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Focus ring distance indicators meaningless?

SVBony SV555 Focuser Beginner
9 replies•179 views
Robert Powell avatar
Robert Powell avatar
My imaging train is: SV555, ZWO CAA (16.5mm), ZWO 54mm Filter Drawer (20mm) and ASI 2600mc Duo (17.5mm).
Total backfocus distance 54mm.
I am able to achieve perfect focus, but when I do, the focus ring indicator on the scope is WAY down towards approximately 50 feet.  (Nowhere near infinity).
Is this normal or am I missing something here. (Certainly possible as I'm very new to this hobby).
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Brian Valente avatar
Brian Valente avatar
Well, accurate focus doesn't lie, so if it's in focus, you know that is optically correct

I doubt the lens barrel markings are meaningful, but I don't know for sure. Having a lens marketed for astronomy that has a focus distance scale (and focuses down to 50 feet) isn't a feature, it may just be repurposing parts from other lens lines. Not necessarily bad, i'm sure it's low priced. If it works optically, that's all that really matters

Brian
Robert Powell avatar
Robert Powell avatar
Well, that's pretty much what I thought, but was thinking that I may have had my spacing off a bit.
Thanks for the reply!

Bob
bigCatAstro avatar
bigCatAstro avatar
Brian Valente:
Well, accurate focus doesn't lie, so if it's in focus, you know that is optically correct

I doubt the lens barrel markings are meaningful, but I don't know for sure. Having a lens marketed for astronomy that has a focus distance scale (and focuses down to 50 feet) isn't a feature, it may just be repurposing parts from other lens lines. Not necessarily bad, i'm sure it's low priced. If it works optically, that's all that really matters

Brian

I’d second that if you can get into and have repeatable autofocus or manual focus results—then ignore the markings.
craigs0522 avatar
craigs0522 avatar
My SV555 focus ring has to be  rotated all the way to infinity for sharp stars. It makes me nervous not having any wiggle room for focus, but it's also super quick to set up haha
Robert Powell avatar
Robert Powell avatar
What’s your back focus distance? Are you using a filter?
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craigs0522 avatar
craigs0522 avatar
Robert Powell:
What’s your back focus distance? Are you using a filter?

I have 55mm of back focus and am using a dual band filter. For whatever reason I cannot achieve focus without using my 2 inch filter. Without a filter, it seems like the focus ring needs another 1/8 of a turn clockwise, but I'm stuck at infinity focus and cannot rotate any farther
Robert Powell avatar
Robert Powell avatar
Wow. I am at 54 also using dual band filter. Would not expect 1mm to change the focus point by almost half the range!
Tony Gondola avatar
Tony Gondola avatar
What you are describing makes sense for a camera lens but it's not what you want for astrophotography. It sounds like you spacing needs to be just a tad shorter to give you some wiggle room for filters.
craigs0522 avatar
craigs0522 avatar
Tony Gondola:
What you are describing makes sense for a camera lens but it's not what you want for astrophotography. It sounds like you spacing needs to be just a tad shorter to give you some wiggle room for filters.

I agree. When I'm using the Bahtinov mask, the diffraction spikes look ever so slightly not centered. My pictures look okay, but I'd like to have a little more control.
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