyepiece??  With the magnification in the eyepiece being the focal length of the telescope divided by the focal length of the eyepiece.  How is that different from the field-stop formula ...?

AFOV/magnification is only an estimation of true field of view.  For it to be accurate, the eyepiece would have to have to be perfectly orthoscopic (free from visual distortion-- no field compression and no pincushion or barrel distortion; note that aberrations like coma and astigmatism are different from distortion) and the AFOV must be accurate.   There are very few eyepieces for which both of these things are true.  Any eyepiece with an AFOV wider than 45 degrees probably will have some distortion, and manufacturers' AFOV numbers are often overstated, or they may be an average for a line of eyepieces with some having a bit more and others having a bit less.

 

The fieldstop formula is as accurate as the measurements of focal length and fieldstop (save for a tiny tiny discrepancy due to the use of the small angle approximation).

 

For example, let's put an Explore Scientific 24mm 68 degree eyepiece (27.2mm fieldstop) into that 1250mm 90mm Mak.

 

AFOV = 68   Magnification = 52X, estimated TFOV = 1.31 degrees (to three decimal places)

 

But 57.3*27.2/1250 yields 1.25 degrees.  

 

And if you actually believe that a 32mm plossl has a 52-degree AFOV and no distortion, the AFOV calculation would tell you that the field produced is 1.33 degrees.   But a 27mm field stop only yields 1.24 degrees.  The AFOV/mag calculation overstated the field width by almost 8%.


Edited by jallbery, 07 December 2018 - 12:36 AM.


#20 BillShakes

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Posted 07 December 2018 - 08:46 AM

Also note that the focal length of these Sinta 90mm Maks is understated.  Measurement of the actual focal length is closer to 1400mm, rather than the spec’d 1250.  Search the “Something for Nothing” C90 thread for details.



#21 PXR-5

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Posted 07 December 2018 - 09:07 AM

I get close to 2° with mine using the Celestron focal reducer and a 32mm Plossl or the 24 pan. Haven't noticed much vignetting.

While it's huge, read the "something for nothing" thread. One can learn everything about this great little scope :)

Edited by PXR-5, 07 December 2018 - 09:08 AM.


#22 Peter Besenbruch

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Posted 08 December 2018 - 02:39 PM

I have an Apex 90mm with a .63 reducer, and you get vignetting (pretty severe, actually) at the nominal 25x that a 32mm produces. That said, it's still usable. The irony is that without the reducer, the 32mm will cut off completely at about 40-45° apparent field, while there will be some illumination at the edge with the reducer in place. All told, that means you can get a roughly 2° true field.


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#23 PXR-5

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Posted 08 December 2018 - 07:16 PM

I have an Apex 90mm with a .63 reducer, and you get vignetting (pretty severe, actually) at the nominal 25x that a 32mm produces. That said, it's still usable. The irony is that without the reducer, the 32mm will cut off completely at about 40-45° apparent field, while there will be some illumination at the edge with the reducer in place. All told, that means you can get a roughly 2° true field.


I've noticed that also Peter, there's more vignetting without the focal reducer with the the 32mm EP.
But yes I get about 2°, its a fun little scope :)

#24 Achernar

Achernar

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Posted 09 December 2018 - 10:17 AM

I have one of these telescopes, which I use mainly for solar observing and as a spotting scope. I find the maximum true field of view for this and other similar telescopes is about 1.2 or 1.3 degrees, and the limit on the true field of view is determined by the baffle tube.

 

Taras


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