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Astro-Tech AT60ED Backfocus Question

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#1 John Verderame

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Posted 21 November 2020 - 11:15 PM

I just received a brand new AT60ED today which I purchased hoping to upgrade the guidescope I was using.  I like both the build and look of it, and the functionality of the rack and pinion focuser, but I'm having a problem that I've never experienced before with any finderscope or guidescope, namely, reaching focus.  I know there have been other discussions about this here, but I couldn't find a sure answer to what I'm experiencing.

 

First thing I did, as I usually do, was put a 26mm Plossl in it.  To reach focus, I had to rack the focuser all the way out and then remove the eyepiece and move it back further outside the focuser till I was able to achieve some semblance of focus.  My first thought was, Uh oh.

 

I then put a 2" nosepiece and T-ring onto my Nikon DSLR and attached that to the focuser in place of the 2"-1.25" adapter.  That just barely reached focus with the focus tube racked out the full distance.

 

This evening I hooked up my ZWO ASI290MM (regular model, not mini) to it and attempted to focus on a terrestrial light miles away.  All I could get at maximum rack distance was a huge, dim round light.  Could not bring it into focus.  And no, in-travel makes it much worse.

 

So finally I took the scope outside, pointed it at the Moon, and did a rough estimate of the distance needed to come to focus behind the focus tube with the tube racked all the way out, and it was approximately 2-1/2 inches.  That's not going to work for me.  I was hoping for a solid guidescope that had just a short travel distance to reach focus, as I'm doing all I can to avoid flexure in my system.  I was using an Astromania 60mm with a rock solid guiding train, but figured the AT60ED would get slightly better guide star fields and might help improve PHD tracking some, as I'm using it with a 9.25" EdgeHD.  (Please no lectures on flexure and using an OAG instead wink.gif .  I tried that and didn't like it, and was getting good images (round stars) with the Astromania.  My goal was longer subs.).

 

So, am I missing something here, or is it just necessary to use spacers to extend the length of the focus tube?  If that's the case, I guess I made a mistake.

 

Thanks for any help or advice.



#2 terrypaula

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Posted 21 November 2020 - 11:50 PM

Yes you'll have to use extension tubes to reach focus. I use an Atik Infinity the back focus on it is 13mm.  I have a TS 503ed.  and it needs an extension for a camera to come into focus thankfully it uses an SCT-T2 adapter.  It seems to take care of any racking.  The AT60ed is a wide field AP scope and isn't really made to use as a guide scope without accommodating the back focus.  But it does look cool.



#3 Redbetter

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Posted 22 November 2020 - 02:56 AM

Visually the AT60ED is well configured for use with 2" and 1.25" diagonals.  The only caveat for visual is that it can require about 5mm of extra extension with standard 1.25" focal position using a 1.25" RA prism--an RA mirror is much further in, and an RACI prism also is also somewhat further in, so neither have demonstrated issues with my 1.25" eyepieces.  Simply sliding out the eyepiece does the trick or a high hat 1.25" adapter or parfocal ring will take care of the necessary extension for an RA prism (star diagonal.)    RA prisms are generally frowned on for f/6, so I haven't used it much this way other than testing.

 

I prefer a 2" mirror for these f/6 scopes anyway giving me full access to 7.3 deg true FOV.  The 2" is racked in well toward the focuser which makes for a much more compact and versatile set up.   This is already a long focuser draw tube for such a diminutive scope.  So I would much rather have the wide visual accessory capability, rather than a scope that runs out of inward travel in 2" visual mode. 

 

I don't do imaging, but I have taken it as a given that imaging requires various adapters.  May I ask what the limitation is with 2 1/2" of extension with the ZWO?  If it is how far the whole apparatus extends rearward on the main OTA, then perhaps the AT60ED (or rings it is mounted in?) could be attached to a longer dovetail.  This would allow the assembly to project further forward. 

 

Good luck!  



#4 John Verderame

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Posted 22 November 2020 - 07:56 AM

Thank you both for the responses.  Just to be clear, I'm not using it for visual at all.  Just usually stick an eyepiece in a finder or guidescope to see where it focuses, and how it looks visually, and with that long drawtube, I figured this was easily going to focus before it was fully racked out.  The limitation (for me) is that if I have to add another 2.25-2.5 inches of length to the drawtube, I can't see how that's NOT going to introduce flexure into the system (and don't want to have to add another support ring to the assembly).  I simply do not want a drawtube that's going to have to come out that far before it will focus with any kind of camera on it.  And I don't want to have to use a diagonal either, as that would be rather awkward.

 

Though this scope would probably easily outdo the Astromania for star field and pinpoint stars, the Astromania did have some nice features, so I guess I'll just stick with it for now, and someone will get a nice deal on the AT60ED, as it's probably not worth bothering to return it.

 

Appreciate the help.

 

John



#5 alphatripleplus

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Posted 22 November 2020 - 08:28 AM

Interesting, but I think you are  correct about the need for spacers. I don't have the 60ED, but I am using a new 72EDII for imaging and have not had any issues with  flexure. I am using spacers to get focus. In one configuration I have the focuser racked out only 19mm, but I then have a couple of adapters, 40mm of extension and a filter wheel in front of a 290MM mini camera. 



#6 John Verderame

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Posted 22 November 2020 - 08:36 AM

Interesting, but I think you are  correct about the need for spacers. I don't have the 60ED, but I am using a new 72EDII for imaging and have not had any issues with  flexure. I am using spacers to get focus. In one configuration I have the focuser racked out only 19mm, but I then have a couple of adapters, 40mm of extension and a filter wheel in front of a 290MM mini camera. 

What OTA are you using?  I'm using a long focal-length 9.25" EdgeHD, so any flexure in the system is going to be a problem for longer exposures.



#7 alphatripleplus

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Posted 22 November 2020 - 08:47 AM

What OTA are you using?  I'm using a long focal-length 9.25" EdgeHD, so any flexure in the system is going to be a problem for longer exposures.

The 72EDII is mounted on a C8. However, I'm not using the 72EDII as a guidescope for the C8, so if there is differential flexure I would not see it. I am doing short exposure imaging directly through the 72EDII, and it is rigid enough certainly for short exposures at its native focal length

 

I think in your situation you might want to try it out as a guidescope, but I agree that at the full focal length of a 9.25 EdgeHD, the rigidity requirement is more severe than in my situation.  



#8 John Verderame

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Posted 22 November 2020 - 08:54 AM

Ok, thanks.  Yes, in your situation I'm sure it's fine, but I know it won't work for my setup.  Thanks for the response.


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#9 ad701xx

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Posted 22 November 2020 - 11:20 AM

Do you have an extra 2" diagonal? That would eat up the back focus and allow you to guide with your 290. It wouldn't hurt to try.


Edited by ad701xx, 22 November 2020 - 11:20 AM.


#10 John Verderame

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Posted 22 November 2020 - 11:25 AM

Thanks, Dave.  Already sold it.  Really did not want to use a diagonal, tho' that probably would work.



#11 alphatripleplus

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Posted 22 November 2020 - 11:35 AM

Yeah, the marketing blurb for the AT60ED is as a wide-field imager, or grab and go visual, but not specifically a guidescope. I never thought about the issue until seeing this topic.



#12 John Verderame

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Posted 22 November 2020 - 11:56 AM

Weeeeel, this is what the blurb says on the Astronomics website:  "The ring/clamshell is hinged so you can remove it with ease and adapt it to your current rig as a guidescope or photographic instrument with the proper photo tripod and camera adapter."

 

So, by "camera adapter" I just figured nosepiece or T-ring or whatever, not spacers.  In any event, I would not recommend it as a guidescope unless you don't have flexure concerns, like on a fast, wide-field instrument, or you have some really secure way of making the extended tube rigid like the Losmandy 3-ring autoguider setup.


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#13 alphatripleplus

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Posted 22 November 2020 - 12:08 PM

Weeeeel, this is what the blurb says on the Astronomics website:  "The ring/clamshell is hinged so you can remove it with ease and adapt it to your current rig as a guidescope or photographic instrument with the proper photo tripod and camera adapter."

 

 

You are right! I read only the first paragraph, and did not see the mention of guidescope in the second paragraph. As they specifically reference guidescope usage with a camera adapter, I would follow up with them as to what the appropriate camera adapter is for the scope to maintain rigidity.



#14 John Verderame

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Posted 22 November 2020 - 12:12 PM

Already have a buyer for it.  I'm just gonna have to come up with Plan B.  (They might have been referring to a tripod camera adapter, not a camera camera adapter, if you know what I mean.  It's a little ambiguous.)



#15 alphatripleplus

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Posted 22 November 2020 - 08:02 PM

Wow that was fast. Those scopes are really popular.



#16 John Verderame

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Posted 22 November 2020 - 09:51 PM

They are.  I actually wanted one for a long time, and missed one or two here on CN, so just decided to go for the new one when they were back in stock.  It really is a nice little scope, and I'm sorry it did not work out for my needs.  Sometimes you just have to try things to find out firsthand.  Win some, lose some.




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