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focaldepth

Choosing a telescope to image M51

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Hi all,

 

One simple question, Which of these 2 telescopes is better for imaging M51 sized objects?

 

  1. Skywatcher 200mm Quattro

  2. Skywatcker Skymax 150p

 

My head is exploding trying to tackle this logically. It is basically resolution vs exposure time.

I do have other equipment but let's keep it simple.

 

Here are all the numbers:

(based on Starlightxpress H18 camera and target M51)

(numbers rounded for ease)

                              SW 200 Quattro                               SW Skymax 150p

Focal Length            800mm                                           1800mm

F Ratio                    F4                                                   F12

 

Object size               11' x 7'                                            11' x 7'

Field of view             1.3 x 1.0 deg                                   0.6 x 0.4 deg

 

Dawes Limit              0.57”                                              0.77”

Pixel scale                 1.4”                                               0.6”

Sky Resolution           2” (???)                                         2” (???)

 

So the:

                              Quattro has better optical resolution,

                              Skymax has better pixel resolution,

                              The Quattro can collect more photons and/or shorter exposures. (eight times more)

                              Skymax has a better framed image size.

 

So which telescope would you choose for producing the “best” image?

 

I could try them both and see but unfortunately I don't get that many clear nights when I am free to do imaging.

We are off to Kielder soon and I want to make the most of any clear skys that may occur.

Other suggestions are welcome as well. (There was a man wandering lost in the woods when he met another traveller and he asked for directions. The reply was, well, I wouldn't start from here)

 

 

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What mount will you be using? Can you guide to 0.6"/px?

TBH I wouldn't use either, instead I'd be looking at a long-ish 'frac or a RC plus reducer.

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If you want to be more likely to come home with something, take the Quattro. At the risk of starting a fire, the F ratio is not very important. (Small targets fall foul of the F ratio myth.) M51 will easily fit on the chip in both cases and the Quattro will catch more photons due to its aperture, not its F ratio. It will catch about 1.8 times as many M51 photons as the Mak if you calculate the area of each objective. It certainly won't catch 8 times as many! Photon count is proportional only to aperture. The Quattro will also put these M51 photons onto few pixels, filling them faster at the cost of resolution. But 0.6"PP is a big ask both of seeing and of mount. You're not going to get a 'big' M51 at 1.4"PP but it will be a decent size, potentially bright and potentially with the considerable tidal extensions if you shoot enough data.

Olly

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Thanks for the suggestions.

DaveS, my portable mount is an HEQ5(belt drive) and yes I can not guarantee 0.6" guiding. Good thought, I had not considered the mount. Hmm.

Ollypenrice, wise words, the old F ratio myth, I am going to have a long think about the maths of this photon counting. I will read your post seven more times before bed, and sleep on it.

The Quattro is easier to set up and focus, and it stays focused forever once set.

Some how I expected this answer so it looks like the Quattro it is.

There are a few similar sized galaxies high up at the moment so I could try for one and include a few extra free galaxies in the frame as well. Probably M51. Or not.

Thanks for the advice.

 

  • Haha 1

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8 hours ago, focaldepth said:

Thanks for the suggestions.

DaveS, my portable mount is an HEQ5(belt drive) and yes I can not guarantee 0.6" guiding. Good thought, I had not considered the mount. Hmm.

Ollypenrice, wise words, the old F ratio myth, I am going to have a long think about the maths of this photon counting. I will read your post seven more times before bed, and sleep on it.

The Quattro is easier to set up and focus, and it stays focused forever once set.

Some how I expected this answer so it looks like the Quattro it is.

There are a few similar sized galaxies high up at the moment so I could try for one and include a few extra free galaxies in the frame as well. Probably M51. Or not.

Thanks for the advice.

 

The maths is easy. The amount of light entering the scope is proportional to the area of the objective which goes as Pi r squared. It cannot be otherwise.

When you image a target like M51, widening the FOV brings in no new M51 photons because they are all there on the chip with the more restricted FOV anyway, and adding light from background sky and field stars won't add anything from M51.

The other factor is flux per pixel. You need to get the signal over the read noise. Whether you'd get the tidal extensions over the read noise in the Mak I don't know, which is why I think the Quattro would be the safer bet.

When folks say that a camera is twice as fast at F2.8 as at F4 they are perfectly correct. But they are correct because, without changing the focal length, they have simply doubled the area of their aperture. If telescopic photographers did the same - used aperture masks to vary their focal ratios, there would be no F ratio myth. But they don't do that (other than in exceptional circumstances.)

Olly

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Thanks again Olly, I'll be sat waiting this afternoon while my wife is at a machine knitters meeting, so I'll be drawing triangles, circles and squares and thinking photons.

So the Quattro it is.

John.

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