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Problems coming to focus on my Orion 90ST

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#1 ericb760

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Posted 04 July 2024 - 12:27 AM

I finally had a chance to take this scope out and immediately had problems coming to focus. There just doesn't seem to be enough back focus. I had to compensate by lifting the EP up and securing it with the screws so that it wasn't bottomed out, thereby giving me an extra inch of focal length (?). I had noticed this previously with a cheapo SGW Baytronix 80/400, but I chocked that up to it being a mass produced, cheap refractor. I didn't expect that with the Orion ST90. FYI, I was using TV smooth sided Plossls in 17, 13, 10.5, and 7.4mm, and I was attempting to split the Double Double. Are my EP choices contributing to this issue? I wasn't able to test it with any other EP's.


Edited by ericb760, 04 July 2024 - 12:28 AM.


#2 tony_spina

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Posted 04 July 2024 - 10:45 AM

Are you using a diagonal? 

 

Also the ST90 has an extension tube on the focuser, are you missing that piece? 

 

Posting a picture of your scope in the configuration you are using it in will help folks here  troubleshoot 


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#3 ericb760

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Posted 04 July 2024 - 02:26 PM

Are you using a diagonal? 

 

Also the ST90 has an extension tube on the focuser, are you missing that piece? 

 

Posting a picture of your scope in the configuration you are using it in will help folks here  troubleshoot 

I am using a standard mirrored diagonal. The scope did not come with either a visual back or an extension tube. I had to cannibalize another Orion scope for the VB. Reading up it would seem that this is not an unheard of problem for these short fl scopes. I will buy an extension tube and see if that corrects the issue.



#4 JIMZ7

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Posted 05 July 2024 - 02:25 PM

I also owned the Orion ST80 & 

ST,90. I've read the tubes and objectives were the same. The extension tube changed the dimensions.

 

 

Jim


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#5 tony_spina

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Posted 05 July 2024 - 03:07 PM

I also owned the Orion ST80 & 

ST,90. I've read the tubes and objectives were the same. The extension tube changed the dimensions.

 

 

Jim

I'n not at home so can't check, but the ST80 has a 90mm diameter OTA, the ST90 has a 100mm OTA.  The cell on on ST90 is bigger to accommodate the bigger lens



#6 deSitter

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Posted 05 July 2024 - 06:07 PM

Get a shorty barlow, which also makes a great extension tube when you unscrew the optics ;) Seriously, this is always an option. I have used my shorty more as an extension tube than as Mr. Barlow intended.

 

-drl


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#7 CHASLX200

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Posted 05 July 2024 - 06:11 PM

Get a shorty barlow, which also makes a great extension tube when you unscrew the optics wink.gif Seriously, this is always an option. I have used my shorty more as an extension tube than as Mr. Barlow intended.

 

-drl

I do that also.  Not sure if my 90mm is the same as the OP's.

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#8 ericb760

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Posted 05 July 2024 - 10:09 PM

I do that also.  Not sure if my 90mm is the same as the OP's.

Looks pretty similar in the pics. Do you find a lack of back focus on yours too?


Edited by ericb760, 05 July 2024 - 10:09 PM.


#9 tony_spina

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Posted 05 July 2024 - 11:39 PM

Looks pretty similar in the pics. Do you find a lack of back focus on yours too?

If you look at the picture that Chas posted you will see the extension.  In the picture the focuser is racked all the way in. What you are seeing is the extension piece with the visual back


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#10 CHASLX200

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Posted 06 July 2024 - 05:28 AM

Looks pretty similar in the pics. Do you find a lack of back focus on yours too?

Never had any trouble when i had it.



#11 ericb760

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Posted 07 July 2024 - 05:52 AM

My cheap Neewer extension arrived today. I am afraid that this is just a band-aid until I can locate an actual Orion ST90 extension tube. It works to bring the scope into focus but I fear that I may be losing critical incoming light due to how it is made. It fits wholly inside the visual back whereas an actual extension tube would thread in. This appears (to me) to be tamping down the amount of light coming in. What do you guys think?
 

Extension


#12 Kasmos

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Posted 07 July 2024 - 01:53 PM

Just a guess, but I'll bet it's fine. How far out does it rack for infinity?

 

I could be wrong but a rough check like this will probably tell ya: Set it up and focus it on something far, stand in front of the objective and look at the EP exit hole. Remove the EP and look again. if it's larger your probably fine.


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