Off axis masks calculations

[Doc]

Following on from Andrew's query regarding off axis masks has got me thinking off building one.

Firstly I want to run a calculation past you to see if I have this right.

My 16" has a 4" central obbstruction so it's possible to make a 6" off axis mask so is the following right.

F4.5 x 16" = 72

72 divided by 6" mask = F12

So scope becomes a 6" F12

Is this correct?

[callump]

Sounds right to me...

/callump

[barkis]

What is your intention here, to expose only 6" of your main mirror?

Making a mask with a 6" aperture offset so as to eliminate your secondary obstruction?

Ron.

Sorry mate. I re read your post, and that's exactly what you are doing.

And your calc. is correct.

[arad85]

Yup, masking changes the aperture, not the focal length so focal length of your 16" is 72". You have a 6" aperture scope with focal length of 72" so the focal ratio is 72/6....

(I know that's what you've said, but just saying why...)

[Doc]

Thanks guys.

After Salisbury I will endeavor to make one.

Would this set up be good for looking at the sun, of course the opening would be sealed with proper baader solarfilm.

[calibos]

I am sure it would be fabulous to look at the sun in white light Mick......if we weren't in the most unusually quiet period of sunspot activity in recorded history :)

In terms of night time astronomy, I have never understood the grab and go, "The best scope is the one you use the most, and you'll use your smaller scope the most because its quick and easy to carry out....' philosophy. Give me 'more hassle to set-up but mindblowing views when I do' over 'easy quick setup but underwhelmed by the views' any day...I mean night of the week.

My point is that even if I bring the big Dob out for a specific reason like looking at a shadow transit on Jupiter, if the seeing is Rubbish or if I got my times wrong, well there are still a million other things to look at seeing as I am out there with the big dob set-up. Not so with using the big dob for solar. Its a big ol' beast to be wheeling out in the day when there is only one thing to look at, the Sun, and the chances are that there are no sunpots visible.

I know the Baader Solar film is cheap enough but I'd still be saving it and putting it towards the price of a PST IMHO

[Doc]

I am sure it would be fabulous to look at the sun in white light Mick......if we weren't in the most unusually quiet period of sunspot activity in recorded history :)

Keith I know nothing about the sun's activity really, but it was just an idea for something to do during the day while waiting for the evening shift at work to start.

I have never seen sun spots so thought it would be pretty cool.

[irishbloke]

I have made a solar filter mask for my 10" dob. I've seen a few sunspots through it and loved it. sweated buckets as I was doing it. Funny that! even managed to take some pics of them with my camera. Something cool or hot about looking at our star.