12 replies to this topic

[patindaytona]

Hello.....I'm trying to set Stellarium's settings to personalize my dob and night vision.   I understand N.V. is effectively a 27mm f.l. eyepiece by itself.

I add 67mm and 30mm eyepieces.  The result says that the magnifications are 27x and 60.9x respecitively.    But the actual image doesn't show any differences in size.

Here's some specs I am using.   My telescope focal length is 1826mm / mirror is 406mm       I want to use the 67mm and a 30mm eyepiece for the N.V.    I put in a 18mm field stop from what I've gathered on that.      But when I choose an object, the resulting magnification of it is identical to both, the 67mm and the 30mm.   Can someone help me out on this?

Edited by patindaytona, 16 May 2025 - 08:54 AM.

[sixela]

If you use it in an afocal stack with a 1x objective, then it has the magnification of the eyepiece and the effective field stop (if you can plug it into a simulator) is close to 18 mm * (eyepiece focal length) / 27*. If you can't plug in the field stop size, then just divide 40° by the magnification.

 

Which, of course, means that an afocal stack and a 27mm eyepiece (with no angular magnification distortion) with a 1x objective yields the same view as a prime focus usage.

If the telescope focal length is 1826 mm:

 

-prime focus (without coma corrector): paraxial magnification 67.6x (1826 mm/ 27 mm), field of view 18 mm / 1826 mm radians or 0.56°

-with TV67: paraxial magnification 27x, field of view 1.4° (note that something with no angular magnification distortion that magnifies 27.3x and has a 40° AFoV shows a field of view of...40°/27.3=1.46°)

-with 30mm eyepiece: magnification 61x, field of view 0.63° (basically...almost what prime focus gives you).

 

--

*inexact because the objective is not distortion free; in 1x usage it has a distortion that is perfectly compensated by that in the eyepiece.

Edited by sixela, 16 May 2025 - 10:30 AM.

[patindaytona]

If you use it in an afocal stack with a 1x objective, then it has the magnification of the eyepiece and the effective field stop (if you can plug it into a simulator) is close to 18 mm * (eyepiece focal length) / 27*. If you can't plug in the field stop size, then just divide 40° by the magnification.

 

Which, of course, means that an afocal stack and a 27mm eyepiece (with no angular magnification distortion) with a 1x objective yields the same view as a prime focus usage.

If the telescope focal length is 1826 mm:

 

-prime focus (without coma corrector): paraxial magnification 67.6x (1826 mm/ 27 mm), field of view 18 mm / 1826 mm radians or 0.56°

-with TV67: paraxial magnification 27x, field of view 1.4° (note that something with no angular magnification distortion that magnifies 27.3x and has a 40° AFoV shows a field of view of...40°/27.3=1.46°)

-with 30mm eyepiece: magnification 61x, field of view 0.63° (basically...almost what prime focus gives you).

 

--

*inexact because the objective is not distortion free; in 1x usage it has a distortion that is perfectly compensated by that in the eyepiece.

Thanks sixela for the information. I have the right specs in stellarium. Using the 30mm and the 67mm doesn't show any difference in visual size/magnifications. Not that important, but curious why.
 

[Joko]

Thanks sixela for the information. I have the right specs in stellarium. Using the 30mm and the 67mm doesn't show any difference in visual size/magnifications. Not that important, but curious why.
 

You can use the calculation formulas on our NV blog :

www.ovni-nightvision.com/en/blog/post/18-formules-de-calculs-pour-ovni-m-ovni-b

 

Hope this helps.

[patindaytona]

You can use the calculation formulas on our NV blog :

www.ovni-nightvision.com/en/blog/post/18-formules-de-calculs-pour-ovni-m-ovni-b

 

Hope this helps.

Thanks. I have the calculations correct in Stellarium. Yet, the telescope view shows no differences in magnification between the two eyepieces (30mm and the 67mm) The specs. are there, but the image shows no increase/decrease in magnification.
 

[sixela]

Could you post some screenshots? Because on our side it really is working...

[patindaytona]

Some screenshots. You can see the actual image size stays the same.

Attached Thumbnails

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Edited by patindaytona, 16 May 2025 - 04:35 PM.

[sixela]

Can't reproduce that, it's working fine on my system:

Edited by sixela, 16 May 2025 - 06:06 PM.

[ButterFly]

Hello.....I'm trying to set Stellarium's settings to personalize my dob and night vision.   I understand N.V. is effectively a 27mm f.l. eyepiece by itself.

I add 67mm and 30mm eyepieces.  The result says that the magnifications are 27x and 60.9x respecitively.    But the actual image doesn't show any differences in size.

Here's some specs I am using.   My telescope focal length is 1826mm / mirror is 406mm       I want to use the 67mm and a 30mm eyepiece for the N.V.    I put in a 18mm field stop from what I've gathered on that.      But when I choose an object, the resulting magnification of it is identical to both, the 67mm and the 30mm.   Can someone help me out on this?

 

Don't set the field stop of the eyepiece, but only the AFOV, and set that to 40 degrees.  Leave the field stop of the eyepiece at 0.00mm.  If you set an eyepiece field stop in Stellarium, that wins over your AFOV setting, so that's your problem here.

With afocal, the eyepiece and scope are the things to focus on - not the device.  The caveat is that all eyepieces now have a 40 degree AFOV.  So, the scope gets set up as per usual.  The eyepiece also gets set up as usual, but its AFOV is set to 40 degrees.  Leave the field stop as 0.00mm, and just set the 40 degree AFOV.

[ButterFly]

If you insist on putting in a value for the field stop as well, for whatever reason, here's the math to get the "effective field stop".

 

The ratio of the effective field stop to the focal length of the scope is the true field one sees in radians, distortions aside.  So, let's say an eyepiece plus scope plus device yields a true field of one degree.  That's about 1 degree / (57.3 degrees per radian) of true field.  That is the ratio of your effective field stop to your focal length, distortions aside.

 

So, your telescope focal length is: 1826mm.

 

Your eyepieces are 67mm and 30mm, both with an effective AFOV of 40 degrees when using the device after them.

 

The magnifications with those eyepieces and that scope is about 1826mm / 67mm and 1826mm / 30mm, respectively, for about 27.25x, and 60.86x, respectively.

 

Those magnifications both appear to span 40 degrees as seen through the eyepiece and the device.  The true fields of each are, therefore, distortions aside, 40 degrees / 27.25x and 40 degrees / 60.86x.  Those true fields are 1.4679 degrees and 0.6572 degrees, respectively.

 

So the effective field stops with your eyepiece and your device are, respectively:

(1.4679 degrees / (57.3 degrees per radian)) * 1826mm ~ 46.77mm (which makes sense for a 67 Plossl!!!); and,

0.6572 degrees / (57.3 degrees per radian)) * 1826mm ~ 20.94mm (which makes sense for fitting twice the mag of the 67 Plossl into the same AFOV of 40 degrees).

 

If you notice carefully, one multiplies by the focal length of the scope to get the magnification, then divides by the focal length of the scope to get the effective field stop.  The telescope itself is thus irrelevant for the effective field stop of that eyepiece plus the device.  Try out the same thing with another one of your telescopes to see that.  Try to figure out a way to avoid needing the telescope focal length at all (hint: think of the relationship between the eyepiece's actual effective field stop and its AFOV, to the relationship with a 40 degree AFOV).

[sixela]

Don't set the field stop of the eyepiece, but only the AFOV, and set that to 40 degrees.  Leave the field stop of the eyepiece at 0.00mm.  If you set an eyepiece field stop in Stellarium, that wins over your AFOV setting, so that's your problem here.

With afocal, the eyepiece and scope are the things to focus on - not the device.  The caveat is that all eyepieces now have a 40 degree AFOV.  So, the scope gets set up as per usual.  The eyepiece also gets set up as usual, but its AFOV is set to 40 degrees.  Leave the field stop as 0.00mm, and just set the 40 degree AFOV.

Of course you can still use the eyepiece field stop, but then you *also* have to adjust it for the fact the NVD in afocal is only going to use 40° of the AFOV (which is what you did, and explained very well at that).

Or if you take the 18mm of the NVD photocathode as a field stop, you need to adjust it by the ratio of the eyepiece focal length divided by 27 (which is the reduction factor of the eyepiece plus NVD objective subsystem). That also works.

 

Both methods are going to give slightly different results, but that is because distortion will actually make small differences.

Edited by sixela, 18 May 2025 - 01:59 PM.

[ButterFly]

Not doing the math is just easier.  Set the focal length as is, and the AFOV to 40 degrees and move on.

 

The "hidden" setting in Stellarium that made it less apparent what was going on was "Scale image circle".

[patindaytona]

Thanks Butterfly and sixela. That worked!