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Need AnotherTube for AT8 to Reach Focus:Source?

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#1 dawziecat

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Posted 18 January 2013 - 10:36 AM

I have only a single 1" and a single 2" tube for my AT8RC.
Seems I need more extension to reach focus with the ST8300M and FW8.
I see Astronomics are out of stock. I don't want to wait an interminable period of time for a tube. Any other sources for these?
Pic attached showing my FT at max extension. I'm still short.

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#2 Dan Crowson

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Posted 18 January 2013 - 11:01 AM

Terry,

one option would be to buy a 2" extension tube to put between the SBIG and the focuser. Here's an example - Agena 2" Extension . Another option would be to look for the AT2FF flattener which would add some distance. If you ever thought about adding a focuser, the Moonline 2.5" also adds to the back-focus, too.

The problem you're going to have is that the Astro-Tech extenders are required with these scopes and even people with extras (I have an AT6RC and AT8RC) are reluctant to get rid of them.

Dan

#3 jmasin

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Posted 18 January 2013 - 01:58 PM

I'd do what Dan says. Just get a 2" t-thread between the cam/focuser.

#4 dawziecat

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Posted 18 January 2013 - 03:40 PM

Thanks, Dan and Jon. I ordered the Agena 2" extension. I hope it will be enough.

#5 pfile

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Posted 18 January 2013 - 07:54 PM

that's actually kind of weird, i don't think i've ever not been able to reach focus on an AT10RC and an AT6RC. the 2 stock rings have always been enough. and right now i think i have only the thinnest ring installed, but that's with AO-8+STT-FW8+STT-8300M.


i wonder if this means your (or my) primary mirror position is wrong relative to the secondary?

#6 dawziecat

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Posted 18 January 2013 - 10:53 PM

i wonder if this means your (or my) primary mirror position is wrong relative to the secondary?


:bigshock:
I hope not!!!

According to this link the focal plane of the AT8RC is located 255 mm behind the back plane of the rear plate.

The front of my FW8 as pictured is about 65mm short of that.
The camera sensor is further back still.
In reading this thread bear in mind I have not achieved focus but have only tried in daylight and with an object some 500 to 700 feet away (more or less). Perhaps I am shooting myself in the foot here?

#7 jmasin

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Posted 18 January 2013 - 11:02 PM

This is what it takes me to reach focus with my AT10...

The MMOAG takes up alot of backfocus

http://www.jemastrop...Image_Train.jpg

Image train with measurements here: http://www.jemastrop...Image_Train.pdf
Again, that's an AT10

#8 pfile

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Posted 19 January 2013 - 12:58 PM

oh - i'm not sure 500-700 feet is really far enough away. can you see something a few miles away?

like jon, i have the 1" spacer, a moonlite focuser, an SBIG AO-8, FW8-STT and STT-8300. focus is reached maybe 2/3 the way thru the focuser travel.

#9 dawziecat

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Posted 19 January 2013 - 10:09 PM

oh - i'm not sure 500-700 feet is really far enough away. can you see something a few miles away?


In a word, "no!"
I just will have to try on a star at night. (What an idea that is, eh? :) )

Trouble is, I am OOF, and don't really know where the thing is pointing. Until I can get a star in the FOV and a finder aligned, I don't know where the thing is pointing. The star has to be close enough to focus to at least be visible. Surely everybody is faced with this with a new system?

#10 JoseBorrero

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Posted 19 January 2013 - 11:53 PM

I think the mallicam RC version has more backfocus, had you researched that?

#11 kfir Simon

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Posted 22 January 2013 - 04:49 AM

I bought a 2" extender from here:
http://www.teleskop-...SO-50mm-exte...

Cheers

Kfir

#12 dawziecat

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Posted 24 January 2013 - 09:08 AM

Hi Kfir:

Looks like a nice spacer but I only tried once to order anything to NA from Europe and it was not a success. The vendor had no interest it seems. :(
Sort of soured me on the experience.

I focused the AT8RC last night, using the 2" Agena extender.
Focus occurs with the SBIG FW8 face plate 227mm back from the rear of the AT8RC back plate. This was with the Astrodon 3mm Ha.

#13 jmasin

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Posted 24 January 2013 - 09:23 AM

I focused the AT8RC last night, using the 2" Agena extender.
Focus occurs with the SBIG FW8 face plate 227mm back from the rear of the AT8RC back plate. This was with the Astrodon 3mm Ha.


Good!

That sounds about right... glad you got it!






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