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Does anyone know the focal ratio of the 1100 EdgeHD vs. backspacing?

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#1 Borodog

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Posted 05 October 2022 - 08:17 AM

At the nominal backspacing of 146mm the focal ratio of the 1100 EdgeHD is f/10. Does anyone know of a source for what the focal ratio is as the backspacing is varied? I'm trying to figure out how much additional spacing is required to get to f/12 - f/17. No, I'm not worried about the degradation in the correction. I can measure it by swapping spacers in and out but that is a PITA and I'm hoping someone just knows or has a reference handy.

 

Thanks.

 

 



#2 Borodog

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Posted 05 October 2022 - 05:10 PM

I guess the answer is "no." ;O)



#3 jdupton

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Posted 05 October 2022 - 05:36 PM

Borodog,

 

   I have never seen any test results for that. Maybe you can gain some insight from the posts by John Hayes who did lots of tests on the Edge 11" and 14" scopes.

 

https://www.cloudynights.com/index.php?app=core&module=search&do=search&andor_type=&sid=954fbc0d94aeb4a8ec9669c438c04bd0&search_author=jhayes_tucson&search_app_filters[forums][sortKey]=date&search_content=both&search_app_filters[forums][sortKey]=date&search_app_filters[forums][liveOrArchive]=live&search_app_filters[forums][noPreview]=0&search_app_filters[forums][pCount]=&search_app_filters[forums][pViews]=&search_app_filters[forums][sortDir]=0&search_app_filters[forums][searchInKey]=&search_term=back+focus&search_app=forums

 

 

John


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#4 hfsarber

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Posted 05 October 2022 - 06:19 PM

There probably is a formula for calculating that.

 

When I had a diagonal and ASI183MC in my C8 it platesolved to 2190mm (f/10.8).  With the camera attached without the diagonal (and no additional spacers for proper backfocus) it platesolved to 1950mm (f/9.6).  If I knew the distance between the two I could probably estimate where f/17 would be located (as long as it's a linear function, beats me if it is)

 

Two questions:

 

1)  Are you sure you can reach focus at f/17?  I think you might run out of room before you get there.

 

2)  You said you weren't worried about the edges so I assume you're not worried about possible vignetting?

 

Harry


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#5 Peter in Reno

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Posted 05 October 2022 - 06:29 PM

Celestron secondary mirror for C8 and C11 act like approximately 5X multiplier. So for every millimeter spacing added can add 5mm back focus assuming the calculation is linear.

 

C925 has longish OTA so I assume the secondary multiplier is less than 5X. Not sure about C14 because native focal ratio is f/11.

 

Peter


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#6 Borodog

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Posted 06 October 2022 - 07:16 AM

Borodog,

I have never seen any test results for that. Maybe you can gain some insight from the posts by John Hayes who did lots of tests on the Edge 11" and 14" scopes.

https://www.cloudynights.com/index.php?app=core&module=search&do=search&andor_type=&sid=954fbc0d94aeb4a8ec9669c438c04bd0&search_author=jhayes_tucson&search_app_filters[forums][sortKey]=date&search_content=both&search_app_filters[forums][sortKey]=date&search_app_filters[forums][liveOrArchive]=live&search_app_filters[forums][noPreview]=0&search_app_filters[forums][pCount]=&search_app_filters[forums][pViews]=&search_app_filters[forums][sortDir]=0&search_app_filters[forums][searchInKey]=&search_term=back+focus&search_app=forums


John


Thank you, John. Will check that out today.

#7 Borodog

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Posted 06 October 2022 - 07:18 AM

There probably is a formula for calculating that.

When I had a diagonal and ASI183MC in my C8 it platesolved to 2190mm (f/10.8). With the camera attached without the diagonal (and no additional spacers for proper backfocus) it platesolved to 1950mm (f/9.6). If I knew the distance between the two I could probably estimate where f/17 would be located (as long as it's a linear function, beats me if it is)

Two questions:

1) Are you sure you can reach focus at f/17? I think you might run out of room before you get there.

2) You said you weren't worried about the edges so I assume you're not worried about possible vignetting?

Harry


1. No, not sure in the slightest.

2. I think vignetting would actually improve at higher focal ratio, but no, I’m not concerned about it. Very small targets.

#8 Borodog

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Posted 06 October 2022 - 07:29 AM

Celestron secondary mirror for C8 and C11 act like approximately 5X multiplier. So for every millimeter spacing added can add 5mm back focus assuming the calculation is linear.

C925 has longish OTA so I assume the secondary multiplier is less than 5X. Not sure about C14 because native focal ratio is f/11.

Peter


Ooh, that checks out, actually, although I think you said it backwards. My backspacing is currently a couple mm too long and my focal length measures out at 2810mm instead of 2800mm. But there is certainly some uncertainty in my measurement. So if I really wanted to get to even f/12, that’s an additional 560 mm of focal length, requiring an additional 112 mm of back spacing, or 4.41”. I could probably do that, but anything higher is probably prohibitive.

The right solution is a Barlow. The problem is that the optical train is currently all M42 back from the Starizona T adapter with filter holder. I’d have to replace the entire optical train to put a Barlow in. After I just spent $65 on the T adapter. :Op

I suppose I could go back to my old optical train, but it was a kludge complete with compression and parfocal rings. I like the all-threaded connections of the new setup.

#9 const

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Posted 07 October 2022 - 03:32 PM

I played with spacers on my 8" Edge recently. With a .7x reducer at recommended 105mm distance I got 1490mm focal distance vs standard 1422mm, whatever that means. Adding 50mm of spacers brought the FD down to 1430mm. The important point here is that adding spacers lowers the f-number, not raises it. Now, it's a different OTA with extra reducer, so who knows.

#10 Borodog

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Posted 07 October 2022 - 05:21 PM

I played with spacers on my 8" Edge recently. With a .7x reducer at recommended 105mm distance I got 1490mm focal distance vs standard 1422mm, whatever that means. Adding 50mm of spacers brought the FD down to 1430mm. The important point here is that adding spacers lowers the f-number, not raises it. Now, it's a different OTA with extra reducer, so who knows.


That’s because you have a reducer in the optical train. In the case of reducers and Barlows, when you add spacing between the lens element and the sensor you get more of whatever the element is doing, reducing or magnifying. If there is no reducer, adding spacing always increases the focal length (in an SCT).
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