102 MAK vs 80mm refractor for imaging?

[mikehab]

Hi,

I've had a Celestron Nexstar 4SE (102mm, 1350mm, f/13) for a year and have been taking images of planets / moon etc. with a Philips webcam. I've been wondering about what could be achieved with an 80mm refractor instead - compared to a 102mm MAK Cas.

My thinking is as follows:

For the 4SE MAK : 102mm diameter, 1350 focal length gives a very slow focal ratio of f/13 ... with a 2x barlow that comes down to f/26.... so extremely slow - hence not much light hitting the webcam sensor and quite dark images. With a webcam giving the equivalent (roughly) of a 6mm eyepiece the 4SE gives a magnification of 225x, which again with the 2x barlow doubles to 450x! So reqsonably but, but very dark image capture.

For an 80mm refractor : e.g. ST80 or ED80 with focal length of say 400mm @ f5 - so much faster. With 2x barlow goes to f/10 which is still significantly better than the f/26 MAK - so more light photons hitting the sensor, but the smaller apperture and shorter focal length only gives magnification of 66x, or 133x with 2x barlow.

So on the one hand the MAK has a larger aperture and vastly more magnification, but is correspondingly slower, so less light hitting the sensor, whilst on the other hand the 80mm refractor has a much smaller aperture (therefore grabbing significantly less light photons in the first place) and less magnification, but potentially faster (more photons) hitting the sensor = more detail?

Would love to see a direct comparison of images taken with the same webcam through a 102mm MAK vs 80mm refractor ... but wondering what other people think...?

Does anyone have any images of Saturn or Jupiter taken with an ST80 or ED80 with a Philips SPC900 that I can compare with my 4SE images?

Many thanks,

Mike

[michael.h.f.wilkinson]

For planets, aperture is king, and the 102mm Mak will best an 80mm refractor. Speed is not an issue, as ideally you should match the focal ratio to the size of the pixels on the sensor (F/24 is near ideal for the SPC900). On DSOs a fast 80mm will beat a mak, because speed is more important than aperture

[RikM]

I agree with Michael. For planetary imaging you need aperture (which gives you the optical resolution) and focal length, to give the imaging scale and chip resolution (arcsec per pixel). For a better option, think about a Skymax 127.

[cgarry]

I have to agree with the others, you need aperture for planetary, for optical resolution and to get a decent amount of light per pixel for a given focal length.

Another thing, calculating an imaging "magnification" by saying the sensor is equivalent to a 6mm EP makes no sense at all. In imaging we talk about image scale not magnification.

Cheers,

Chris

[mikehab]

Thanks for the replies - they all seem to be pretty consistent.

Regarding the comments re focal ratio --> imaging scale and chip resolution (arcsec per pixel) ... are there any resources / documents which explain that in a little more detail? Presumably this argument holds for why people prefer to use webcams rather than DSLR for video capture - though I am starting to see some pretty good results from people using an EOS 1100D these days - so presumably it's a combination of the focal length, aperture, sensor & pixel size that matters. Presumably there's some magic formula?

Mike

[IanL]

Here's a really good one regarding aperture, aberration and obstruction (which is also a consideration in the refractor vs reflector/mak/SCT equation):

http://www.damianpeach.com/simulation.htm

[cgarry]

Hi Mike,

This link seems simple and quite good at explaining the relationship between focal length and image scale:

http://www.astropix....23/image-scale/

Note that all that matters for image scale is focal length and pixel size.

Cheers,

Chris

[mikehab]

Thanks both - will do some further reading ;-)