Friday, April 20, 2018

Kowa TSN-PA8 for DSLR digiscoping

I have previously written about digiscoping using a 50mm lens on the DSLR camera body. However, an alternative is to mount the camera body directly on the telescope, using the telescope's optics as the lens system. My scope is a Kowa TSN-823M, which is a fairly old scope that is no longer in production. Kowa have offered an adapter of this type (TSN-PA7) for their current 'scopes, but their adapter for the legacy range of 'scopes - including mine, is a bit more recent. This is the TSN-PA8.
TSN-PA8 mounted on the 'scope eyepiece with Canon 60D fitted.
The adapter is in two parts: a collar that screws on to the accessory mounting thread around the eyepiece aperture of the telescope and a tube which fits over the eyepiece, slips onto the collar and is held in place by a pair of screws. The camera is mounted using a T2 mount which screws on to the outer end of this tube.
from left to right, telescope with the eyepiece removed, collar, tube and the Canon T2 mount at the bottom
The outer end of the tube is closed by a piece of coated, optical glass which covers the eyepiece and prevents any muck, dust or moisture which might be on it from getting into the mirror box of the camera and hence on to the sensor. This was added to the PA7 adapter about 2015 (and it became the PA7A) but, as far as I know, has always been a feature of the PA8.
The assembled mount in position over the eyepiece. Note the optical glass covering the end of the eyepiece to prevent anything getting into the mirror box of the camera.
The cut-outs on the side of the tube allow access to the eyepiece's zoom ring so that you can still operate the zoom whilst the camera and adapter are in place.
Ports in the side of the tube allow access to the eyepiece zoom ring.
Overall, it is a well made and well designed, if rather expensive, piece of kit and does the job of mounting a DSLR on the 'scope very well. It is fairly quick and easy to fit. Swap the camera lens for the adapter, slip it over the eyepiece and do up the two securing screws and you are ready to go. The camera can be oriented either horizontally, in landscape mode or vertically, in portrait mode, just by loosening the screws and twisting and, unlike my previous DSLR digiscoping setups, the zoom function is accessible whilst the camera is mounted.

In use, the usual problems of digiscoping remain. Firstly, the field of view is very small, so finding your target is often quite difficult and it can be rather tricky to keep a moving subject in view. Secondly, the magnification is considerable and the whole setup rather cumbersome, so camera shake is a big problem. It needs a pretty fast shutter speed and you really want to trigger the shutter without touching the shutter button. I usually use either the 2 second delay timer or the Canon RS-60E3 remote shutter release. I find one difference with my previous digiscoping attempts is that I don't get any vignetting, even at the lowest zoom magnification setting.

According to info on the Kowa web site, at minimum magnification this set up is roughly equivalent to a 1200mm, F12 lens on a 35mm camera and a maximum zoom a 2900mm lens at F34. On an APS-C sensor camera like mine, you need to multiply these focal lengths by 1.6x to appreciate the sort of reach you get.

Exposure Settings

At these very small apertures, even on a bright day, a pretty high ISO is needed to get a decent shutter speed. I usually try to stick to 1/1000s to counter subject movement and camera shake, so the ISO rarely goes below 1600. I put the camera in manual mode (M), set the shutter speed to 1/1000s and select Auto ISO. I generally find I have to dial in between minus one and minus one and two third stops compensation to get a correct exposure, otherwise the metering system seems to over-expose everything. I am not sure why this should be so.

Focusing is, of course, manual and done with the telescopes focus knob. If the subject is fairly static, live view is very useful for adjusting the focus accurately. I use as much magnification as I can get on live view to critically set the focus. Otherwise, make sure that the dioptre setting on the viewfinder is properly adjusted for your eyes before you start and focus as best you can visually, through the viewfinder. You don't get a lot of depth of field, so I generally take a series of shots, adjusting the focus each time - unless I have been able to set it up accurately on a static subject using live view.


These examples are derived from straight RAW shots taken using my Canon 80D on the setup described, at minimum zoom unless otherwise stated.
Gadwall female, 1/1000s, ISO 2000, -1⅓ exposure compensation

Great Spotted Woodpecker on feeder, 1/1000s, ISO 6400, -1⅓ exposure compensation
These two shots of a coot were taken seconds apart, the first at minimum zoom and the second at maximum.
Coot at minimum zoom, , 1/1000s, ISO 1600, -1⅓ exposure compensation
same Coot at maximum zoom, , 1/1000s, ISO 5000, -1⅓ exposure compensation
I find that shots taken at higher zoom levels are always rather soft. I think this is because diffraction becomes a very significant issue at such tiny apertures.

This shot of a  Heron catching a fish illustrates the problems of capturing action with such a cumbersome setup and without the benefits of auto-focus. Here, I haven't got the focus quite right - I think the plane of maximum sharpness is just in front of the bird and, despite the fast shutter speed,  camera movement is obvious (I think I jabbed at the shutter button with my finger to try and capture the moment!).
Heron catching a fish, 1/1000s, ISO 2000, -1 stops exposure compensation