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Prime focus solar imaging at f/12

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#1 jurdea

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Posted 27 February 2024 - 03:54 PM

Getting ready to go after the upcoming eclipse with the following optical train:  Solar filter, F6 SV ST80, 2" TV Big 2X Barlow and Canon EOS RP.  As my system is effectively operating at f/12, would a field flattener make much of a difference?



#2 michael8554

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Posted 28 February 2024 - 06:15 AM

A FF is added to an astro setup to improve star shape at the edges of the frame.

 

A few stars will be visible at Totality, but I'd suggest you're more interested in the Sun, the few stars can be removed if distracting.

 

So no need for a FF.



#3 jurdea

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Posted 28 February 2024 - 08:37 AM

Thanks.  Good point on few stars being visible, but I was worried about smearing of the corona at the edge of the field.



#4 michael8554

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Posted 29 February 2024 - 04:42 AM

"I was worried about smearing of the corona at the edge of the field."

 

I hadn't considered that

 

The distortion that FFs fix is subtle, I don't think you'd notice the difference on the corona.



#5 james7ca

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Posted 01 March 2024 - 08:51 AM

I wouldn't try to combine a field flatterer with a barlow. Field flatteners are designed to work "best" at certain f-ratios and focal lengths and barlows may introduce their own field-related artifacts (which may work opposite to the field flattener). So, unless you are particularly lucky those two optical components aren't likely to work well together. The other issue could be added flare, reflections, or loss of contrast from the extra optical components (could be critical during a solar eclipse). However, if you already have the components it might be worth trying a test using either the moon or a star field.

 

Your best bet would probably be a long focal length doublet refractor like the Stellamira, 80mm, f/10 ED scope. However, that scope is only "flat" across a so-called one inch sensor (like Sony's IMX183 or IMX533, as used in the ZWO ASI183 and ASI533 and several other cameras). But, I'd expect that a short-focus SV ST80 with any barlow won't even do as well as that.

 

Moderately long focal length, wide and flat-field refractors aren't that easy to come by or to make. Thus, if you could combine a field flattener with a standard barlow and get a wider and flatter field everyone would probably be doing it (for the relative "cheap"). That said, something (ST80 with barlow) is probably better than nothing to get additional image scale (if that is what you really need). But, you may want to rethink your scope or the image scale and/or field size that you are after. For example, the ST80 with a 1X field flattener may be a better option (and skip the barlow).


Edited by james7ca, 01 March 2024 - 07:58 PM.



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