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How to start shooting the planets

Posted: 01 May 2017, 13:27
by Dariusz
Many people start their adventure with astrophotography from the images of the planets and the moon. After few unsuccessful attempts, lots of folks give up in a favour of visual observations. In my opinion, it is a pity because one doesn’t preclude the other and the different forms of astronomy are only enriching our hobby
This post is intended to make it easier for beginners to start in planetary photography by an indication of the most important factors and techniques affecting the final effect.
I will tell you about my own experience in this area, however, it should not be treated as such the current approach but as the direction to go.
Everyone should develop their own methods most appropriate for the equipment used, available conditions and personal preferences.

Equipment for planetary imaging.

In fact, each telescope and each detector are suitable for planetary and lunar photography.
However, if we want to photograph the details of Jupiter or Saturn we will need a very long focal length and a camera capable of capturing a large number of frames in
the shortest time possible and the best is a camera with a resolution of 640x480 pixels or slightly larger. If our goal is Moon and especially its large areas,
more resolution will be more suitable. Cameras with 1280x980 pixel allows you to register multiple craters simultaneously and they allow you to make excellent mosaics of the entire shield of our satellite. You can also take pictures of mirror moons but I did not deal with it so I do not have much about it to say.

Depending on what we want to capture in the picture we will need the right focal length depends on the size of the detector pixel and area of detector itself.
Photos of the entire moon compare to Mars will require completely different focal lengths and detectors.Using an SLR camera with focal length from 500 mm to 1500 mm.
we can successfully photograph the moon disc at any stage but for shooting Jupiter it is better to use a simple webcam and a telescope with a focal length of 2500 mm or more.

The focal length of the telescope and the camera must be chosen in the right way.
They are basic criteria to follow when processing signal analog to digital. They say that in order to do not lose anything from the analog signal the sampling frequency should be more than double of the highest frequency in the original signal. Probably most of us know this technique where audio sampling rate for music on CD is 44.1 kHz at maximum 20 kHz sound frequency.

For digital processing of the image from the telescope this maximum limit of image resolution is due to the limitation of the optics called diffraction and is approximately equal to the resolution of our telescope, which is 138 / D where D is the telescope diameter in mm.
For telescope D=138 mm resolution will be 1 " (arc second)
In this telescope we should select a camera that will record these tiny details on at least 2 pixels. In practice, slightly larger scale scales are used like 3-4 pixels. Greater scaling does not make sense as nothing new will be bring to final image. ínstead it will only reduce the brightness of the image what we will have to compensate with longer exposure times or increases gain which in turn will result in increased noise and deterioration.
On the other hand reducing the focal length will result with loss of some details that we could register.

Knowing the resolution of the telescope and the pixel size of the detector, we can calculate the optimum focal length meeting the sampling criterion

F = 206 x A x P / R

Where F is the focal length of the telescope
P is the pixel size of the detector in micrometers
R is the resolution of the telescope in arc seconds (138/D)
A is a factor that should be larger than 2 or better 3/4

For a D=150 mm telescope and a P= 3,7 micrometer pixel size camera like the QHY5L-II Colour we can calculate the optimum focal length ( Fl ).
R= 138/150mm = 0.92’’
‘’A’’ 2 Fl =206x2x3,7/0,92 = 1656mm
‘’A’’ 3 Fl =206x3x3,7/0,92 = 2485mm
‘’A’’ 4 Fl=206x4x3,7/0,92 = 3313mm

Focal length o 1650mm is the minimum focal length that allows you to register all the details available through your lens. In practice we usually use A = 3 and then we have F = 2500 mm. However focal length of 3313mm would be best for ideal seeing conditions.

With a bigger pixel size the optimum focal length will be larger and for P = 5.7 microns and A=3 it will reach 3828 mm.
Let's note the changes with the optimum focal ratio (F) , it is the focal length (Fl) / lens diameter (D)

3,7 micrometer pixel size camera
‘’A’’ 3 Fl =206x3x3,7/0,92 = 2485mm The focal ratio is F= 2485mm/150mm = 16,5

5,7 micrometer pixel size camera
3 Fl =206x3x5,7/0,92 = 3828mm The focal ratio is F= 3828mm/150mm = 25,5

In my case;
Fl/ focal length of the scope is 1000mm
Lens/mirror diameter is 200mm
Focal ratio is 1000/200 F=5
Telescope resolution 138/200 R=0.69
Pixel size is 3,75 um
So using the rule the optimum focal length I should achieve would be
Fl = 206 x A x P / R
206x3x3,75/0,69 Fl=3358mm where the focal ratio F= Fl/D 3358/200 is F=16.7
However at this moment I use TV Power mate x4 that increase telescope focal length by x4 what is a focal length of 4000mm the A factor close to 3,5 and focal ratio of 20.
The planet size on the detector is around 200 pixels in diameter and it also could be a reference point when choosing the equipment.
Probably increased it more would not give any more details but instead the image will get dimmer. Please remember that effective focal ratio depends of seeing and should be adjusted as seeing conditions change, the better seeing the more effective is greater focal ratio.

And now in a just few words
Using most popular available on the market cameras our telescope focal ratio should be between F=15 to 25
The 25 is challenging but it is the way to get maximum of your set up when F=15 will offer wilder field of view and it is better if poor seeing.
There is many other factor that will affect the final image but collecting the light is always most important and therefore is most demanding,
I need to mention that using monochromatic cameras vs color offers better resolution but I haven’t tried it personally.
Another part is what we do with the collected data and the processing to final image.
The amount of catching and processing software is huge and often offers great support.
Hope it helps a little instead of complicate the things more.
Do not hesitate if you have any questions
Clear Skies