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Focal length

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Here are some real 'getting started' questions which I'm confused about.......... and I hope I'm in the right forum!


What is the significance of the focal length of a telescope without an eyepiece? 


Is a telescope with a focal length of, say. 650mm equivalent to a camera telephoto lens of 650mm?


What is the relationship between the focal length of the telescope and the size of the image captured by a prime focus eyepiece camera and is that a fixed relationship?



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I realise that your question relates to photography whereas I only observe, therefore I can not comment on the photography specific bits of your question, but my understanding of telescope focal length is as follows:


the magnification is the focal length of the telescope decided by the focal length of the eyepiece (EP). Therefore a scope with a longer focal length will give a greater mag compared to that with a shorter focal length. However the amount of light gathered is dictated by the lens diameter:

A telescope with a long focal length but small diameter, although giving high mag, would be like looking through a long dark straw.


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I('m going to say the principle is the same.  Longer focal length - smaller field of view, greater magnification.

However just like a lens and DSLR combination the actual view and image scale is dependent on the eyepiece or camera sensor.  I'm not super good at explaining it - but here's a link that will allow you to model different options.


In general if you put a camera on the end of 1000mm F/L scope, it will act like 1000mm lens. 

One important thing to remember - magnification is the least important power of a telescope.  Light gathering and resolution are more important.

For a lot of space stuff, magnification works against you. Many objects are large and faint, so wider a scope with a faster focal ratio can be a better option.

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Thank you for the interesting and helpful replies and the very useful links too.


I have my eye on buying the ZWO ASI224MC camera rather than using my Sony A700 DSLR and when I put the ZWO   into the FOV calculator with Mars selected  as the target and my SkyWatcher 130P it gives me a FOV of 0.43 x 0.32 degrees and a resolution of 1.19" x 1.19"/pixel  very different from my Sony A700 which gives a FOV of 3.14 x 2.1 degrees and a resolution of 2.67" x 2.67"/pixel.  So do you think the ZWO camera would be a good buy for planetary work?



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The FOV is not only related to the focal length of the telescope, but also the size of the sensor in the camera.


Your Sony A700 has an APS-C size sensor (aka Crop Sensor) which will produce a smaller FOV than a Full Frame sensor.


The sensor in the ZWO is much smaller than the sensor in your A700, and as such will give a smaller FOV.



Edited by Dave_S
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When imaging planets smaller pixels and high frame rate video capability are very desirable. The 224 is designed for that. The image of Mars is going to be small with either camera. Shooting planets is very different to DSOs. The A700 would be good for galaxies and clusters. 


I think the 130p is f5 (650mm) so you are going to need a good Barlow to get a reasonably sized planet and Mars is one of the small ones. 


Challenging but fun😊

Edited by paul
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