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#AskPS: How do I find focus on the sun with no sunspots?

F asked:

I’ll try to photograph the ISS transit over the sun tomorrow. While rehearsing today I found that there is not a single spot on the surface of the sun that can be used to focus my digiscope. The only part with “contrast” is the limb of the sun against the background (space). Do you have any advice on how to confirm/grab focus in this circumstance? My equipment: Sony a7rii + Leica APO Televid 77 (800mm f10.4) + Tele.x1.4 + stack of {ND3.0 – ND1.8 – IR/UV cut} filters @ APS-C.

Thanks in advance. Best regards.
-F


Hi F,

Focusing on the sun without sunspots is difficult, but doable.
 
What you need to remember is that in addition to the contrast of the limb, there is SLIGHT contrast in the granulation of the sun. So, exposing properly is as important as focus. DO NOT overexpose the image, I’d say rather err on the side of underexposing, so you can focus enough to barely see slight granulation. It’s difficult to do, but doable nonetheless.
 
See the below image I captured a while back, with no sunspots. Please note the granules are much more pronounced after processing.
 
Nothing to focus on except for faint solar granules and the limb.
 
Use the limb for main focusing, and expose low enough that you can dial in to the granules as best as possible. They are barely visible in good seeing conditions, depending on the focal length and resolution of your setup as well.
 
However, regarding your equipment — I don’t see a proper solar filter — that is important! Do not trust a stack of ND filters, rather please use a proper solar film or glass solar filter. Please don’t hurt your eyes or camera sensor! (This is a disclaimer I HAVE to include.)
 
All the best, and clear skies!

Cheers,
Cory


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About the author

Cory Schmitz

Co-founder of PhotographingSpace.com, co-owner of several telescopes and mounts, too many cameras, and not enough hard drives, Cory is an American expat living in South Africa with his wife, Tanja Schmitz.

An avid astrophotographer for timelapse, deep-space imaging, lunar, planetary, and star trail imagery, he is an all-around jack-of-most-trades for night-sky photography.

He is also an internationally published and commissioned astrophotographer, where his photos have been used in multiple online and print publications.

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