In this video I explain how you deal with Backfocus & a reducer.
0:00 Intro
0:32 Unboxing
3:38 Optics: What a Reducer does
5:09 Optics: Reducer/Flattener vs. Barlow lens
6:24 Backfocus
7:12 Attaching Reducer/Flattener - What you'll need
8:27 Attaching Reducer/ Flattener and ZWO camera (& Filter Drawer)
15:04 Flied rotation using TS Triplet Apo
16:03 Proof it works
16:09 Reducer/Flattener Weight
16:26 Outro, Remarks
ZWO ASI Backfocus information:
[ Ссылка ]
Question:
Is the back focus distance affected noticeably by introducing extra glass as filters in front of the sensor?
Answer:
The simple answer is: Yes, the back focus is affected by introducing extra glass (/filter). Personally and currently I'm only using an Optolong L-Pro broadband light pollution filter and I did not see any (noticeable) effect.
The longer answer is: Your backfocus is affected (in mm) by the one third of your glass(!) filter thickness.
delta f (focal change) = t (thickness of your filter) * ((n (refractive index of your filter) - 1)/n (refr. ind. o.y. filter)) So if you are using the usual filter material glass which has a refractive index of about 1,52 - the equation is t = (1,52-1/1,52) which is about 0,33 which is equal to 1/3. So your thickness t = 1/3 = t/3. So a 3mm thick filter (only the glass itself of course) would give you a shift in the backfocus of 1mm. So you would need to ADD 1mm in backfocus distance. But as I said. I didn't do that and I achieve perfect focus.
#Astrophotography #Backfocus #FocalReducer_Flattener
Enjoy & subscribe. :)
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