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Off aperture or not?

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#1 Substrate

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Posted 15 March 2013 - 09:58 PM

Hello,

I have been viewing the sun with a hydrogen-alpha scope for years now, and suddenly realized it's about time to take the big leap into whitelight viewing ;)

I just received the Baader Astrofilm in the mail today and thought I'd ask you all about how to go about applying it to my scope. The scope I will use it on is a Vixen 95mm modified cassegrain. The vixen came with a nice plastic dust-cover that I think will work nicely for the filter cell.

I ordered a 6 inch square of film which is more than enough to cover the whole thing, but I read off hand somewhere that it is better to use only a portion of the aperture with reflectors.

Is this an issue with a 95mm scope? Also I bought some crazy glue which bonds plastics. Will it be alright to use this?

I appreciate your insights

#2 old_frankland

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Posted 15 March 2013 - 10:56 PM

Go full aperture, and google for ideas on mounting the filter material. There are a number of good sites showing how to build the filter. Do note that the Astrofilm needs to be mounted with wrinkles because it tightens up in sunlight. If it is drawn up too tight when you assemble the filter the film can actually stretch or rip. Also, I used silicon adhesive which stretchs if needed when the filter heats up. Good luck with the white light viewing.

#3 drksky

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Posted 16 March 2013 - 06:36 AM

More than anything, it's said to use off-axis filters with reflectors to avoid the central obstruction, thereby increasing the contrast.

#4 BYoesle

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Posted 16 March 2013 - 09:39 AM

That's a good general rule of thumb, but only if the off-axis aperture is say larger than 75-100 mm or so...

With only 95 mm of total aperture to begin with, an off axis filter would be less than 40 mm. Despite a little decrease in contrast due to the central obstruction, the increased 50+ mm of aperture if used full aperture will more than make up for any contrast effects, especially with daytime seeing conditions being less than ideal most of the time.

So I'd go with full aperture for anything less than about 8 inches, and consider off-axis only above that...

Addendum: And for imaging, always go full aperture. You can use a mask if the seeing is poor. Witness some of the unbelievable WL images being captured with C14s at full aperture!

#5 Substrate

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Posted 16 March 2013 - 03:21 PM

Full aperture it is then.. It hadn't occurred to me I can simply mask it off to suit the particular day. Thanks for your help gentlemen!

#6 marktownley

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Posted 16 March 2013 - 03:26 PM

Full aperture, definitely!

#7 George9

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Posted 17 March 2013 - 02:12 PM

Even with a 10" reflector, I always go full aperture. In mediocre seeing, the view sometimes crystalizes and with the 10", you get the full benefit of the fleeting moment.

(I've never tried the sun in my 18", but there I am sure I would go for off axis.)

George






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