Barlow use for imaging the moon and planets

[Robp]

Hi all,

I currently image the moon and Jupiter with a 2.5x Revelation Barlow, and that's fine for normal seeing.

I want to add a stronger Barlow for when the seeing is very good, specifically for imaging closeup on the moon and increase the size of the planets.

I have already tried adding the top of another 2x Barlow into the current 2.5x Barlow to increase the magnification a bit and this was fine but want to push it a little more if possible.

Is there any formula or guidance that will tell me whether to go for a x3, x4 or x5 or would it be a case of trial and error?

If it helps, I'm using a Explorer 200p and the camera I will be using with it is a QHY5L-ii (Colour).

Many thanks,

Rob

[Cornelius Varley]

Generally, for Lunar / Planetary imaging the ideal f ratio is about f25. This normally works with a 5.6micron sensor. The QHY5L-II uses a smaller pixel (3.75 micron) so the optimum f number would be a little lower (@f20).

The formula goes something like this :

(206.265 x p) / ((116/a) x 0.3) = F

206.265 is a constant, 

p = pixel size (microns)

a = telescope aperture (mm)

F = ideal focal length (divide this by the telescope's aperture to get the required barlow size)

The 0.3 can be replaced with 0.5 to give a  shorter focal length and faster f number.

[Robp]

Many thanks for the detailed response.

Running my numbers through the formula you provided I get F=4445.37    (using a=200mm, p=3.75)

When I divide this by the aperture I get 22.23 so I'm guessing the top end Barlow would be a 4x?

I used to love mathematics....

[SimonfromSussex]

This FOV calculator will also give you an idea of image size with the desired barlow:

http://www.12dstring.me.uk/fov.php?aperture=90&flength=621&telescope=William

[lukebl]

I've never understood why Focal Ratio is considered important. Surely it is Focal Length which matters and focal ratio is irrelevant?

That's solely what determines the size of the image. A 400mm f5 telescope and, say, a 5x Barlow will produce a 2000mm FL and FR of 25. A 1200mm f5 telescope would produce a 6000mm FL and also a FR of 25. Clearly they have the same focal ratio, but the latter will obviously be more suited for imaging, having a longer focal length and bigger aperture = much bigger image and better resolution.

[riklaunim]

Optimal f/ratio of a given camera will be valid for every telescope. Focal length only for given aperture.

[sharkmelley]

I've never understood why Focal Ratio is considered important. Surely it is Focal Length which matters and focal ratio is irrelevant?

The aperture of the scope determines the maximum resolution (i.e. detail) that can be obtained from it (Dawe's limit, Airy Disk etc).  As you continue to increase magnification you reach the point where the sensor (or indeed the human observer) cannot extract any more detail - this is where you are oversampling.  For sensor pixels of a certain size this point of oversampling is reached at a certain fixed F-ratio.  This is why F-ratio is frequently quoted in this context.

Mark

[Dyptorden]

Many thanks for the detailed response.

Running my numbers through the formula you provided I get F=4445.37    (using a=200mm, p=3.75)

When I divide this by the aperture I get 22.23 so I'm guessing the top end Barlow would be a 4x?

I used to love mathematics....

Hi Robp,

How did you get from the value of 22.23 to the 4x Barlow... ? Am I missing something?

Thanks,