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jambouk

Solar scopes focal length

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I am thinking of getting a Lunt 50mm scope. I want to image as well as observe. I am thinking I could put my ZWO on the scope and capture video. My question is, does focal length in a solar scope relate to the FoV as it does for a non-solar scope? The field of view calculator (http://www.12dstring.me.uk/fovcalc.php) suggests I could just about get a full disc with a focal length of 350mm (the 50mm Lunt), but not with a focal length of 500mm (the 60mm Lunt). Is my logic correct?

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Sort of but you also need to consider the size of the blocking filter. The sun's disc is roughly the focal length in mm divided by 100. So 500mm fl = 5mm and 1000mm = 10mm disc. Therefore if you have a 5mm BF you'd maybe just get full disc with the former but around half the disc in the latter no matter what you do. An exception to this seems to be when using binoviewers as this seems to allow full disk with a 5mm BF even with my 1000mm Tal based PST for a reason I have never been able to definitively confirm.

 

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Your logic is correct but the operational field of view can be increased for a given solar telescope by using a larger blocking filter. Be aware though, that this does not increase the "sweet spot".

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Thanks.

Is there a list of the relationship between the values of the blocking filters Lunt give (B600, B1200, B1800  etc) and the size of the blocking filter in mm?

James

 

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yes, indeed 600=6mm      1200= 12mm       BF5 = 5mm      BF10 = 10mm

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Thanks again. Looks like the Lunt 50 with B600 is the one for me. Cheers.

James

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