Using a focal reducer with a C9.25 and DSLR to image eclipses?

[kornfeld]

Hi all,

I have a question about imaging eclipses in general, and even though it's relevant to the 2024 eclipse cutting Mexico and the US, I figured this would apply to all eclipses, so I wanted to ask it in a more general subforum.

I imaged the 2017 eclipse with a 5D Mark II mounted to my C9.25, and was very happy with the images it produced. The disk of the Sun fit well onto the image sensor, as did prominences.

However, a small issue arose when trying to get the corona. I was able to fit enough of the corona in frame in the wide-dimension of the image sensor to be happy; however, I had to slew the telescope back and forth slightly along the shorter dimension of the image sensor to get shots of the corona in those directions, and then stitch them together after the fact.

For 2024, I'm considering using a focal reducer. However, I'm at the I-don't-know-what-I-don't-know stage. If I use a focal reducer, will I actually get more field of view imaged onto the sensor? I'm seeing some mentions here and there of there being vignetting issues, but I don't know how this actually will play out. I'm worried that I'm going to reduce the magnification, but also reduce the size of the imaging circle, so I end up not really gaining much.

Thoughts? What am I not considering? Like I said, I'm early in exploring this specific topic, so I'd love some input. Thanks in advance.

[cybermayberry]

You will get vignetting on that full frame sensor.
Flats may correct the issue or at least minimize it.
The link below will allow you to see the effects of how the reducer will effect your FOV as well as changes to the pixel scale.
https://astronomy.tools/calculators/field_of_view/

[RodAstro]

Hi
If you are getting good information across the wide plane of the chip then a focal reducer will give you the same information across the short plane of the chip.
The information across the wide plane will all still be there as before only smaller.
The only place you may get noticeable vignetting is on each side of the wide plane but this is information you could not see before so is ok to crop it out to a square image.
What you will also benefit from is a brighter image so faster FPS that helps with the pixel scale difference.
So in the end a focal reducer will not give you any more vignetting than the telescope is already giving you, the best way to think of this is the focal reducer is making your chip larger so it shows the vignetting in the telescope optics.
SCTs and Maks suffer with vignetting caused by the central baffle tube just the design problem in the optics.

So in the end you will be fine with a focal reducer to get a full single disk and corona as you were getting before.

Hope it all makes sense you may have to read it a couple of times.

Cheers Rod

[kornfeld]

Great, thank you for the response! I'm more curious about the physical dimensions of the light cone/imaging circle itself, and how that may or may not be impacted by the physical components of the telescope/reducer/camera/etc.

For example, in this post, it is mentioned that the baffle tube within a C9.25 might be the limiting factor that determines how large the field of view is for some configurations, regardless of what a calculator might tell you.

So my question is more about the physical limitations of these specific components. Does anyone have an idea if the usage of a focal reducer will actually allow the camera to see more of the sky? Thank you again in advance!

[kornfeld]

Oh wow---Rod, I started typing my response as you posted yours, so I didn't see yours until I had already posted my previous message. Thank you, that is more or less the information I was hoping to have. Perfecto, thanks

[cybermayberry]

Ultimately everything you will online find will just be a close but rough guide of about what to expect.
The exact reducer used and the exact placement along in the image train is going to have an affect as well.
Individual mileage is going to vary
Bottom line is unless your spending big bucks on research grade equipment, you should expect some deviations from manufacturers listed specs as well.
This may be of some interest to you as well
https://cdn.shopify.com/s/files/1/0327/ ... 1611096646

[christian viladrich]

Hi,
Are you using the C9.25 Edge HD or the "classic" C9.25 ?

What kind of focal reducer are you planing to use ? The usual f6.3 reducer is of rather bad optical quality.

The reducer of the C9.25 EgeHD est excellent, but very expensive.

What about using a telescope with about 1000 m focal length ?

[Dennis]

I dont understand the concept of imaging the suns full disk with a c9.25. Do you have other telescopes?

[kornfeld]

Ultimately everything you will online find will just be a close but rough guide of about what to expect.
The exact reducer used and the exact placement along in the image train is going to have an affect as well.
Individual mileage is going to vary
Bottom line is unless your spending big bucks on research grade equipment, you should expect some deviations from manufacturers listed specs as well.
This may be of some interest to you as well
https://cdn.shopify.com/s/files/1/0327/ ... 1611096646

This is great, thank you for the link!

[kornfeld]

Hi,
Are you using the C9.25 Edge HD or the "classic" C9.25 ?

What kind of focal reducer are you planing to use ? The usual f6.3 reducer is of rather bad optical quality.

The reducer of the C9.25 EgeHD est excellent, but very expensive.

What about using a telescope with about 1000 m focal length ?

It's the classic, not an Edge.

To be honest, I absolutely loved how much detail I got from the circumference of the moon, and the flares around the Sun, with this focal length. I liked the images of this stuff more than the images I saw that were only the corona, to be honest. So I was hoping to possibly split the difference with a reducer.

[kornfeld]

I dont understand the concept of imaging the suns full disk with a c9.25. Do you have other telescopes?

Just responding here as well, because I do appreciate everyone sharing their thoughts.

I actually really liked how much detail I was able to get with the higher magnification. I liked my images better than the ones I saw with low magnification that only caught the corona. So my thought was that a reducer might split the difference. Possibly?

[cybermayberry]

I dont understand the concept of imaging the suns full disk with a c9.25. Do you have other telescopes?

Just responding here as well, because I do appreciate everyone sharing their thoughts.

I actually really liked how much detail I was able to get with the higher magnification. I liked my images better than the ones I saw with low magnification that only caught the corona. So my thought was that a reducer might split the difference. Possibly?

I think what Denis is alluding to is....

Using your current camera with a .63 reducer is going to end up with a resolution at approx. 0.89 arc/sec per pixel.
When shooting the sun you will benefit greatly with shooting video vs single exposures due to atmospheric distortions. With the data I could find on your camera that would limit you at 1920x1080 at 30 FPS in video mode. This will not change your resolution but would dramatically reducer your FOV. (video mode is also typically highly compressed from DSLR) This also under-samples at f6.3

Now contrast that to using a short tube 80 APO with a IMX183MM sensor and a 1.5x barlow. That combo would give you a resolution at approx. 0.83 arc/sec per pixel resolution and allow almost the same FOV as the setup you describing, while giving you 19 FPS (still slow) off the full sensor (5496×3672) @ 8 bit ( not compressed ). Again this also under-samples at F 7.5. You may even find sharper images based on seeing by not using Barlow and using ROI to increase FPS despite increasing your pixel scale, further lower sampling at F5. ( You can also try drizzle that data to recover some of that lose ) This setup would also not give you vignetting issues as the c9.25 with reducer would.



I do understand full well budgetary constants, all to well. With that said you may want to look further into a wider APO refactor, and dedicated mono astro camera. The slight loose of resolution in some (difference individual pixel size) may be made up with by greater FPS. This may ultimately be cheaper than a decent reducer, if this is the primary reason for buying the reducer.



Everything is a trade off.
Don't forget to factor in your local seeing as to how much resolution you can actually get vs theoretically get.

[Dennis]

I dont understand the concept of imaging the suns full disk with a c9.25. Do you have other telescopes?

Just responding here as well, because I do appreciate everyone sharing their thoughts.

I actually really liked how much detail I was able to get with the higher magnification. I liked my images better than the ones I saw with low magnification that only caught the corona. So my thought was that a reducer might split the difference. Possibly?

Sampling is the problem is see, too. You will have to magnify the suns image a lot to reach critical sampling with a C9.25. Once you are at critical sampling this will be far away from a full disk image. Full disk imaging with that telescope will be heavy overersampling (no gain vs a smaller aperture) and therefore not a good choice, since the bigger aperture is much more sensitive to seeing. Apart of this if you dont have perfect seeing all the time: To actually use the resolution potential of the C9 you need to do lucky imaging with a mono camera (>100 fps and exposure times lower then 7-8 ms).

Another consideration: If you use a non edge hd version of the C9 also the diffraction limited field is very small for such large camera chips. So you will get coma and field curvature abberations with these cameras outside of the "good" field.

So you get the idea.. the sct is generally very limited when it comes to wider fields.

[kornfeld]

Ok this is all really excellent food for thought---thank you to everyone who contributed their knowledge, I genuinely appreciate it.

I'll keep thinking about what to run for next year. For some reason, I find the DSLR more appealing in this setting than my higher speed dedicated astro cameras, but I'm not sure why.

Just for future reference in case anyone is curious about this setup (now, or in the future), here were a few of the shots I took in 2017.

Straight out of the camera, this was the field of view:





By slewing up and down a few times, I could get enough shots to stitch together at different exposures (this was very, very minimal processing--just auto-stitched together, without combining different exposures, and without doing much to remove stitching artifacts):





But I was surprised that my favorite images were the punched in ones, instead of the wide views. This is the main reason I'm not ready to abandon the full magnification of the c9.25:

[marktownley]

Excellent shots!

[cybermayberry]

Those proms!
The shots are excellent.

You may find this link very helpful in weighing out all your options.

https://www.wilmslowastro.com/software/formulae.htm#FR

[kornfeld]

Those proms!
The shots are excellent.

You may find this link very helpful in weighing out all your options.

https://www.wilmslowastro.com/software/formulae.htm#FR

I had not seen that page before--thank you!

(I'm glad I started this thread, if for no other reason that it brought a bunch of great info out of the woodwork. )

[Dennis]

Very nice for a one shot camera. Against my idea to go for a different setup, with results like this its hard to tell what could improve things even more.

[RodAstro]

Personally I think the Celestron 6.3 reducer at£120 new will do the trick, no way could you get another scope and camera to do this for that price.
Remember this is only going to be used occasionally during a total solar eclipse, maybe only once.
The reducer will do the job of fitting in the whole sun and corona on one image and remove the stitching artifacts that can be seen in the last beautiful image.
Field distortion will be minimal as you have a huge big black blob in the middle of the image so you just focus to what detail you can see and use a little pincushion distortion to correct it if you can even see it.
Sorry if I sound a bit blunt but it was said he was happy with the results he got and would just like to fit the whole image on the camera chip of the camera he already owns.

[RodAstro]

Sorry I should have put the last corona image.

Rod

[kornfeld]

I may end up getting a focal reducer even for visual use, since it seems like something that a lot of C9.25 owners think highly of. Thank you for the input!