Cannot set F-Stop with no lens for Astrophotography

[willseadawg]

Bought a Rebel T6 with no lens for Astrophotography just T-Ring adaptors - camera at F00 and will not change when moving the wheel when Av button pressed. Would like to preset the camera before attaching it to my Orion 8" Newtonian- 1000m FL, F4.9 - read that I should set the camera F-Stop at 4.9 to match the telescope. 

[jrhoffman75]

 Just set the camera to manual exposure and adjust shutter speed and/or ISO as required to get proper exposure. The camera can’t read the lens info since there is no signal connection, thus the “00” indication. 

[willseadawg]

Thanks, I'm gonna assume the camera will receive the signal connection once the camera is mounted to the telescope vis T-Ring  - love your town - my cousin lives in N. Conway  -  Will Minden, NV

[wq9nsc]

You are missing what the previous posters are telling you, the camera body isn't talking to the telescope which is acting as your lens because there is nothing for it to talk to in the telescope.  The aperture assembly is in the lens, not the camera, and telescopes do not have an aperture assembly.  All your T ring adapter is doing is physically connecting your camera to the telescope at the proper physical distance for focusing.

The effective aperture of the telescope is fixed by the size and design of its optical components.  Camera lenses have an aperture assembly with blades that close in steps to allow aperture from the lens (NOT the camera itself, it only controls the lens) to be stepped down from wide open (the lowest f setting provided by that particular lens) to a lower light transmission setting (higher f number).  With the telescope, exposure is controlled only via the length of time the shutter is open and the ISO setting which are of course both internal to the camera.  The telescope also must be manually focused since it has no AF capability.

Below are a couple of photos I took of the moon.  One is with a Canon EF 800 f5.6 lens attached to the body and stepped down to f 7.1 with a shutter speed of 1/160 @ ISO 100, all three legs of the exposure triangle are controllable with a camera and lens.  The second is taken with my Celestron CPC 1100HD telescope which is equivalent to a 2800mm f10 lens which is fixed by the optical tube design.  I used the same exposure time and ISO setting as the EF 800 lens and adjusted for the slight differences between the f7.1 Canon lens and the f10 Celestron telescope in post rather than resetting ISO for RAW file capture since the exposure latitude of the 1DX III sensor easily handles this slight deviation from a standard exposure.  The EXIF data for the image captured with the Celestron shows no aperture data because the telescope has none to report with its fixed design.

Rodger

[AtticusLake]

I think you're misunderstanding what aperture is. You can't set the camera's aperture because the camera doesn't have an aperture. The aperture is in the lens -- it's a combination of the lens' entrance pupil (i.e. the physical size of its optics) and the diaphragm setting. Your camera doesn't have optics, and it doesn't have a diaphragm. Look in the front of the camera with no lens attached, and you'll see, it's just the sensor.

Nothing is going to change when you attach a telescope, for a bunch of reasons:

• There is no communication through a T adapter. Look at it and you'll see that there is not a bunch of electronic contacts round the edge.
• There is no computer in the telescope to communicate with (at least not one controlling the telescope's diaphragm).
• The telescope doesn't have a diaphragm.

Whoever told you "set the camera F-Stop at 4.9 to match the telescope" -- I'm baffled. This would only make sense if the telescope was attached to the end of the camera's lens, and that's not how T adapters work. In any case, as I've explained, the camera doesn't have an F-stop. The telescope has a rated F-stop, like f/4.9, because that's the amount of light it lets in. But there's no diaphragm you could turn to stop that down to f/22 -- at least not on any telescope I've ever seen. Usually, in astrophotography, too much light is not a big problem.

The camera is always going to display "F00" because as far as the camera's concerned, there is no lens attached. Anytime the thing attached to the camera doesn't have those electronic contacts, the camera sees that as no lens. For example this would apply when using an all-mechanical cine lens. BTW make sure your camera's settings are set to release the shutter with no lens attached.

To control exposure, it's as jrhoffman75 said -- use shutter speed and ISO. Both of those things are actually in the camera.

[willseadawg]

Thank You both very much, this clears up a lot of my of my misunderstandings,  Will