How to Calculate Sun diameter inside Light Cone?

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Brett
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How to Calculate Sun diameter inside Light Cone?

Post by Brett » Sun Sep 20, 2015 1:00 pm

How does one calculate the Sun's diameter inside a light cone? I am familiar with the concept of calculating the Sun's diameter at the focal plane, eg for a 1000mm focal length telescope, the Sun would have a diameter of just under 10mm at the focal plane.

The reason I am asking, is if I add a negative lense with diameter of 13mm inside a light cone with diameter 25mm at a specific point, do I crop out light from the Sun and effectively making the telescope objective smaller, or is the Sun diameter inside this light cone still within 13mm? :?:

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Re: How to Calculate Sun diameter inside Light Cone?

Post by Merlin66 » Sun Sep 20, 2015 5:23 pm

Similar triangles plus the 10mm solar disk....
Say a 80mm, 1000mm fl, f12.5
10mm diameter (approx.) at 1000mm distance behind the objective....
then (1000-500)/1000 x 80 = 40mm +10mm = 50mm at 500mm behind the objective
and (1000-800)/1000 x 80= 16mm+10mm = 26mm at 800 behind the objective etc.

You can use a barlow to extend the focal lengths, but an achromat is needed as the collimator to get a focus on the rear of the entrance slit and provide the parallel beam to the grating.
Gratings: as large as needed to catch the field angle of the solar image and allow for the foreshortening due to grating rotation maximum l/m - 1800 or 2400 l/mm
In the files area of the yahoo group look for the SimSpecV4
https://groups.yahoo.com/neo/groups/ast ... copy/files
spreadsheet which allows you to calculate all the sizes and gives the probable resolution outcome.
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Re: How to Calculate Sun diameter inside Light Cone?

Post by Brett » Mon Sep 21, 2015 2:57 pm

Can I use this formula to calculate the width of the light cone coming from the main objective? Reason I am asking is I am considering a negative lens, added at its focal lentgh inside the light cone. With a 1000mm FL and 80mm apperture, say a 150mm FL negative lens (13mm diameter) is added 150mm inside the light cone from its focus point.

The light cone should then be 12mm wide at this point? Am I correct? And the negative lens should then give a parallel beam?

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Re: How to Calculate Sun diameter inside Light Cone?

Post by Merlin66 » Mon Sep 21, 2015 10:44 pm

At (1000-150)=850mm behind the objective the light cone would be (1000-850)/1000*80=12mm.
You then add the size of the solar disk at this point - the more accurate formulae is (1000-150)/1000*10 = 8.5mm giving a total diameter of 20.5mm
A -150mm barlow placed here would give a parallel beam.
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Re: How to Calculate Sun diameter inside Light Cone?

Post by Brett » Tue Sep 22, 2015 12:31 am

Thank you for the reply. Only one thing that still does not make sense to me: if the light cone has a diameter of 12 mm at 850 mm behind the objective, how can the Sun's diameter here be 20.5 mm? This is then larger than the diameter of the light cone? In other words, if I were to add the Barlow or negative lens here, and say for argument sake that the Barlow was 12 mm in diameter, would I clip any light coming in from the main objective?

My apologies if I'm missing something simple here, just trying to wrap my head around this.

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Re: How to Calculate Sun diameter inside Light Cone?

Post by Merlin66 » Tue Sep 22, 2015 1:05 am

Brett,
The light cone you have in your mind (the 12mm diameter @ 150mm) is ONLY for a stellar point source. The size of the solar image (30 arc min) increases the overall diameter of the light cone (at the focus the image is 10mm diameter and not a few microns) and you end up with an extended 20.4mm diameter "cone" @ 150mm.
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https://groups.yahoo.com/neo/groups/ast ... scopy/info
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Re: How to Calculate Sun diameter inside Light Cone?

Post by Brett » Tue Sep 22, 2015 12:36 pm

Mmm, okay that is interesting. I have never thought about it like this. But then, to bring me to the collimator lens, be it a positive or negative lens, if the f ratio needs to be the same as the main objective focal ratio (in this case f/12.5) one can never fit the "full diameter" of the sun in the collimator lens.

Let me give you examples of what I mean:

Let's say we have a positive or negative lens of 12mm diameter at f/12.5. This needs to be placed 150mm inside light cone (for negative lens) or 150mm after focal point (positive lens). At either of these points, the light cone will be 12mm diameter, but the sun will have a disk of roughly 22mm. This means the collimating lens is too small?

The same goes for any other size collimating lens. We always end up with the Sun diameter too big for the collimator. So does this really matter then?

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Re: How to Calculate Sun diameter inside Light Cone?

Post by Okay-1 » Thu Jan 05, 2017 3:43 am

Brett - thanks for posing the questions.
Merlin66 - thanks for all the answers.
This Thread has been very useful to me - revealed that replacing the 50mm objective lens of my Lunt LS50 with a Televue 2X Powermate and then breach loading the Lunt scope into my Canadian Telescope 152mm would require an additional extended focus of near 300mm!! Not going to happen. It's much easier to use a C-mount teleconverter. Thus, the means to calculate herein saved me from doing a lot of experimental work.
Thanks guys!

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Re: How to Calculate Sun diameter inside Light Cone?

Post by kornfeld » Mon Apr 03, 2017 7:43 pm

I recently picked up a combo quark, and have been using it with a 3x Focal Extender from Explore Scientific. I used it with a 38mm two inch eyepiece to attempt to see a full disk, and I'm getting edge darkening around the entire circumference of the field of view. The majority of the disk of the Sun is equal brightness, then it fairly abruptly starts dropping off in brightness as you approach the edge. The prominences are either fully cut off, or are so dim as to be invisible. If I move the telescope such that the edge of the Sun is away from the edge of the field of view, the prominences are immediately visible.

Is it possible that it's caused by the opening at the base of the focal extender clipping the edge of the light cone? The opening at the base of the focal extender is somewhere between 11 and 12mm.

My set up is as follows:

-OTA: 70mm/700mm f/10 scope
-3x focal extender
-Combo Quark
-2" 38mm eyepiece

I'm having a hard time telling for sure where the field stop of the focal extender ends up in relation to the objective lens, but I think it's somewhere around 640ish mm behind it (i.e., 60ish mm from the focal plane).

Using the equations listed above, I think I would enter these values:

(700-640)/700 x 70 = 6

6 + 7 = 13mm

Is that all correct?

Assuming that is correct, the outer 1mm or so of the circumference of the light cone would get clipped off.

What would I visually see if that was what was happening?


(As an aside....according to this post from DaystarJen: https://www.cloudynights.com/topic/5346 ... ?p=7177835

I should be able to pass a full disk through the quark itself with my current setup. She lists a 2160mm focal length assembly as passing a full disk; and I think should be just under that at 2100mm.)

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Re: How to Calculate Sun diameter inside Light Cone?

Post by marktownley » Mon Apr 03, 2017 11:39 pm

Sounds like vignetting to me. The quark combo free diameter is 25mm(?), I'm guessing the field stop of the 38mm 2" eyepiece is more?
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Re: How to Calculate Sun diameter inside Light Cone?

Post by kornfeld » Fri May 05, 2017 1:30 pm

The telecentric barlow was definitely vignetting the light cone, and the quark itself is not. I moved up to a 2" 2x diagonal with a ~26mm clear aperture, and I can now easily see the entire solar disk plus prominences. The 1.25" 3x focal extender I first tried has a clear aperture of ~11mm. I also have a 2.5x powermate with a clear aperture of 18mm, and there appears to be some edge darkening toward the outer circumference of the FOV.

I also tried attaching the lens element from one of my regular barlows to the end of the 2x telecentric. This should add an additional 1.5x magnification which brings me up to 3x or so (I think). I could no longer fit the entire disk into the FOV; however, it had relatively even brightness all the way out to the edge of the FOV, as opposed to the significant dimming I saw with the 1.25" 3x telecentric barlow.

This leads me to believe that that the Explore Scientific 3x telecentric barlow was vignetting the light cone itself as it entered into the optical system at the diagonal. I'm not 100% confident that that is accurate, but I am 100% confident that the telecentrics with larger clear aperture work dramatically better.

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