Afocal Astrophotography
An easy and productive method of imaging the Sun, Moon and planets, can also be used for limited deep sky objects and comets with digital cameras of the fixed lens variety.
Olympus C740UZ
This method provides far better results that eyepiece projection with a digital SLR as there is more control over f/ ratios and magnifications with the added benefit that no dirt gets on your sensor when changing lenses, any dirt on the camera or eyepiece can be whisked off with a lens brush or blower bulb. First off, the camera used must be capable of being used in a manual mode so that exposure, sensitivity and color balance can be controlled as well as a zoom feature to vary the focal length of the lens to enhance detail and control vignetting.
Olympus SP510UZ
Another important feature is an external thread mount normal used for filter or telephoto attachments in order to mount the adapter that will attach it to the eyepiece and thus the telescope. The goal is to get the eye lens of the eyepiece as close to the camera lens as possible, to center the object being imaged along the telescope optical axis and to accommodate different focal length eyepieces. So a system of adapting and attaching the camera to the optical train must be devised.
Olympus CLA-4 adapters
The best system that I have used to this point is manufactured by TeleVue and consists of eyepiece adapters for camera threads of 28, 37 and 49mm thread sizes as well as a standard ‘T’ thread adapter. The system also includes an adapter to fit their Radian line of eyepieces that have large eye lens diameter couple with 20mm eye relief that works well especially with long focal ratios.
The camera system I have come to like for this work is the Olympus C series from the 720 model up through the 760 models and the SP500 to SP510 models. These are available through used outlets and online vendors as well as most camera dealers that carry this brand. These cameras all feature an excellent Zuiko lens that can zoom from 6.3mm to 63mm focal lengths providing up to 10X zoom, and mega pixel counts from 2.1 up to 7.1. An adapter sold by Olympus such as the CLA-1 which has 52mm threads or the CLA-4 which has 55mm threads is attached by the 43mm thread on the camera. Step rings can be used to adapt the 49mm TeleVue adapter which is attached to the eyepiece.
Before proceeding further, we should look at the math involved in afocal projection to see what the possibilities are:
There are three equations used to calculate the necessary information:
1/ magnification = camera lens focal length / eyepiece focal length.
Magnification meaning the telescope and eyepiece magnification plus that of the camera lens)
2/ system focal length = telescope focal length * magnification.
3/ system f/ ratio = system focal length / telescope objective diameter.
So if we want to create a lot of magnification to say image Mars, then we pick an eyepiece that will provide a bright image at sufficient magnification that will allow the camera to further magnify it where we can get a detailed image that requires a short exposure to minimize seeing effects. Using the formulas and my own telescope, a TV102 as an example with an 8mm Radian eyepiece and maximum zoom of 63mm we get:
1/ 63mm / 8mm = 7.875X
2/ 880mm * 7.875 = 6930mm
3/ 6930mm / 102 = 67.94 system focal ratio
This works well for Mars which is relatively bright resulting in an exposure of 1/4 to 1/2 second at iso 200 which yields a clear image with little noise, using a lower f/ ratio and longer exposure for Jupiter and Saturn would yield exposures of ½ second for Jupiter at f/45 to f/50 and 1 second for Saturn at f/40 to f/45. But more on exposures later.
Conversely, if one wishes to image a wide field with a lower magnification such as an open cluster, bright nebula or comet then one can use an eyepiece of longer focal length with a shorter zoom setting on the camera to do this, allowing very short f/ ratios thus lowering exposure times to less than 10 seconds. It can be seen that your telescope mount can track for these short periods with little guiding.
Using the formulas and a TV76 with a 55mm eyepiece, the results will be surprising:
1/ 6.3mm / 55mm = 0.1145X
2/ 480mm * 0.1145 = 54.98mm
3/ 54.98mm / 76 = 0.723 system focal ratio
Is this fast or what? It is advisable to shoot a dark frame or two to subtract from your images to cancel out noise and sky glow.
If you want the equivalent of prime focus then you simply match the eyepiece and camera lens focal lengths to be the same, for example:
1/ 40mm / 40mm = 1X
2/ 480mm * 1 = 480mm
3/ 480mm / 76 = 6.3 system focal ratio the same as the telescope alone.
Calculation image brightness scales can be calculated using the B (brightness values) below with the formula:
Time(exposure) = (system f/ ratio * system f/ ratio) / (iso * B value)
Example 1/ Mars at f/68 and iso 200: (4624) * (12000) = 0.385 or ½ second.
Moon crescent = 20
Moon Quarter = 40
Moon Full = 200
Mercury = 60
Venus = 400
Mars = 60
Jupiter = 30
Saturn = 10
Total lunar eclipse, dark = .005
Total lunar eclipse light = .05
Partial eclipse umbra = .25
Partial eclipse penumbra = .5
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