11 replies to this topic

[Paul Schroeder]

Does it make sense to use a 2” diagonal on a Celestron C5?  The consensus seems to be no.

 

Why not?  Well, the rear axial port on a C5 is approximately 25mm in diameter, far smaller than the opening in a 2” diagonal.  This, in turn, would suggest that eyepieces with 40mm+ field stops (say a Nagler 31mm Type 5 or Explore Scientific 30mm 82 degree) would be quite limited - an argument that makes a fair amount of sense to me.

 

But from time to time I do use a 2” diagonal and an ES 30mm 82 on my C5.  Whenever I do this, the field seems far wider than it should be - if the axial port were actually limiting the field of view.

 

So today I decided to *attempt* to measure what was going on.

 

I set up my late-model C5 and a TV 85 approximately 60 feet from a wall where I’d taped two yardsticks end to end.  I wanted to measure the visible field of view using various eyepieces, and then compare the ratio of the measured fields with the ratio of the corresponding theoretical fields of view.

 

Here’s my logic, such as it is .  The shortest FL eyepiece I’d use was a TV 11mm DeLite, whose field stop of 11.7mm wouldn’t be constrained by the C5’s axial port.  I’d set the measured field of view for the 11mm DeLite as my baseline, and then see if the measured FOV for wider field eyepieces (with field stops in excess of the 25mm axial port) scaled up as fast as they theoretically should.  If not, my assumption would be that the axial port did indeed limit things.

 

On to the measuring!

 

First up was the C5 with a 32mm Meade plossl and an 11mm Tele Vue DeLite, in a 1 1/4” diagonal.  I calculated the theoretical field of view at 1.24 degrees for the 32mm Meade, and .54 degrees for the 11mm DeLite.  This gave a theoretical ratio of 2.30 (Meade FOV as a multiple of the DeLite).

 

I then used the yardstick to measure the actual FOV through the eyepieces, and got 14.15” for the 32mm and 6.3” for the 11mm.  This resulted in an actual ratio of 2.26, which seemed close enough to the 2.30 to indicate there was no limitation imposed by the axial port (see table below).  An expected result.

 

Next I tried my TV 85 with an ES 30mm 82 (43mm field stop) and the TV 11mm DeLite.  In theory, the clear aperture of the TV 85 shouldn’t limit the FOV observed with the ES 30mm.  Using the same approach as above, I came up with a theoretical FOV ratio of 3.67 (ES as a multiple of the DeLite), and a measured ratio of 3.56.  Again, close enough given my less-than-exact measurements to suggest that actual equaled theoretical, and to seemingly confirm that this measurement methodology worked OK.

 

Then it was time to try the 2” TV diagonal on the C5.  As a first step, I tried to estimate the focal length of the C5 with the 2” diagonal.  I measured the FOV through the 32mm Meade Plossl at 12.75”, which was roughly 89% of the FOV I’d measured using the same eyepiece with a 1 1/4” diagonal.

 

Based on this, I estimated that the C5 with 2” diagonal was operating at about a 1400mm focal length, instead of its standard 1250mm.  I used this FL to calculate the theoretical FOV on the C5 using the various eyepieces.

 

Finally, I went ahead and measured the field of view on my yardstick while using a 30mm ES 82, 20mm ES 100, and 11mm TV DeLite.  As you can see in the table below, in all cases the ratio of the measured FOV very closely tracked the ratio of the theoretical FOV - which was not exactly what I’d expected.  

 

I didn’t try to measure how bright the image was at the edge of the field vs. the center.  There was probably some light fall off, but it wasn’t dramatic and it wasn’t hard to read the numbers at the edge of the field.

 

I don’t doubt that there are shortcomings with this simple approach.  In any event, I’d love to have the optical experts out here on CN either a) point out the mistakes I’ve made while trying to measure this, or b) explain the following: why doesn’t the small axial port on a C5 act as more of a limiter on the field of view?

 

Thanks!

 

Paul

Attached Thumbnails

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[jallbery]

 why doesn’t the small axial port on a C5 act as more of a limiter on the field of view?

 

My guess (I'm not an expert) is that the answer is two-fold:

(1) The rear baffle is far enough away from the focal plane that it isn't in focus and therefore is not a hard fieldstop-- therefore it only produces fall-off and not a hard vignette

(2) human eyes and brains are remarkably good at compensating for this sort of fall-off.

[jallbery]

Let me also add that I've also experimented with using a 2" diagonal on a C5 and found similar results.  I don't generally use a 2" diagonal with my C5, though, for the following reasons:

1. The focuser knob and my XLT 2" SCT diagonal don't get along particularly well
2. Big 2" eyepieces plus a 2" diagonal can weigh as much as the OTA, causing balance issues
3. Using the F/6.3 reducer/corrector allows you to get the same fields with 1.25" eyepieces, but with better ergonomics.

As I noted in another thread, I was very surprised how well the C5+R/C+35mm Ultima combination worked.  That combination ought to push the C5 to almost 2 degrees.  In a daytime test, I could see little apparent vignetting, with the eyepiece field stop being very clearly defined and the full width of the field illuminated.  I'm sure there was falloff, but the combo was much more useable than I expected it to be.   When I tried with a wider field/higher magnification option (24mm ES 68 + R/C, yeilding about 1.9 degrees, but at 46% more magnification), field curvature was a more apparent problem than vignetting.

 

I'm still looking to try some tests at night to try to measure the amount of falloff-- I just haven't had the time and opportunity yet.

[Sarkikos]

Similar threads on this topic have been coming up regularly.

 

IME, I prefer not to use 2" diagonals and 2" eyepieces on the C5.  Too much weight and bulk for a telescope that is best for maxing out aperture in a small, compact package.  And there will probably be vignetting.

 

Just get an f/0.63 Reducer/Corrector and a 24 Pan or 35 Ultrascopic (Ultima) and be done with it.  That's as good as it gets for low-power wide-field in a C5.  Period.

 

So I don't see the need for any more threads on this topic.  Alright?  

 

Mike

Edited by Sarkikos, 07 August 2017 - 10:20 AM.

[Sarkikos]

Here's a PDF showing a C5 with various eyepieces, without and without a 0.63 R/C.

 

Mike

Attached Files

•  C5.pdf   18.32KB   273 downloads

[jallbery]

Similar threads on this topic have been coming up regularly.

 

IME, I prefer not to use 2" diagonals and 2" eyepieces on the C5.  Too much weight and bulk for a telescope that is best for maxing out aperture in a small, compact package.  And there will probably be vignetting.

 

Just get an f/0.63 Reducer/Corrector and a 24 Pan or 35 Ultrascopic (Ultima) and be done with it.  That's as good as it gets for low-power wide-field in a C5.  Period.

 

So I don't see the need for any more threads on this topic.  Alright?  

 

Mike

The R/C+1.25" won't vignette less than the equivalent 2" option--  e.g., a 32mm plossl with the R/C yields almost exactly the same field and magnification as a 2" diagonal and a 55mm plossl, with the same amount of falloff.   Same for a 24mm Pan+R/C vs. the 41mm.    Perhaps you didn't intend to imply otherwise, but it kind of sounded like you were...

 

I prefer the lower power view with the 35mm Ultima over the 24mm 68 (I don't have the Pan, but I have the ES 68) because, at least for my eyes, the higher power view makes the significant field curvature of the C5 significantly more evident.  But that's going to vary from person to person.

[Sarkikos]

The R/C+1.25" won't vignette less than the equivalent 2" option--  e.g., a 32mm plossl with the R/C yields almost exactly the same field and magnification as a 2" diagonal and a 55mm plossl, with the same amount of falloff.   Same for a 24mm Pan+R/C vs. the 41mm.    Perhaps you didn't intend to imply otherwise, but it kind of sounded like you were...

 

I prefer the lower power view with the 35mm Ultima over the 24mm 68 (I don't have the Pan, but I have the ES 68) because, at least for my eyes, the higher power view makes the significant field curvature of the C5 significantly more evident.  But that's going to vary from person to person.

 

No, I'm not sure if it is correct that a 41 Pan + 2" diagonal will not vignette more than a 24 Pan + R/C, or that a 55 Plossl + 2" diagonal will have the same amount of vignetting as a 32 Plossl + R/C.  Maybe in my spreadsheet I should be increasing the effective field stop of the eyepieces used with R/C by a factor of 0.63?

 

In any case, on a C5 I'd much rather deal with a 24 Pan or 32 Plossl + R/C, than a 41 Pan or 55 Plossl + 2" diagonal.  Can you imagine a 2" diagonal + 41 Pan sticking out the rear of a poor little C5? 

 

To my eyes, lower power and/or wider field always shows the field curvature more obviously.  Since a 35 Ultima (Ultrascopic) has a narrower field (49 degrees) than a 24 Pan (68 degrees), but lower power, it's hard to predict which will show FC more than the other.  You just have see for yourself and make a choice.  

 

But I like the 24 Pan more under my bright red zone skies because with the R/C it gives a 3.8mm exit pupil rather than 5.6mm exit pupil of the 35 Ultrascopic + R/C.  An exit pupil around 6mm or wider has a washed-out background here at home.

 

Mike

Edited by Sarkikos, 07 August 2017 - 01:27 PM.

[jallbery]

 

The R/C+1.25" won't vignette less than the equivalent 2" option--  e.g., a 32mm plossl with the R/C yields almost exactly the same field and magnification as a 2" diagonal and a 55mm plossl, with the same amount of falloff.   Same for a 24mm Pan+R/C vs. the 41mm.    Perhaps you didn't intend to imply otherwise, but it kind of sounded like you were...

 

I prefer the lower power view with the 35mm Ultima over the 24mm 68 (I don't have the Pan, but I have the ES 68) because, at least for my eyes, the higher power view makes the significant field curvature of the C5 significantly more evident.  But that's going to vary from person to person.

 

No, I'm not sure if it is correct that a 41 Pan + 2" diagonal will not vignette more than a 24 Pan + R/C, or that a 55 Plossl + 2" diagonal will have the same amount of vignetting as a 32 Plossl + R/C.  Maybe in my spreadsheet I should be increasing the effective field stop of the eyepieces used with R/C by a factor of 0.63?

 

Look at it this way...   the scope makes an image circle (ideally) about 100mm behind the rear cell.   Whatever vignetting is caused by the rear baffle and/or secondary appears on that image circle.  The eyepiece simply provides a window to this image.  Since the image circle is at the eyepiece's fieldstop when things are in focus, the diameter of the fieldstop controls the true field.   However, when the R/C is in place, the image circle gets shrunk by a factor of 0.63X.  In an interesting bit of fate, 1.25/2.0 is approximately 0.63 as well.    So a maxfield 1.25" eyepiece with the R/C is rather similar to the view offered by the comparable maxfield 2" eyepiece without the R/C.  One small wrinkle in this analysis is that the walls of the eyepiece barrel are a more significant fraction of a 1.25" eyepiece than they are a 2" eyepiece.   However, this little wrinkle that gives a small advantage to the 2" eyepiece gets undone by the fact that the 2" diagonal lengthens the focal length, reducing the net field.

 

If we assume the FL of the C5 with a 2" diagonal at 1375mm,  a 41mm Pan (46mm fieldstop) gives you

(1375/46)*57.3 => 1.92 degrees

 

And if we assume the C5 is 1280 without the R/C (the 1250 has been reported as a bit a short with the 1.25" diagonal in place) and 806 with it, the R/C and a 24mm Pan (27mm fieldstop) yield...

(806/27)*57.3 => 1.92 degrees

 

So the fields are for all practical purposes the same.  And since neither the R/C (41mm clear aperture) or the eyepieces are adding additional vignetting, the limiting factor to field illumination in both cases is that rear baffle tube.   The R/C just reduces the effective size of the tube.  Whether you look at it from the perspective that the R/C increases the effective fieldstop of the eyepiece by a factor of 1/0.63, or look at it as the R/C reducing the effective size of the rear port by a factor of 0.63X, the net result is the same.   The image circle with the R/C is smaller, but percentage wise, it shows the same vignetting.  

 

 

>>In any case, on a C5 I'd much rather deal with a 24 Pan or 32 Plossl + R/C, than a 41 Pan or 55 Plossl + 2" diagonal.  

 

As would I.   I've put my 38mm Q70 on a C5, just to see how it worked.  It worked better than I expected, but it sure looks silly.    In the case of a 24mm Pan + R/C vs a 41mm Pan and 2" diagonal, the 24mm+R/C option is almost 2 pounds lighter, if memory serves me.

 

>>To my eyes, lower power and/or wider field always shows the field curvature more obviously.

Well, the wider the true field, the more field curvature you'll see-- that certainly should not be up for debate.   However, if you keep the true field the same, and increase magnification (all other things being equal), the higher mag version of the field will have to show you every aberration of scope in greater detail.  You are providing more magnification to the circles of confusion created by the field curvature.

 

The only way the higher magnification rendition of the same field wouldn't show more field curvature is if the eyepiece in question has field curvature OPPOSITE that of the scope.

 

Of course, in the case of the 35mm Ultima, the field is getting a bit wider, so there is a bit more curvature there.   It just is less obvious due to the reduced magnification.   I've also read that the Ultima/Ultrascopics have a slight bit of negative field curvature (relative to an SCT), so this may be factoring into the equation as well (it is also possible that the ES 68 24mm may be adding some field curvature, too).   But different people have different sensitivities, so, as you say, you have to try out different options and see what works best for you... 

 

>>But I like the 24 Pan more under my bright red zone skies because with the R/C it gives a 3.8mm exit pupil rather than 5.6mm exit pupil...

That's why it's nice to have both options.   Conversely, the brighter view may be beneficial when used with nebula/light pollution filters.

[GlennLeDrew]

In the same vein...

 

A friend had to toss the whole plastic rear end on his little Meade 90mm Mak because the three internal (plastic) studs through which attaching screws passed had all broken. So he adapted a 2" helical focuser. In spite of the soda straw-like 17-18mm diameter rear port on the scope, a 2" 32mm Koenig's 32mm field stop did not reveal any 'hard' vignetting, although the edge-of-field illumination was down to no more than about 30%.

 

A field stop well removed from an otherwise limiting baffle has that obstructor well out of focus, with the result that its shadow is made quite diffuse. Furthermore, any of the system's off-axis chief rays, which pass through the center of the entrance pupil, necessarily diverge outward and away from the optical axis as they proceed to the focal surface. This results in a larger circle of illumination for any selected illumination level, which in turn permits to use a larger field stop.

 

Of course, moving-primary telescopes introduce their own wrinkles which result in varying field illumination (and at some point, aperture reduction even on axis) as the focal surface is made to move in/out. But to first order, throwing the focus farther back does permit to utilize eyepieces with surprisingly 'oversized' field stops relative to the rear port aperture.

 

In a less 'extreme' case, binoviewers can allow the use of field stops somewhat larger than the rear prism aperture if the field stop can be located more than 2 prism apertures distant from said prisms. For such longer f/ratio systems as SCTs and Maks (which do not come close to suffering aperture reduction with BVs), which moreover can provide sufficient effective in-travel via primary mirror focusing, short 1.25" extension barrels might be utilized in order to move the field stops of longer f.l. eyepieces farther back from the BV's rear prisms. In this way such venerable oculars as the 24mm Panoptic might well be largely rid of hard vignetting with 21mm aperture BV prism apertures.

[Riccardo_italy]

Your test was not with focus at infinity, isn't it?

 

I guess that's the reason. From Telescope Optics (Rutten & Van Venrooij), for a 8" SC with a focal lenght of 2000mm, focusing at 10mt implies a movement of the primary mirror of 16.87mm.

 

Of course this changes the path of the light and reduces the importance of the exit of the baffle bottleneck.

 

Repeat the same test focusing at infinity and you probably get different results.

[Jon Isaacs]

I have a C-5 in the mail and it should arrive tomorrow.  I parted ways with my previous C-5 because it was a Nexstar 5 and I couldn't find rings to fit it. I did enjoy the views with the 2 inch eyepieces, in particular the 32 mm TV Wide Field. 

 

There is more to the view than just the magnification, the AFoV, the sharpness, there's the combination of aesthetic factors one might call "presentation.". This is where eyepiece's like 32 mm WF can be preferred over a shorter focal length eyepiece used with a reducer/corrector..

 

Jon

[Sarkikos]

Here's the mounting solution I found for my NexStar 5:  ScopeStuff Strap Tube Mounting System http://www.scopestuff.com/ss_stms.htm   

 

A DIYer could rig up something similar themselves.  But often if there is something available off-the-shelf ready-to-go, I'd rather just go with that than devote my time and effort to it.  The time and effort I would have put into it would not have been worth it.  

 

Previously, I had the NexStar 5 in rings with thick felt padding to fit the OTA.  It even worked OK with just one ring.  But the ScopeStuff solution is more compact and lighter, and IME works better, is more secure.

 

Mike

Edited by Sarkikos, 14 August 2017 - 08:59 AM.