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pilgrimprogressive

Solar observing - does aperture help?

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Hey.

I have invested in some Baader solar film but I don't know which scope to put it on. I have a 3" short tube refractor and an 8" dob. I realise I don't need the aperture for capturing light but will it give me any more detail?

Thanks, PP

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Even with solar...the bigger the apature the better the image...For imaging you would be better with the 8", you'll pull a lot more detail out of those sunspots with the 8" than the 3"...

For observing the 3" would be fine.

You could use the 8" Dob and make up a 6" filter..A good compromise.

As a grab and go solar scope then it would have to be the 3"....It's a case of experimentation..

Be careful though..Tis a dangerous target for the observers without respect. Just check and double check everything.

Greg

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Just to add a two penn'orth about Dobs. in general. Solar observing with a truss type Dob. is potentially hazardous as you can damage stuff when the 'scope isn't pointed at the sun. The light will get through the truss tubes and the primary mirror could focus it (although not perfectly) on something and damage it.

Most people use sub-aperture filters off axis with the larger Newts., though I seem to remember that Magician made a full aperture one for his 200mm newt. recently. http://stargazerslounge.co.uk/index.php?topic=11285.0 is a link to it. I haven't seen any review of it yet so he probably hasn't had a chance to test it.

Captain Chaos

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I'd be tempted to make an off-axis sub-aperture filter for the Dob' (positioned between the spider vanes).

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I use a off axis one for my C8 , works fine

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I thought I read somewhere that anything over a 4" aperture was not really ideal for solar work.

I suppose in one respect, conditions are rarely good enough in this country to use a really large aperture anyway (for any sort of observing or imaging)

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I'd be tempted to make an off-axis sub-aperture filter for the Dob' (positioned between the spider vanes).

OK but won't that defeat the object of using a large aperture to gain detail? :)

P

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I'd be tempted to make an off-axis sub-aperture filter for the Dob' (positioned between the spider vanes).

OK but won't that defeat the object of using a large aperture to gain detail? :)

Its only what I would do.

Though I understand and accept the fact that maximum resolution is determined by aperture, I am not convinced that you will benefit from anything over 3-4 inches. Whereas, a sub-aperture placed between the spider vanes overcomes the loss of contrast/acutence caused by the obstruction from the spider-vanes and secondary mirror. I think you are more likely to appreciate that than any increase in resolution from using a full-aperture solar filter :)

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I'd make a filter for your refractor, it'll be easier to set up than the full aperture Dob and more aperture than a masked 8" Dob. For white light solar viewing anything over 5" starts suffering too much from the poorer 'seeing' caused by the warmer temperatures in the day time (when the majority of solar observing is done :)) which is why a lot of people buy cheap 4" long focal length refractors to use a dedicated solar scopes.

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Would it make any difference if I said my refractor is short focal length, 400mm?

Perplexed, P

The 'problem' will be that you'll be mainly looking at sunspots and high magnifications are desirable when viewing these. Maybe a x4 barlow would be the answer? I just think masking off the Dob will give you a ~2" solar scope thats still pretty heavy (you'll also need to track the sunspots at high mag).

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Anything over ~6" for solar viewing is overkill. Larger apertures are used scientifically so they can split the solar spectrum into a longer strip. The longer the strip, the greater the resolution. If you mask off part of your Dob, you must be certain the scope is perfectly configured-ALL of the light coming off the main mirror is reflected by the secondary into the EP. If any of it misses, it can, and will, set your mask on fire. I have personal experience with this. I recommend a full aperture filter ONLY, regardless of aperture.

A larger aperture, as mentioned, will be susceptable to bad seeing, so smaller is somewhat better. It's really pretty astounding how much you can see with a 40mm PST, and a 6" scope, which is my main solar scope, doesn't show all that much more detail, just better contrast under less than ideal conditions. Careful dark adaptation goes farther than aperture.

HTH.

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I can see the newspaper headlines now.....

"Over enthusiastic solar observer burns hole right through his house" :)

Remember thats 10" of mirror concentrating all that light in to one place :shock:

Matt

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I did try the solar viewing bit with the 200mm, to be honest it was not better, if not worse than when i used to use my 105mm. Photography through the 105 was far superior and the clarity excellent. I now just take off the peep hole cover on the protective cover to view.

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