Aparent FOV vs TRUE aparent FOV, and max true FOV of an SCT

[michael.h.f.wilkinson]

In a recent discussion on max FOV for an SCT, I came across a remark about the Vixen LVW 42 mm, which with its 72 deg aparent FOV (AFOV) would yield 1.49 deg true FOV (TFOV). This is considerably more than my 40mm Paragon with its 69 deg AFOV, yielding 1.36 deg TFOV. However, the Burgess website on the Paragon states a 68 deg TRUE AFOV (TAFOV), corresponding to 1.34 deg TFOV. The difference between the two lies in the fact that (like most super wide EPs) the Paragon has pincushion distortion, which increases magnification towards the edge of the FOV. In the Paragon it is very small, yielding just one degree difference between the size of the illuminated circle seen through the EP, and the circle seen in the sky after applying the correct magnification has been applied (50.75 in my case).

Now back to the LVW. The TFOV is claimed to be 1.49 deg at a focal length of 2030mm for my C8. However, the field stop diameter FS can be calculated as

FS = 2030 * 2 * tan(1.49/2) = 52.8 mm = 2.08"

This is evidently impossible with a 2" barrel. In fact, the largest field stop possible is about 48mm. This yields a max TFOV for a C8 of

TFOVmax = 2 * arctan(48/(2*2030)) = 1.355 deg

i.e. very close to what is provided by the Paragon 40mm. If we stretch to 50 mm field stop, we get TFOVmax = 1.41 deg, but that would leave just 0.4 mm thickness of metal for the barrel.

I read a review on CN that the LVW 42 showed a great deal of pincushion distortion, which may explain these matters. At a TFOVmax of 1.355 deg the LVW has a TAFOV of just 65.5 deg. At a TFOVmax of 1.41 this rises to 68.15 deg. The difference of 4-7 degrees is pincushion distortion (quite a bit of it).

Conclusion: the real concern for TFOV is the field stop diameter, or the TAFOV, not the normally rated AFOV of the EP. Burgess is the only firm I have seen listing the TAFOV for an eyepiece. Others do list field stop diameters.

After doing the math I am no longer tempted by the LVW 42mm. My Paragon shows (almost) all there is to see through a C8. Thus, a little math can save you a lot of money

[Merlin66]

Did you consider the vignetting from the SCT baffle tube?

[michael.h.f.wilkinson]

Did you consider the vignetting from the SCT baffle tube?

I left that out, as I found that in the C8 the Paragon's FOV shows no serious vignetting. Out of focus stars show a little, but I would say under 20% light loss (i.e. less than 0.24 mag, which is visually not disturbing)

[Mr Spock]

The LVW does have pincussion distortion, but it is considerably less than a Nagler.

Try using the field drift method to measure field of view. It's much more accurate than calculations

[michael.h.f.wilkinson]

The LVW does have pincussion distortion, but it is considerably less than a Nagler.

Try using the field drift method to measure field of view. It's much more accurate than calculations

True, but at a 2030mm focal length a 48mm field stop cannot hold more than 1.355 deg, unless there is considerable barrel distortion in the SCT itself (not likely). I also checked through Stellarium whether the computed FOV matched that seen at the EP. In the Paragon, if using the field stop or TAFOV, rather than the AFOV it does, visually at least. With the Naglers I did not check as thoroughly, but my impression is that they are further off than the Paragon (which has very low pincushion distortion).

[John]

The impact of the diameter of the rear aperture of cassegrain scopes on eyepieces with large diameter field stops has always intrigued me and I don't feel I've ever really fully understood it, despite reading lots of view points on it.

My hunch is that the way the optical system works, vignetting, if it's happening, will be evenly spread across the whole field of view rather than at the edges but I may well be wrong in that. I'm fairly certain it does have some impact however.

When I was limited to 1.25" eyepieces only I got rather obsessed by squeezing the maximum true field of view from them and the scope and took to measuring the field stops using calipers (I could not afford eyepeices with internal field stops back then !). I found a number of cases where the actual field stop diameter did not quite match the published specification for that eyepiece which explained the true fields of view that I was getting

I agree that the star drift method is the ultimate test.

On the scopes themselves, I thought the schmidt-cassegrain design produced some field curvature itself to add to the complexity

[Adrian]

I agree that the star drift method is the ultimate test.

On the scopes themselves, I thought the schmidt-cassegrain design produced some field curvature itself to add to the complexity

Also changing the focal point changes the focal length of the system But that means to get more TFOV all you need to do is remove the diagonal so that the focal point moves inwards. This reduces the focal length so you see more sky in a given eyepiece:D

[great_bear]

My hunch is that the way the optical system works, vignetting, if it's happening, will be evenly spread across the whole field of view rather than at the edges

Unfortunately not - the edges get very distinctly fuzzed out and vignetted when overly-long f/l eyepieces are used.

On the scopes themselves, I thought the schmidt-cassegrain design produced some field curvature itself to add to the complexity

In practice, the FC won't affect the FOV to any significant degree.

[michael.h.f.wilkinson]

Unfortunately not - the edges get very distinctly fuzzed out and vignetted when overly-long f/l eyepieces are used.

In practice, the FC won't affect the FOV to any significant degree.

As I noted before, in the C8 vignetting is not severe even in the Paragon 40, with its 47.8 mm field stop. This can be verified by studying the shape of the out-of-focus "doughnuts," which show the shape of the effective aperture. At the edge, some vignetting is evident, but no more than about 20% (or at most 25%) which boils down to 0.24 magnitude (0.3 mag). Visually this is not a huge deal. In a C6 the effect will be much stronger, I suppose.

Vignetting is much more pronounced when using a focal reducer (obviously).

[cs1cjc]

I measure field of view by viewing graph paper through my refractor. I can then calculate the field stop diameter by scaling from eyepieces where I have measured it with a vernier caliper. It is quick, repeatable and does not use up scarce viewing time.

[great_bear]

As I noted before, in the C8 vignetting is not severe even in the Paragon 40

On the Mak 180 it varies - which I think has something to do with the precise position of the focal plane with any given eyepiece. Generally speaking I'm not too bothered, since the main use I have for this in the Mak is to fit the whole moon in a single view without clipping either side of it. It's not always possible with a 32mm Plossl (depending on apogee / perigee) but a 40mm widefield does the trick - even if the field edges are fuzzed out.

It'll be interesting to see how the Aero fares when I get one in the new year...

[Macavity]

I think it has to be an *experimental* subject? Suck it and see? <G> I always found it rather depressing that (until recent, more enlightened, times) "MAK detractors" dismissed experiment over (their) theorising: MAKs were always "planetary only", but SCTs were somehow more... "general purpose". One sees both sides of the argument, but...

There seems to be both "soft" and HARD (absolute limiting) vignetting. The former, open to experiment and subjectivity. I think the addition of default 2" capability with (bigger) Skymax MAKs was overdue. Exit aperture? Internal baffling? Without pukka ray tracing, I doubt one can ever be sure? They are never going to be truly wide-field though...

I previously agonised over "field of view" (bought far too many eyepieces!) - Less so, now. The only eyepiece my "theoretical calculations" (LOL!) agreed with was the old Vixen LV. Shame I had to part with a [potential] set of them...

[michael.h.f.wilkinson]

On the Mak 180 it varies - which I think has something to do with the precise position of the focal plane with any given eyepiece. Generally speaking I'm not too bothered, since the main use I have for this in the Mak is to fit the whole moon in a single view without clipping either side of it. It's not always possible with a 32mm Plossl (depending on apogee / perigee) but a 40mm widefield does the trick - even if the field edges are fuzzed out.

It'll be interesting to see how the Aero fares when I get one in the new year...

To get the whole moon in one FOV I sometimes use the 14mm UWA (145x and 82 deg AFoV). Just goes to show how different the Mak180 is compared to the C8. If your image is fuzzy towards the edge, vignetting is probably not to blame. More likely is the astigmatism prevalent in the Erfle-like wide angle EP designs.

The Aero is a Paragon clone I am told. If it works as well as my Burgess-TMB original it should be tack-sharp to the edge.

[michael.h.f.wilkinson]

I think it has to be an *experimental* subject? Suck it and see? <G> I always found it rather depressing that (until recent, more enlightened, times) "MAK detractors" dismissed experiment over (their) theorising: MAKs were always "planetary only", but SCTs were somehow more... "general purpose". One sees both sides of the argument, but...

There seems to be both "soft" and HARD (absolute limiting) vignetting. The former, open to experiment and subjectivity. I think the addition of default 2" capability with (bigger) Skymax MAKs was overdue. Exit aperture? Internal baffling? Without pukka ray tracing, I doubt one can ever be sure? They are never going to be truly wide-field though...

I previously agonised over "field of view" (bought far too many eyepieces!) - Less so, now. The only eyepiece my "theoretical calculations" (LOL!) agreed with was the old Vixen LV. Shame I had to part with a [potential] set of them...

The longer focal length of the Mak will reduce its max TFOV, but on the vast majority of DSOs it is of no consequence whatsoever. Visually on about 350 out of 373 DSOs I have seen to date the only things that matter are the sizes of aperture and exit pupil. The focal ratio of the scope does not come into it.

Things are different on the dark side

[great_bear]

If your image is fuzzy towards the edge, vignetting is probably not to blame. More likely is the astigmatism prevalent in the Erfle-like wide angle EP designs.

No, I'd have to be very confused about optics indeed not to know the difference between vignetting and astigmatism. When I say "the field edges are fuzzed out" I mean exactly that - the edge fuzzes into blackness early - with no sharply-defined field stop. I didn't mean to imply there was any lack of sharpness in the view content (well - apart from the field-stop, or more correctly, the lack of it).

[michael.h.f.wilkinson]

No, I'd have to be very confused about optics indeed not to know the difference between vignetting and astigmatism. When I say "the field edges are fuzzed out" I mean exactly that - the edge fuzzes into blackness early - with no sharply-defined field stop. I didn't mean to imply there was any lack of sharpness in the view content (well - apart from the field-stop, or more correctly, the lack of it).

OK, I got confused by the word "fuzzed" which I interpreted as meaning lack of sharpness.