Solar aperture and eyepiece size

[fauxpas]

Knocking up a home made solar filter and someone said regardless of the aperture of the scope, you only need 3"... Is that true? If so should the hole be offset like I see on a lot of scope end caps?

Secondly, if I want to view the entire sun in the eyepiece and I have a 4" Dob at 640mm focal length, what eyepiece would I need? I'm imagining 30mm or more...

Thanks...

[Merlin66]

For viewing the sun, apertures above say 100mm can sometimes be used- depends very much on the local seeing conditions (better in the early morning).
A 100mm aperture will allow the recording of solar surface granulation.
A good camera is equally imprortant. The "weapon of choice" still seems to be the IS DMK series cameras.
The size of the solar disk is approx 0.5 degree....if your eyepiece has a AFOV of say 50 degrees then you could use an eyepiece which gives around x80-x100 and it would fit.
CCDcalc is a great freeware resource to check out the field coverage of various camers.....

[brian nordstrom]

:) Hi Tony I use my lil 60mm FS60C Tak and Lunt Hershel wedge , and with my dedicated 12,5mm thats about 30x the views are awsome , I just 2x barlow to 60x if I need more power , buy daytime observing wont allow much magnification as the seeing is always really , really BAD!!! , you will have a great solar scope there BUT ! , just be careful :help: and check the filter for pin holes by holding it up to the sun at arms length while looking thru it at the sun and moving it around , you should not see any star like sparkles thru the filter , if so discard it as those sparkles( UV and all the bad radiation ) will be magnified in the scope to you only set of eyes .:sadeyes: you have , bad news :eyepop: .
If it blocks the sun without light passing thru using your naked eyes at arms length , then it will be safe .
What type of material is it by the way ?
You have to be VERY CAREFUL here !! as we only have one pair of eyes .
Brian.

[fauxpas]

I'm getting some thousand oaks optical stuff...

[malclocke]

Hi Tony,

The maths to give you an approximation of the true field of view is quite easy.

true fov = eyepiece apparent fov / (telescope focal length / eyepiece focal length)

For example, if you had an 8mm plossl eyepiece with a 52 degree apparent field of view then with your 640mm focal length dob you get:

52 / (640 / 8) = 0.65

Which is just over the 0.5 degree of the full solar disk.

Obviously, an eyepiece with a different apparent field of view will give you a different result, so you'll need to take that into account.

Malc

[brian nordstrom]

:thumbsup: sweet a real life colo0ur'd sun ( dark yellow Orange , Cool ) yellow is Sol's real colour. , enjoy mate .
Brian.

[Poita]

Or to bypass the math, the Moon and the Sun are about the same size in a scope, so check out the moon through any given eyepiece and you will know exactly how big the sun will look.

[fauxpas]

Cheers... Still waiting for the solar film to come... :(