Sorry to come straight to the point but action on my part is required immediately.
I would like to have a telescope which could be used for astrophotography, particularly of the planets, and hopefully of nebulae as well, but my urgent requirement is to have a very powerful lens for a camera to make a terrestrial observation of targets at about 1km, 2 km, 4km, and 8km distances, each of which will have an angular size of about 0.01 degrees. As I explain below I believe that if I want to have the targets form a 100 pixel high image in a photograph this requires a telescope/camera combination with a focal length of about 3.8m.
I believe I can get a camera/telescope combination to achieve this focal length with a Meade 8" SCT as shown below in the pictures and here https://www.ebay.co.uk/itm/8-Meade-SCT-Telescope-Motorised-EQ5-Mount/303633495209?hash=item46b1f774a9:g:cw0AAOSwMmRfHGE2#viTabs_0
The telescope, which has a focal length of 2m is not sufficient by itself, and would be acting as a prime lens i.e. the camera replaces the eyepiece of telescope and would be at the primary focus of the telescope, so I would need a 2x adaptor between the scope and the camera to get the focal length up to 4m. Is this a viable option?
I would like to make a number of terrestrial observations with his telescope in addition to the one described, perhaps nature observations, and I think I read on some website that this sort of telescope would NOT be suitable of nature observation, could anyone tell me why? Is it
1) The default 2000mm focal length is too high? So everything is magnified too much? Can one not put a reducing eyepiece on it to halve the focal length?
2) The f/10 focal ratio is too small?
3) It is too bulky/heavy/ difficult to handle
4) Is there a focusing issue? Can this telescope be focused at the distances I described above and could I focus on objects as close as 100m?
Additionally, the telescope I am considering comes with a motorised equatorial mount, does this make it difficult to use for nature observations where the telescope would be near horizontal and where I would not be using the motor?
Is there a better way to achieve the terrestrial observation with a different method, perhaps just using a horrible Canon P1000 camera which has a focal length of 3000mm?
Thanks in advance for any assistance, and sorry for the urgency in my questions.
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Here is my calculation for determining a required focal length of the telescope for target of 5 feet high at 5.5 miles and I wish the target to form at least 100 pixels in an image taken by a camera.
Specifically there will be a 5 feet high target at 5.5 miles (8800m), so size of target in metres is 5*12*2.54/100 which needs to be divided by distance to the target to get the angular size in radians, and finally multiply by 180/Pi to get degrees, so that gives
angular size = 60*2.54/100 / 8800 * 180 / Pi = 0.01 degrees
The above calculation does not feature in my calculation below but the angular size I am dealing with is useful to consider.
I wish the target to form at least 100 pixels pixel height in an image, and so if using a full size 24mm high sensor in a Canon DSLR camera with the ability to output images of 3700 pixels in height, I believe that gives a magnification (the ratio of image height to target height) of magnification
mag = 100 / SensorPixels * SensorY / TargetHeight ,
where
100 = desired pixel height of target in final image
SensorPixels = 3700, the number of pixels vertically in the sensor
SensorY = 24mm, the size of sensor in mm vertically
TargetHeight = 1.5m
This gives a magnification
mag = 0.0004
or 1/2312
Using the lens maker equation 1/f= 1/u+1/v,
where
f = focal length
u = image distance,
v = object distance,
and using that magnification is the ratio of the u and v distances, and v = 8800m, therefore knowing that the magnification is mag = u/v, so u = v * mag
1/f = 1/(v * mag)+1/v
that gives the focal length f
f = 1/ ( 1/(v * mag)+1/v)
putting in the numbers
= 1/ ( 1/(8800 * 0.0004)+1/8800)
= 3.8m