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White Light Filters, Refractors, and CA?

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#1 Michael Rapp

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Posted 28 November 2020 - 03:08 PM

Hey all,

 

Now that we're getting out of solar minimum, my interest in solar observing is returning significantly.  While I've had several solar-capable scopes over the years -- I have a Lunt 35, and 8" and 5" white light filters -- I've never been able to keep up a systematic observing program because in Houston, it gets a little warm outside during the daytime.  :D

 

My Lunt is wonderful.  It's a dedicated solar scope and it is grab and go for even just a 5 minute session in the middle of the summer.  I want to do the same for white light.  I want to minimize thermal effects, so I'm thinking refractor.  I'm even thinking of one of those relatively inexpensive 70mm f/10 achromats on an "okay" alt-az mount.  (I'm aiming for 50-60x magnification and dedicated to white light solar.)

 

I've never used a white light solar filter on an achromat before.  Is chromatic aberration even a factor on such a scope with a white light filter?  



#2 gustavo_sanchez

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Posted 28 November 2020 - 03:12 PM

Based on what I remember, CA due to the refractor optics shouldn’t be an issue. Eyepiece CA from cheap EPs could still be present.

#3 MalVeauX

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Posted 28 November 2020 - 03:33 PM

Heya,

 

Photospheric observation is most of the visible spectrum, so you can indeed see CA on the limb with an achromatic doublet if its not long focal-ratio to produce near apochromatic results. A fast achromat will have CA in white light. Film filters and wedges, etc, merely pass a lower transmission with some trimming to safe levels, but the basic RGB is passing so there's room for CA still there. A 70mm F10 refractor will not have CA, but an 80mm F5 or 120mm F5 will for example. But to be quite frank, it's not bothersome, as there's nothing to see on the limb of the disc in the photosphere anyways.

 

Very best,


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#4 Sketcher

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Posted 28 November 2020 - 04:21 PM

The CA is still there; BUT this is one situation in which the addition of a yellow filter (like the Baader LongPass 495) will serve double duty.  It'll tame the CA while producing a more comfortable and aesthetically more pleasing yellowish sun -- a win-win situation.

 

With all other things being equal, there's really very little advantage in going with an apochromat here.

 

Even when using telescopes without any CA, I'll often plop in a yellow filter for visual observation when using a white-light solar filter.

 

Also, depending on one's aperture, stopping down the aperture a little can be beneficial in taming daytime seeing -- while also reducing the effects of an achromat's CA.

 

I've made white-light solar filters for most of my telescopes; and for the above reasons I made a sub-aperture, 120mm, white-light solar filter for my 152mm f/6.5 achromat.  Add a yellow filter you'll likely not know that you're looking through an achromat.  This is one area in which an achromat can perform near enough to that of an apochromat that monetary savings (for most people) will outweigh any remaining advantage of the apochromat.



#5 Michael Rapp

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Posted 28 November 2020 - 04:56 PM

The CA is still there; BUT this is one situation in which the addition of a yellow filter (like the Baader LongPass 495) will serve double duty.  It'll tame the CA while producing a more comfortable and aesthetically more pleasing yellowish sun -- a win-win situation.

 

With all other things being equal, there's really very little advantage in going with an apochromat here.

 

Even when using telescopes without any CA, I'll often plop in a yellow filter for visual observation when using a white-light solar filter.

Interesting!  I do have a really nice #12 filter already.  If I were to see any CA, I could just use that.




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