Back-Focus distance

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John_Gill
Posts: 9
Joined: Wed Jan 23, 2019 6:33 am

Back-Focus distance

Post by John_Gill » Mon Jan 28, 2019 6:06 am

Hi,

I am using an APM apo 107/700 scope, a 2" TS flattener and the ASI1600MM-Pro with the 7 position filter wheel and 36mm filters. The ZWO site shows the 55mm back-focus but the APM site shows 111mm for back-focus.

Which is the correct distance for proper back-focus and are the filters par-focal (NB & LRGB)?

Regards
John

artem
Posts: 2
Joined: Mon Dec 31, 2018 11:49 pm

Re: Back-Focus distance

Post by artem » Mon Jan 28, 2019 9:59 am

John_Gill wrote:
Mon Jan 28, 2019 6:06 am
Hi,

I am using an APM apo 107/700 scope, a 2" TS flattener and the ASI1600MM-Pro with the 7 position filter wheel and 36mm filters. The ZWO site shows the 55mm back-focus but the APM site shows 111mm for back-focus.

Which is the correct distance for proper back-focus and are the filters par-focal (NB & LRGB)?

Regards
John
Hello John, the fact that ZWO mentions 55mm is related to a sample case ( if you have a flattener which needs 55mm back-focus )

In your case and since you claim to have an 2" TS flattener, and that it needs 111mm BF, then you have to calculate the BF in accordance to the specification given by TS or in your case APM..

There are many flatteneres and reducers out there with different BF.

PS! I also have the APM apo 107/700+ TSFlat2,5 and the ASI1600MM-Pro with the 7 position filter wheel and 36mm filters

Regards

Martin

John_Gill
Posts: 9
Joined: Wed Jan 23, 2019 6:33 am

Re: Back-Focus distance

Post by John_Gill » Tue Jan 29, 2019 9:25 am

Hi Martin,

I had a look at my TS flattener and it is the 2.5". What is your BF distance with the APM and TS Flattener. I also use the Riccardi 0.75 reducer/flattener (this is the one that fits inside the scope) and I think the BF is 80mm.

If you have other info, it would be appreciated.

Regards
John

Katherine.Tsai
Posts: 9
Joined: Fri Dec 07, 2018 10:24 am

Re: Back-Focus distance

Post by Katherine.Tsai » Tue Feb 12, 2019 7:28 am

I agree with artem.

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