Which goto for planetary work: cat or refractor?

[blusky]

Considering either an 8" cat or 4"-ish (apo?) refractor on the basis that the latter can yield better contrast.

Opinions appreciated. Thanks.

[Odd Thomas]

Hi Blusky,

I've got an 8" SCT and a 4" refractor, I'd say the 4" every time for planets.

Tom

[pilkinn]

I too have a 4" refractor and a SCT. The refractor is around f6 so has a shortish focal length. For visual use the refractor is excellent. However for imaging using a webcam I use the SCT. It all comes down to aperture and focal length. In my refractor I have to use a 5x barlow whereas in the SCT I almost get the same image size at prime focus (no barlow or eyepiece) due to the much longer focal length. Resolution is better as well due to the larger apature. With the refractor I soon get to the maximum useable magnification beyond which I can get the image bigger but there's no more detail (actually there's less). The ideal is a long focal length refractor with a reasonable size aperture.

[John]

A well collimated SCT should out perform a 4" apochromat by a noticable margin, except perhaps under really poor viewing conditions. Trouble is I'm not sure all SCT's in use are collimated accurately enough. My best ever views of Saturn were with a well colimated Celestron C8 and a Tele Vue 8mm plossl eyepiece.

That said, I use a 4.7" ED refractor for my planetary viewing now and find it very satisfactory.

Musn't forget the maksutov-cassegrains - they deliver excellent planetary performance as well and come in a compact package.

[pilkinn]

A well collimated SCT should out perform a 4" apochromat by a noticable margin,...

That's the important bit. As I've found, an even slightly out of collimation SCT gives poor results. That said,it's not really that hard to collimate a SCT. You just need to practice and remember to keep checking it regularly.

[rwilkey]

I have an f/9 doublet refractor and it works very well for planetary use, never tried an SCT, but I will never exchange my refractor on an AZ mount, it's perfect for me.

[blusky]

Thanks, do goto refractors also perform better for planetary imaging?

[barkis]

Resuolution is a function of aperture. All things being favourable, Collimation, seeing conditions, Quality oculars.

The SCT's with their inherrent long focal length, which translates to magnification for a given eyepiece, or webcam, if you are imaging. Wins the battle. No way am I rejecting Apo's, no sir, but in a contest the SCT for me.

If in doubt, look at Damian Peach's Jupiter Image. Astrophotographer of the year winner. I know it was a 14" SCT, so not fair, but Io and Ganymede resolved as discs with detail.

You Betcha!.

Ron.

I agree with Pilkinn's Point, but it's worth the effort to ensure perfect collimation before embarking on serious work, whether imaging, or observing

[ollypenrice]

For imaging, the SCT. For visual it depends very much on the night and on the individual. I have a perfectly collimated 10 inch SCT (not being arrogant but it was checked by a professional and, anyway, collimating SCTs is very easy) and a 5.5 inch TEC apo. To my eye the TEC usually wins. There is a debate over whether, perhaps, the SCT gives more information but in a slightly 'scruffy' form. This excellent description isn't mine but was offered by BrianB, who likes SCTs. However, I have both and would, without hesitation, take out the TEC for planetary observing. Illusory or not, I find it looks sharper and more contrasty and a lot more convincing.

The word 'resolution' both in a visual context and a photographic one is a bit of a minefield and I try to avoid it and talk about detail.

Olly

[blusky]

Thanks. Are there any good apo goto/tracking refractors under £1000 or is it too much to ask for?

[garryblueboy]

You will struggle for a apo with goto for under 1k poss but would be a small scope i have had the pleasure of owning a vixen 180 apo and it was with out a doubt better than my scmdt 11" if you want a good image and stay in budget then a good secondhand 8" goto scmdt should be in your budget and give a good clear image just not as much detail as an apo.

[26Left]

...snip... I have both and would, without hesitation, take out the TEC for planetary observing. Illusory or not, I find it looks sharper and more contrasty and a lot more convincing.

Olly

I would echo what Olly wrote. I spent many nights comparing my 5" refractor with my 11" SCT on Jupiter. Almost every night, I found the contrast and detail with the refractor significantly better. The only night my SCT outperformed the 'frac was a night of exceptional seeing. My 4" doublet refractor also performs very well on planets.

But, for planetary imaging, it's the SCT every time.

[Tinker1947]

Can we do a price thing, Olly's 5.5" and 26Left's 5", there gonna be a bit bigger than Blusky may be-able to afford so a £ for £ comparison might be a better idea.

[garryblueboy]

id say having owned lots of scopes pound for pound as ive just found out a dobs gives you more grunt for the money add in goto and you will have half the price spent on drives etc so not so much left for optics but good all rounder is a 8"SCMDT look around second hand £1k or less good all rounder on planets and dso or if you can a 7" mak .

[LukeSkywatcher]

Hi Blusky,

I've got an 8" SCT and a 4" refractor, I'd say the 4" every time for planets.

Tom

I have the same and agree.

[kniclander]

why goto for planetary?

ps I second (third?) the thumbs ups for the 4" F9s:)

[Moonshane]

would either of these options beat your current 8" dob on planets by a large margin? I suspect not. if I were you, I'd find someone local and make a direct comparison with your suggested options before buying.

[LukeSkywatcher]

My 90EQmm refrac is an absolute planet killer regarding views. Best views ever of planets.