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Extension before telextender or vice versa?

astrophotography dslr moon optics refractor
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#1 jeffry7

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Posted 12 August 2019 - 09:33 AM

Hi folks,

 

I am having a weird, to me, experience and hope y'all can help.

 

I am trying to take pictures of the moon with a Celestron d70 Traveler. Since it is a refractor, some extension is required. I also have a t-mount telextender in the mix. Does it matter where the telextender sits. Does it need to be close to the camera, or should it not matter? I am having trouble getting good focus with the arrangement Telescope->telextender->extension->camera. But I had good focus with Telescope->extension->telextender->camera.

 

EDIT

The teleextender is a Rokinon 2x Multiplier for T Mount. You can see more here:

https://www.adorama.com/RK2X.html

CLOSE EDIT

 

But maybe this doesn't matter and I am just being too impatient with the humidity where I am. (I wait for the condensation on the telescope front lens to evaporate, but maybe there is condensation inside, or possibly on the telextender?

 

That is the short version. The long version follows.

 

A few months ago I did a comparison between my cheap refractor telescope and my expensive mirror lens. The refractor plus 2x telextender has a focal length of 800mm. The mirror lens is 800mm by itself. The cheap telescope was clearly sharper. I did two separate tests and it came out this way both times. The telescope arrangement then was to use an extension meant to work with eye piece holders. This is held in with typical three screws. On the end that doesn't fit into the eyepiece are threads meant for a t-mount. So the arrangement is Telescope -> eyepiece extension -> telextender -> camera.

 

More recently I was trying this again, but this time on a goto mount rather then an ordinary tripod. To help with alignment, I had a red dot finder in the hot shoe of the camera. This did not work. I could not align the red dot with the center of the field of view. I decided that the reason for this was that the eyepiece extension was not giving a rigid connection to the telescope and the camera end was drooping. (The red dot was always to high to align on the centered feature.) So I got some t-mount extensions.

 

Now try again with the t-mount extension. The eyepiece hardware comes off and the extension tube screws on. So now I have Telescope -> telextender -> extension -> camera. I did not have a particular reason to switch the order. I just did it. For three nights I try this arrangement and am disappointed by the results. The focus always looks off. On the third night I take special pains to get the focus, but it is only a slight improvement on the previous nights, and nowhere near as good as the images when I first tried the telescope.

 

I would simply go try changing the order but for several problems. The weather turned. Last night was overcast and tonight looks to be the same. One of the extensions "welded" itself to the camera t-mount adapter and I can no longer get it off. (Well I probably could with vice grips, but I figure I would damage it in the process.) I partially disassembled the t-mount adapter by unscrewing the set screws. This didn't help, and now the screws won't hold anymore. (This is something like "If it don't fit, beat it. If it breaks, you needed a new one anyway.)

 

So while I wait for a new set of extensions and a new t-mount adapter for the camera, I thought I would ask the folks at cloudynights.

 

(If you read this whole rambling bit of nonsense, bless you.)


Edited by jeffry7, 12 August 2019 - 11:43 AM.


#2 james7ca

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Posted 12 August 2019 - 10:21 AM

You don't say what kind of camera, but I assume that it is a DSLR. I'm also a little confused by your term "t-mount telextender." Do you mean a barlow or a telextender designed to be used with a camera lens?

 

So, if using a DSLR and if you are using a telexender designed for a camera lens then I'd put the "telextender" right before the camera. Meaning: telescope -> extension -> telextender -> camera

 

However, if it is a barlow then the placement will determine your effective magnification, which will change with the spacing between the barlow and the camera. But, even in this case I'd probably attach the barlow directly to the camera (if a DSLR).

 

As for removing a "welded" extension tube you may want to review the following thread:

 

  https://www.cloudyni...s/#entry9486436



#3 jeffry7

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Posted 12 August 2019 - 11:44 AM

You don't say what kind of camera, but I assume that it is a DSLR. I'm also a little confused by your term "t-mount telextender." Do you mean a barlow or a telextender designed to be used with a camera lens?

 

So, if using a DSLR and if you are using a telexender designed for a camera lens then I'd put the "telextender" right before the camera. Meaning: telescope -> extension -> telextender -> camera

 

However, if it is a barlow then the placement will determine your effective magnification, which will change with the spacing between the barlow and the camera. But, even in this case I'd probably attach the barlow directly to the camera (if a DSLR).

 

As for removing a "welded" extension tube you may want to review the following thread:

 

  https://www.cloudyni...s/#entry9486436

 

The camera is a Sigma sd Quatro h. This is a mirrorless camera, but not a specialty astronomy camera.

 

The telextender is this:

https://www.adorama.com/RK2X.html

 

Rokinon 2x Multiplier for T Mount.

 

Whether this is optically like a barlow or not, I could not tell you.



#4 james7ca

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Posted 12 August 2019 - 08:48 PM

T-mount lenses have a 55mm backfocus specification, so you want 55mm of spacing between the telextender and the sensor in the camera. If you have a T-mount adapter for your camera that should give you the required spacing.
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#5 jeffry7

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Posted 12 August 2019 - 11:18 PM

Of course. I had read about back focus requirements and why some lens mounts cannot be adapted to some cameras, but had not made the connection that the telextender would have the same requirement.

Thank you!


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