Off Axis Solar Dob with Dust/Cover plate - What is the new F ratio?

[TheButcher]

Hey All,

I have a question about using my AD12 Dob with Baader Solar Film in the Off-Axis circle that came with the Top Lid/Dust cover. Now I am back with one scope, my Trusty AD12, and with the eclipse coming I wanted to at least attempt to image it. I wish I could throw it on my Cem70, but I had to sell the Tri-Pier for more Bills last week . So I will probably end up using the Dob with Dob Base since it already has Baader Film that I put in it.

Anyway, here is the question. By putting the Lid on the DoB and only allowing the 1.5"-2" hole (I got to measure it when I get home) with the Baader film for the Aperture, Now, What estimate would my New focal ratio be around? I am curious because I am going attempt aFocal imaging for fun with my smartphone (Had to sell all my Astro cameras too lol) and looking for an eyepiece to get the whole sun in and have a little extra wiggle room for when it drifts.

Thanks in advance,
Brian

[JayTee]

You would still use the formula focal length divided by aperture of the dust cover opening. This will give you that new focal ratio.

[TheButcher]

You would still use the formula focal length divided by aperture of the dust cover opening. This will give you that new focal ratio.

Thanks. JT, I got rusty with my Astro, and trying to make a comeback, I should have thought of that lol.

Now what if I 3D printed a 12" Filter to cover the Front and maybe have a Hole 5",6", or 7" in the center? Would that be better?
Also, keep in mind I am using just a smartphone this time with an eyepiece but I will look through the Eyepiece as well.

Thanks Again
- Brian

[JayTee]

Remember, as with any telescope when you increase the aperture you will increase the brightness of what you're looking at plus you will also get better resolution. Increasing the aperture makes your object appear brighter for two reasons, you are gathering more photons and you are decreasing the focal ratio both of those make the object appear brighter.

I have used a mask with a 4-in opening on my 11" SCT. It produced a very usable and photographable image of the sun. Obviously, with Baader solar film over the opening.

[TheButcher]

Thanks Again JT,

I think I may start my 3D print for it. At least experiment