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2" EXTENSION AND BACK FOCUS

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15 replies to this topic

#1 rodney j johnson jr

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Posted 13 April 2021 - 01:37 AM

i bought it from orion while waiting for my st80 to come in.

orion has a 38 second video that explains nothing, and their online catalogue is unhelpful

also. both mention back focus, but

WHAT DO IT DO?

 

definitions talk about auto-focus cameras, but that's not relevant, is it?

 

thanks!



#2 Deep13

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Posted 13 April 2021 - 02:30 AM

Did you buy a 2" extension tube? Why?

#3 sg6

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Posted 13 April 2021 - 03:32 AM

If it as I read it is simply a 2" piece of tube that you are adding to either the rear of the focuser or between the focuser and the OTA body.

 

In effect all it is doing is making the "focuser travel" a bit longer. Used when there is insufficent rearward travel with a focuser to get the camera or eyepiee far enought back. You simply a a 2" extra spacing to move the item that much further back. If it is as the post reads a simple 2" extension tube then the term "back focus" is irrelevant and meaningless.

 

Not sure why an ST80 would want any extension, they are not directed at imaging so should be right for visual. So why did you buy it? Are you thinking of trying imaging on an ST80.

 

In a somewhat generalisation "back focus" is only a relevant term when you are placing a flattener or reducer or coma corrector before the eyepiece or camera. It is a combination of 2 or 3 conditions. And is really a requirement of a separation or spacing between the camera/eyepiece and some datum face. The term "back focus" sounds nice and technical, whereas "separation" doesn't. But separation is more accurate.


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#4 michael8554

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Posted 13 April 2021 - 04:47 AM

Hi Rodney

 

What camera are you planning to attach to the ST80 ?

 

Maybe you aren't, I'm not clear what you want the ST80 for.

 

Images of the ST80 show it with a 90 degree or a 45 degree Star Diagonal attached to the focuser.

 

This is an extra piece of glass that Imagers discard  - with Imaging, every piece of glass in the system cuts down the transmitted light a small amount.

 

To compensate for the removed Star Diagonal, you need an Extension the same light-path length as the missing Diagonal.

 

Without it you may not be able to rack the focuser out enough to get focus.

 

For Visual use you don't need the Extension, just use the Diagonal and eyepieces.



#5 spereira

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Posted 13 April 2021 - 07:15 AM

Moving to Equipment.

 

smp



#6 epee

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Posted 13 April 2021 - 07:16 AM

Is the extension 2" in diameter or 2" long?



#7 PatrickVt

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Posted 13 April 2021 - 07:50 AM

A 2" extension is just a hollow tube used to extend the length of the focus tube.  You would replace the diagonal with an extension tube when imaging or for any other reason you might not want to use a diagonal.  

 

Now, the other question is whether the extension tube is for a 1.25" focuser or a 2" focuser (so 2" would refer to diameter in this case).  These tubes come in various lengths for various configurations some of which are 2 inches in length.  

 

Patrick



#8 PatrickVt

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Posted 13 April 2021 - 08:56 AM

I should add one more thing to my comment, above.  

 

Some telescopes (but I don't know anything about the ST80) will require an extension tube even when using a diagonal especially if the diagonal is a 1.25" diagonal on a 2" focuser.  So, it is possible that you will require an extension tube just to come to focus with eyepieces.  As I said, I don't know if this applies specifically to the ST80 or not but I know it does apply to a number of refractors.

 

In case you are wondering why an extension tube would be required even in a typical observing configuration, it is to make the telescope as versatile as possible.  If the optical tube assembly (OTA) is long, then you could end up with situations where the configuration ends up being too long and you would have no way to shorten it.  Then you will have problems coming to focus in these situations.  My Skywatcher ED 72mm refractor comes to mind...  the OTA is about 7mm too long so I cannot come to focus in certain configurations.  So, it is better to have an OTA that is a little short and require an extension tube sometimes rather than cut down on the telescope's versatility.  

 

So, you might need an extension tube just to come to focus when observing.  Hopefully someone with the ST80 will add some info specifically about the ST80.

 

Patrick


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#9 davidmalanick

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Posted 13 April 2021 - 01:51 PM

I use the Orion ST80  and Orion SSAG camera.  The only way it comes into focus is to use the extension tube.

 

Here is a link to the system

https://www.telescop...ts?keyword=st80

 

It comes with the extension.

I have tried other cameras and all need the extension.

 

David


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#10 rodney j johnson jr

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Posted 14 April 2021 - 11:54 AM

Did you buy a 2" extension tube? Why?

part a: i did!

part b: not sure, now...



#11 rodney j johnson jr

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Posted 14 April 2021 - 12:02 PM

Hi Rodney

 

What camera are you planning to attach to the ST80 ?

 

Maybe you aren't, I'm not clear what you want the ST80 for.

 

Images of the ST80 show it with a 90 degree or a 45 degree Star Diagonal attached to the focuser.

 

This is an extra piece of glass that Imagers discard  - with Imaging, every piece of glass in the system cuts down the transmitted light a small amount.

 

To compensate for the removed Star Diagonal, you need an Extension the same light-path length as the missing Diagonal.

 

Without it you may not be able to rack the focuser out enough to get focus.

 

For Visual use you don't need the Extension, just use the Diagonal and eyepieces.

ok, nowwwwww i'm tracking!

in defense, advertising used the word 'back focus' and i guess i don't need it if i'm not

going to take pics or look straight through the tube.

 

will send it back to orion.

 

THANKS, ALL!!!



#12 rodney j johnson jr

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Posted 14 April 2021 - 12:03 PM

I use the Orion ST80  and Orion SSAG camera.  The only way it comes into focus is to use the extension tube.

 

Here is a link to the system

https://www.telescop...ts?keyword=st80

 

It comes with the extension.

I have tried other cameras and all need the extension.

 

David

thank you for the enlightenment!



#13 rodney j johnson jr

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Posted 14 April 2021 - 12:07 PM

I should add one more thing to my comment, above.  

 

Some telescopes (but I don't know anything about the ST80) will require an extension tube even when using a diagonal especially if the diagonal is a 1.25" diagonal on a 2" focuser.  So, it is possible that you will require an extension tube just to come to focus with eyepieces.  As I said, I don't know if this applies specifically to the ST80 or not but I know it does apply to a number of refractors.

 

In case you are wondering why an extension tube would be required even in a typical observing configuration, it is to make the telescope as versatile as possible.  If the optical tube assembly (OTA) is long, then you could end up with situations where the configuration ends up being too long and you would have no way to shorten it.  Then you will have problems coming to focus in these situations.  My Skywatcher ED 72mm refractor comes to mind...  the OTA is about 7mm too long so I cannot come to focus in certain configurations.  So, it is better to have an OTA that is a little short and require an extension tube sometimes rather than cut down on the telescope's versatility.  

 

So, you might need an extension tube just to come to focus when observing.  Hopefully someone with the ST80 will add some info specifically about the ST80.

 

Patrick

i got it sorted out, partly thanks to you!



#14 rodney j johnson jr

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Posted 14 April 2021 - 12:10 PM

Is the extension 2" in diameter or 2" long?

 

Is the extension 2" in diameter or 2" long?

i guess i should have been more precise! 2" long.



#15 rodney j johnson jr

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Posted 14 April 2021 - 12:38 PM

If it as I read it is simply a 2" piece of tube that you are adding to either the rear of the focuser or between the focuser and the OTA body.

 

In effect all it is doing is making the "focuser travel" a bit longer. Used when there is insufficent rearward travel with a focuser to get the camera or eyepiee far enought back. You simply a a 2" extra spacing to move the item that much further back. If it is as the post reads a simple 2" extension tube then the term "back focus" is irrelevant and meaningless.

 

Not sure why an ST80 would want any extension, they are not directed at imaging so should be right for visual. So why did you buy it? Are you thinking of trying imaging on an ST80.

 

In a somewhat generalisation "back focus" is only a relevant term when you are placing a flattener or reducer or coma corrector before the eyepiece or camera. It is a combination of 2 or 3 conditions. And is really a requirement of a separation or spacing between the camera/eyepiece and some datum face. The term "back focus" sounds nice and technical, whereas "separation" doesn't. But separation is more accurate.

i shouldn't have even been looking at the darned thing, SNORT!

my desire to expand my horizons got the better of me!

i have no intention of taking fotos with the scope.



#16 rodney j johnson jr

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Posted 14 April 2021 - 12:48 PM

what we have here is a classic case of getting 'equipment happy'.

many times a salesperson will convince a customer to buy more than he/she needs.

in this case i sold myself via a catalogue page.

back in the old days one saw a lot of this in camera shops.

now you see it in places that sell telephones- accessories and totally un-needed

applications. "oh, and you 'need' the extra 300M terabyte expansion"....

 

let us not forget the up-sell on cars.


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