Focus and Exposure setting for Afocal Astro-photography.

The following are the procedures I followed for doing photography through my telescope with good success. These procedures are for afocal projection where both the camera lens and the telescope eyepiece are kept in place. Before I made a mounting for the camera on the telescope, I used a tripod to hold the camera looking into the eyepiece, as close as possible but without actually touching. This is a little fiddly but it works.

With the advent of digital cameras, mostly having fixed lenses, the focus procedure described below applies again.


The best way to achive good focus is to adjust the telescope eyepiece to produce parallel rays which then means that the camera lens is simply set to infinity. But how do one know when the rays coming from the telescope are parallel?

This can be done in two ways:

  1. Although this method also works for TTL (through the lens) cameras, it is best suited for cameras without TTL viewfinders like simple aim-and-shoot cameras and digital cameras. First focus a pair of binoculars to infinity on a distant object (like the Moon). Then aim the telescope at a bright object (like the Moon) and, using the binoculars, look through the telescope eyepiece and adjust the telescope eyepiece for best focus. This is a little difficult until you get to know what to look for since the magnification is quite large. It is be best to first train one's eye during the day when there is enough light avaliable.

  2. This method is for TTL cameras only. Although you would think good enough focus is possible by just using the camera's reflex system, it is not that easy and this method is more accurate. You first have to make your own "quick focus" as can be seen advertised in S&T. You simply cut a cardboard mask the size of the telescope aperture. Into this mask you cut two holes at equal distance from the centre point and one on either side of it. The accuraccy of these cuts does not need to be too very high (it seems). With this mask installed and looking through the camera viewfinder through the telescope, you will see two overlapping images while it is still out of focus. Adjust the telescope focus until these disks merge into one and then remove the mask.


The following formulae serve as a good guideline for calculating exposure but it is still recommended to "bracket" either side of the calculated values, ie. take one longer and one shorter exposure as well. Although the values for the Moon have been tested, the numbers for the planets were taken from a different system and tranformed to this system (see sources below), but some have not been proven yet. If the reader finds better/other/more constant K values, please let me know.

Exposure   =   ------------------------    sec.
                       K   x   ISO film speed

Where: K is   20 for a crescent Moon
                     40 for first/last quarter Moon
                   200 for the Full Moon
                  13.6 for Saturn
                  32.5 for Jupiter/Mars
                 1310 for Venus

                            System focal length
and   (f-ratio)   =   --------------------
                            Telescope aperture

System focal length    =    Camera lens focal length    x    Telescope magnification

                                            Telescope focal length
Telescope magnification   =   -----------------------
                                             Eyepiece focal length

This might look complicated, but most of the values become constants for a particular telescope and need only to be calculated once.

As a worked example, take the following which gave me the correct exposure before: My telescope has an aperture of 220mm and focal length of 1420mm. Doing afocal projection on the first quarter Moon through a 19mm eyepiece with a standard 50mm lens and 100 ASA film. First Quarter Moon

Telescope magnification    =    -------    =    75 times

System focal length    =    50   x   75   =   3750mm

f-ratio   =   -----   =   17

                        17   x   17
Exposure   =   -------------   =   1/14 sec.
                        40   x   100

Setting the camera to 1/15 sec. and the lens to full aperture gave me the correct exposure when compared with exposures bracketed on either side of it as can be seen here from the picture.

To decide what focal length eyepiece to use, simply set up the telescope and camera and have a look to see what field of view each one gives. In general the lower power (longer focal length) ones work better.

Download a handy little computer program by Eric Smestad, based on these formulae.

Source: The exposure calculation formula first appeared in Astronomy Now, October 1992, p.18 in a slightly different form, with only the full Moon constant given. The values for the other lunar phases and planets were transformed to this system from graphs published in the 1959 book; Astronomical Photography at the Telescope by Thomas Rackham.

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Willie Koorts

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