12" Dob vs 10" LXR SCT - solar system shoot out!

[MartinB]

On Saturday night my 12" dob, in good seeing, had given me a stunning view of Saturn - good colour, clear banding on the disc and an incredibly sharp and black Cassini. My SCT couldn't compare, largely because the seeing was much worse through this scope. I eventually put this down to an over vigorous dew strap which was warming things up too much.

Too much moon for DSO images on Sunday but the night was so clear that I decided I would have some fun with some lunar observing, also taking in Saturn and the disappearing Mars. 2 scopes out - 12" F5 Skyliner dobsonian and a 10" LXR SCT. The dob is a fast scope ideal for low powered deep sky views whereas the SCT is a long focal length beast well suited to effortless high power viewing. So should be no contest really. Use of the dew strap on the SCT was strictly limited

The dob was set up in 5 mins. The SCT...rather longer - take out mount and counterweights, attach scope (not the easiest of jobs) polar align.

Conditions - excellent transparency, moderate seeing and flippin cold.

First up - the moon. I wanted to see if I could pick out those diddy craters in Plato. Seeing was limited to about x250 with both scopes. I was mainly using a Pentax 10mm xw and a Pentax 7mm xl but also had a 5mm orthoscopic to really push magnification. Best views with the SCT were with the 10mm xw, with the dob the 7mm was ideal although a little more detail was brought out by the 5mm ortho.

At x250 with the SCT could just make out 2 craters, intermittently and with pretty fierce concentration. Moving over to the dob at x210, despite the lower power 3 craters were seen quite effortlessly. With the eye of faith I could pick out a 4th.

I went on viewing more of the moon with views along the terminator. Both scopes gave superb high powered views but the dob just gave that extra bit of resolution. Subjectively the contrast was slightly superior with the dob but this wasn't a significant issue.

Next up was Mars. Here, the SCT managed the same level of detail as the dob with slightly more muted colour. A dead heat.

Finally onto Saturn, still lowing lying at the time of observation, the seeing didn't match the night before and was fairly wobbly. I could just discern Cassini and disc banding with both scopes. Once again the dob displayed slightly more colour and the contrast probably did aid viewing of the banding slightly. The dob had it by a nose.

Obviously with the SCT the target just sat there and never shifted whereas the dob needed constant adjustment. However, after a little while this becomes automatic and I hardly have to think about it. Although a dob mount isn't ideal for high powered viewing it isn't the problem I thought it would be. Viewing comfort is generally superior with the alt az dob. The dob is much quicker hopping from target to target.

So, both scopes performed well but, for me the 12" F5 newtonian definitely shaded it over the 10" F10 SCT. Interesting!

[MikeP]

Very interesting Martin. You have confirmed a suspicion of mine.

I sold my Skywatcher 250 and bought a C11 last year. Whilst I never had both available at the same time for comparison, I have never felt that the C11 is markedly better visually, but it is vastly more convenient to use. Perhaps it would have been better if I'd used the 250 as a dob and kept the EQ6 for imaging with my ED80.

Mike

[MartinB]

One thing I forgot to mention, the wide angle Pentax EPs make a big difference using the Dob at high powers since the target stays in the FOV much longer. Using an orthoscopicis a much more inconvenient matter altogether. A cheaper and excellent alternative would be Hyperion EPs

[MartinB]

Oh and both scopes were well collimated!!

[Beyond_Vision]

Sounds like you had an interesting nights worth of observing Martin. In an ideal World it would be a fairer comparison if both were 10". With the 12" you would have better resolution than the 10".

Regards

Kevin

[MartinB]

I know that in ideal conditions the larger aperture scope should be able to resolve more detail than the lower powered on but I thought that with these relatively high powered views the F5 newt would be busting a gut optically whereas the F10 2500mm focal length SCT would be breaking into a sweat.

Very impressed with the performance of the Skyliner

[GazOC]

The difference between 10" and 12" is pretty noticable, I'd have been surprised if you'd have found anything else than the 12" scope edging out the 10".