Solar image size affected by focal extenders/reducers

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DavidP
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Solar image size affected by focal extenders/reducers

Post by DavidP » Thu Dec 03, 2020 11:14 pm

I’m baffled.
Can someone explain how the use of focal extenders and reducers will affect the size of the image of the sun in my image train?
I am looking at using a Solar Spectrum 18mm clear aperture filter on my 102mm f/8.1 refractor. I’ve been recommended to use 4x telecentric first and then I’m assuming some kind of focal reducer after the filter. So what do I end up with in terms of FOV. I plan to image.



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Re: Solar image size affected by focal extenders/reducers

Post by DavidP » Thu Dec 03, 2020 11:27 pm

My 826mm focal length scope with a 4x converter gives me an effective focal length of 3304. If I decide this by 110 I get a 30mm diameter of the sun. Right?
Then. My filter has a clear aperture of 18 mm giving me a little over 1/2 the suns diameter.
So then what happens when I add a focal reducer ?



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Re: Solar image size affected by focal extenders/reducers

Post by MalVeauX » Fri Dec 04, 2020 1:15 am

Heya,

Your 102mm F8.1 (826mm focal length) will produce an 8.26mm disc image. That's without the telecentric amp. Your 4x amp will generate a 3304mm focal length at F32.4 through the etalon (good). And yes, the new disc image is huge now, too big to image in a single shot. So you can use a 0.5x reducer to bring it back down to F16 which will sample well on 3.75~3.8um pixels. But, it still will not be a full disc image as the disc will be around 15mm or more still. This is happening after the clear aperture of your filter though, not before it. Basically with this system, you will not image a full disc even with a reducer. You will end up having to do a mosaic to do that.

If you want to get a full disc image, you'll need to use a smaller aperture and shorter focal length. Targeting around F7.5 to keep you at F30 at the etalon. You would need to get the focal length down to around 400mm~430mm or so. And to keep F7.5, that means 53mm~57mm aperture, or just call it 50mm and you're good, at just over F7.5, keeping it at F30 or longer. This will then be focal reduced after the filter, with a 0.5x and if you fit it to a 1" sensor or so, you can pull off a full disc this way. It just will be low resolution.

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Re: Solar image size affected by focal extenders/reducers

Post by marktownley » Fri Dec 04, 2020 6:42 am

You can use the fov calculator here https://astronomy.tools/calculators/field_of_view/ to get an idea of what you'll get.

I use a 50mm etalon on the front of a 60mm f6 scope, followed by a Quark and then the Baader 0.7x telecompressor which gives me a full disk on a PGR GH3 with a 3376 x 2704 pixel array. I can just about get a full disk on the chip with this setup. Here's an image from earlier in the year:

ImageHa-FD-DS-bw by Mark Townley, on Flickr

With your 800mm scope, assuming you used the same camera and reducer you would get about a quarter of the disk visible.


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Re: Solar image size affected by focal extenders/reducers

Post by DavidP » Fri Dec 04, 2020 7:03 pm

Y’all are so much help. Thanks for your time.
I’m not trying to get a full disk image. This is for high resolution close ups. I will also want to use the set-up with a 150mm f/12 Mak casa with a ERF. I was just unsure as to whether using a focal extender, then an aperture restriction (18mm through the Ha filter), then a focal reducer complicated the math somehow in a way I wasn’t understanding. I’m guessing that the spacing of the initial telecentric needs to be figured out.



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Re: Solar image size affected by focal extenders/reducers

Post by DavidP » Tue Dec 29, 2020 8:28 pm

Still thinking about this. So I will be using a 4x Telecentric on my 102mm f/8.1 scope, and then a 18mm aperture solar filter. If I were to add a .4x telecompressor (https://alpineastro.com/products/solars ... ee3b&_ss=r)
I then have a 7mm image. Correct?
I have a camera with a 1.1” sensor and 3.45um pixels which should sample well at f/12.8.
But is it silly to capture a 7mm image on a
28mm sensor?
Also. It seems extravagant to send that 12mm image from the filter through a 2” focal reducer. It certainly would be more economical to use a 1.25” .5x focal reducer. I just don’t know on any quality ones.



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Re: Solar image size affected by focal extenders/reducers

Post by Merlin66 » Tue Dec 29, 2020 9:45 pm

Most of the generic 0.5 reducers seem to work OK.
I use an ATiK reducer and also have a GSO version.


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Re: Solar image size affected by focal extenders/reducers

Post by SimonM » Sat Jan 09, 2021 9:22 pm

marktownley wrote:
Fri Dec 04, 2020 6:42 am
You can use the fov calculator here https://astronomy.tools/calculators/field_of_view/ to get an idea of what you'll get.
I use this for general “stuff”, but can this be used without factoring field stops in Etalons and blocking filter sizes?



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Re: Solar image size affected by focal extenders/reducers

Post by rsfoto » Sat Jan 09, 2021 10:17 pm

marktownley wrote:
Fri Dec 04, 2020 6:42 am
You can use the fov calculator here https://astronomy.tools/calculators/field_of_view/ to get an idea of what you'll get.

I use a 50mm etalon on the front of a 60mm f6 scope, followed by a Quark and then the Baader 0.7x telecompressor which gives me a full disk on a PGR GH3 with a 3376 x 2704 pixel array. I can just about get a full disk on the chip with this setup. Here's an image from earlier in the year:

ImageHa-FD-DS-bw by Mark Townley, on Flickr

With your 800mm scope, assuming you used the same camera and reducer you would get about a quarter of the disk visible.
Hi Mark,

You forgot a very important detail and that is the pixel size in micron.

It is not the same thing a chip with 1000x1000 pixels with 10 micron or a chip with 1000x1000 with a 1 micron pixel size :shock: and so that depends if the Sun fits in your camera or not at a certain focal length ;) ;) ;)


regards Rainer

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Re: Solar image size affected by focal extenders/reducers

Post by marktownley » Sun Jan 10, 2021 6:35 am

Hi Rainer

The camera I am using has 3.69um pixels.

https://www.flir.co.uk/products/grassho ... U3-91S6M-C

Mark


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