C8 vs Refractor for planets?

[tico]

Hello
Currently I have a C8 with which I observe the Moon, the planets and some very bright Messier, this is what I can observe from my little corner of observation, which is from where I usually observe, the planets and the Moon are the most apparent objects, Lately I am thinking if it is worth changing the C8 for some refractor that would give me better lunar and planetary images, the truth is that with the Moon I have no complaints, the image is very good, I observe with binoviewers, but with the planets I do not have So lucky, I have a hard time getting an enlarged and stable image..that is why I would like to ask you if it is really worth changing the C8 for an 80ED or an ED100 refractor..for visual planetary and Moon.
Very kind for your comments.
Tico.

 

[inFINNity Deck]

Hi Tico,

That the Moon gives better result simply is due to its massive size, all the planets are much smaller: the largest planet (Jupiter) is only about 2.5% the apparent diameter of the Sun and Moon (depending on its distance to Earth), the smaller planets only a fraction of that.

The advantage of a reflector over a refractor are the additional aperture and focal length for the same price. More aperture means more detail, more focal length means more magnification. Downside of the refractor is that the aperture comes at a price... 😞

Below images or Saturn were shot on one night using a SkyWatcher Esprit 150ED refractor (150mm aperture f/7, but thanks to 2x PowerMate f/14) and a Celestron C11 EdgeHD reflector (280mm aperture f/10, no Barlow).

Personally I find the refractor giving a better image with more details and less artefacts. But then I have to say that the C11 was undersampling at f/10, so I should have added a Barlow and then reduce the size after processing.

Nicolàs

Edited October 12, 2020 by inFINNity Deck

[vlaiv]

Not worth it in my opinion.

In most cases it is about optimizing planetary views rather than choosing particular scope design.

Do internet search on optimizing your planetary views and you should get at least couple of results (very nice youtube video). In nutshell it is about minimizing local seeing conditions impact by careful thermal management and choosing your observing location / direction.

If you suspect that your C8 is not giving you the best image - check collimation first. There is sample to sample variation between scopes and some are less sharp. If you suspect your scope is like that, then maybe think about swapping it for this one:

https://www.firstlightoptics.com/stellalyra-telescopes/stellalyra-8-f12-m-lrs-classical-cassegrain-telescope-ota.html

It has some advantages over C8 with respect to planetary viewing and in general.

[Prism]

46 minutes ago, tico said:

Hello
Currently I have a C8 with which I observe the Moon, the planets and some very bright Messier, this is what I can observe from my little corner of observation, which is from where I usually observe, the planets and the Moon are the most apparent objects, Lately I am thinking if it is worth changing the C8 for some refractor that would give me better lunar and planetary images, the truth is that with the Moon I have no complaints, the image is very good, I observe with binoviewers, but with the planets I do not have So lucky, I have a hard time getting an enlarged and stable image..that is why I would like to ask you if it is really worth changing the C8 for an 80ED or an ED100 refractor..for visual planetary and Moon.
Very kind for your comments.
Tico.

 

I have an 8" SCT and recently got an ED 102mm refractor. I 100% agree with the comment above from vlaiv (focus on optimising your view first by viewing when the planets are high in the sky with nights of good seeing + checking your collimation , thermals etc.). I had many of the same issues with the refractor as with the SCT (the major benefit being quick cool down and being relatively lightweight), Mars even at its closest on Oct 6th was still pretty tiny even at 250x (which is what the refractor was maxing out at), with very little visible detail. 

[inFINNity Deck]

7 minutes ago, vlaiv said:

If you suspect that your C8 is not giving you the best image - check collimation first.

That indeed is a key point: the Celestron SCTs do not hold collimation very well, and may not 'survive' a meridian flip due to excessive mirror-flop. I currently have two C11s here, a XLT and a XLT EdgeHD, and both show collimation goes astray when going through the meridian (see the link in my previous line, it is in Dutch but should translate properly when opened in Chrome). I had the EdgeHD serviced by Celestron in the US and apart from a big hole in my pocket it did not have any effect. Celestron did not want to specify the mirror-flop, so I could not check if it was damaged by the return shipment, and apparently this is not something Celestron has control over.

Ever since I got it back I first slew to a nearby star, collimate the SCT, and then do the imaging. It then gives reasonable results. I also see nice planetary images being taken with Maksutovs, perhaps that is a good alternative.

Nicolàs

[Owmuchonomy]

I have an ED80 and a C9.25.  The C9.25 naturally takes less effort to achieve a reasonable image scale when viewing.  The ED80 would need a very short f/l EP to get the same image scale and this brings its own issues, particularly eye relief.  If you widen the debate to SCT vs refractor  in general then it becomes more difficult to choose.  I am fortunate to also own an ED150; put that against the C9.25 and it's a different matter, the views being very hard to distinguish.  If the seeing is poor or there is slight cloud cover, the refractor wins at that aperture.

[Owmuchonomy]

The OP is asking about visual comparisons.  If the debate is about imaging then it's a different scenario to that I describe above.

[vlaiv]

3 minutes ago, Owmuchonomy said:

I have an ED80 and a C9.25.  The C9.25 naturally takes less effort to achieve a reasonable image scale when viewing.  The ED80 would need a very short f/l EP to get the same image scale and this brings its own issues, particularly eye relief.  If you widen the debate to SCT vs refractor  in general then it becomes more difficult to choose.  I am fortunate to also own an ED150; put that against the C9.25 and it's a different matter, the views being very hard to distinguish.  If the seeing is poor or there is slight cloud cover, the refractor wins at that aperture.

Given that OP asked about ED80 and ED100 and that you have ED80 and ED150 and C9.25 as well, I would say you are in perfect position to advise if swapping C8 for twice the smaller diameter refractor is going to give satisfactory views.

[jetstream]

28 minutes ago, vlaiv said:

In most cases it is about optimizing planetary views rather than choosing particular scope design.

Totally agree.

[johninderby]

Or another contender. 🙂

https://www.firstlightoptics.com/stellalyra-telescopes/stellalyra-8-f12-m-lrs-classical-cassegrain-telescope-ota.html

[mikeDnight]

1 hour ago, tico said:

 I have a hard time getting an enlarged and stable image..that is why I would like to ask you if it is really worth changing the C8 for an 80ED or an ED100 refractor..for visual planetary and Moon.
Very kind for your comments.
Tico.

 

For visual planetary observing I would say "yes"! From personal experience, and I've literally used hundreds of SCT's, they are the absolute worst visual scope in existence for planetary. In forty years I've only once seen an SCT give an acceptable planetary view. Everything else is a better option!  As an imaging scope SCT's can be great, but visually they give the softest, definitionless view imaginable.  For visual planetary definition is everything, and a Newtonian will outstrip the same aperture SCT, but so will a smaller aperture Maksutov Cassegrain. So it's not the Cassegrain design that's the problem, its the fourth order curve of the Schmidt corrector plate. They are a visual observers nightmare! If you're in any doubt, try comparing star images between the SCT and the Newtonian, Maksutov Cassegrain, classical Cassegrain and refractor. 

As for changing the SCT for a 102mm ED refractor, the view will be sharper but dimmer. Image scale can be increased but local seeing conditions may not always allow the use of very high powers. 

[inFINNity Deck]

9 minutes ago, Owmuchonomy said:

The OP is asking about visual comparisons.  If the debate is about imaging then it's a different scenario to that I describe above.

Indeed he was asking about visual observing, I was confused by his line "...give me better lunar and planetary images".

The larger the aperture the sooner seeing is going to play a negative role in the stability of the image, so less is more when it comes to aperture and seeing as Chris already stated.

Nicolàs

[jetstream]

39 minutes ago, vlaiv said:

. If you suspect your scope is like that, then maybe think about swapping it for this one:

https://www.firstlightoptics.com/stellalyra-telescopes/stellalyra-8-f12-m-lrs-classical-cassegrain-telescope-ota.html

It has some advantages over C8 with respect to planetary viewing and in general.

Vlaiv did you see that it has only 29% obstruction? The f12 must help it out on this. These scopes look good. IME every bit under 30% CO makes a nice difference.

[inFINNity Deck]

3 minutes ago, mikeDnight said:

For visual planetary definition is everything, and a Newtonian will outstrip the same aperture SCT, but so will a smaller aperture Maksutov Cassegrain. So it's not the Cassegrain design that's the problem, its the fourth order curve of the Schmidt corrector plate. They are a visual observers nightmare!

Next to my C11 I have a SkyWatcher 300PDS Newton (304mm f/4.9) on a Dobson mount, but even though the SCT is not my best friend, I have to say that I find the views trough the SCT more pleasant than with the larger aperture Newton.

Nicolàs

[nfotis]

The consensus seems to be that a big Maksutov gives very good planetary observation (refractor like), and the Skymax 180 is termed "planet killer", as it offers extremely long focal distance and (usually, provided good atmospheric conditions) quite sharp views. You may want to add a dual speed focuser to it and an insulated jacket from Reflectix if you have problems with thermal equilibrium.

 

The GSO Classic Cassegrain, especially the 8", is a good contender, if you don't want to wait for thermal equilibrium (open tube). It may need some careful collimation for best results, and includes a dual speed focuser.

 

N.F.

 

Edited October 12, 2020 by nfotis

[vlaiv]

1 minute ago, jetstream said:

Vlaiv did you see that it has only 29% obstruction? The f12 must help it out on this. These scopes look good. IME every bit under 30% CO makes a nice difference.

I'm not sure what is true - if that figure on FLO or this one from TS:

If you look at specs for 6" model at FLO:

https://www.firstlightoptics.com/stellalyra-telescopes/stellalyra-6-f12-m-crf-classical-cassegrain-telescope-ota.html

it also says 58mm and I've seen these specs also mentioned on other websites. I suspect copy/paste error somewhere along the information pipeline (maybe GSO rep that sent out specs or similar).

@johninderby

has both, or at least 8" at the moment and awaiting 6" version as well.

Maybe direct measurement of secondary obstruction could settle this?

[jetstream]

6 minutes ago, jetstream said:

Vlaiv did you see that it has only 29% obstruction? The f12 must help it out on this. These scopes look good. IME every bit under 30% CO makes a nice difference.

scratch this I just saw its 34% obstructed.

[jetstream]

7 minutes ago, vlaiv said:

I'm not sure what is true - if that figure on FLO or this one from TS:

 

Thanks Vlaiv. Right now I switched my 75mm from the 200mm f3.8 for the 63mm from the VX10 with both running around 30% now. The VX10 is still giving VG lunar views and the illumination of the 200mm is acceptable.

Anyway, a good refractor in the 100-120mm range will offer good viewing opportunities eventhough detail will suffer from the C8. A good C8 is no slouch however if it is cooled and collimated.

Edited October 12, 2020 by jetstream

[johninderby]

Specs from the Orion US site for the CC.

[vlaiv]

2 minutes ago, johninderby said:

Specs from the Orion US site for the CC.

This matches with 38% and 33% given by TS. It also matches 58mm given by FLO for 6" version, so I guess problem is that this value ended up with 8" version as well instead of 68.

[dweller25]

Hello @tico

SCT’s are challenging at high visual powers but there are a couple of things you can do to optimise your scope....

1. Wrap the scope with a couple of layers of reflectix from B&Q - this will bring much better thermal stability inside the scope and it will also slow dew formation on the corrector.

2. Accurately collimate the scope.

If you are still not happy then I would suggest a Skywatcher 120ED may give you the results you seek.

[Chris]

Definitely worth throwing a Classical Cassegrain into the mix for consideration, it could be the best compromise of aperture, contrast, sharpness, and cooling time. 

[RobertI]

I recently bought a 102ED refractor to complement my C8 (and various other scopes). I agree with some of the comments about SCTs giving 'soft' views, but so far the 102ED has not been able to show more detail than the C8 on Mars (though I have only had a couple of side-by-side sessions) and I have had some breathtaking detailed views of Jupiter with the C8 in the past. As mentioned, SCTs do need to be cooled and collimated and sometimes they just don't seem to perform well under certain conditions, but when they do they can be excellent. It should be noted that planetary observing is often a matter of waiting for those fleeting moments of clarity when the atmosphere steadies, so planets often don't look good at first glance (sorry if you already know this, I don't know what your level of experience is). I don't know if you have looked through other scopes to see what they are capable of in terms of planetary, but I wouldn't rush to buy anything else yet without trying some other scopes, and also trying to get the best out of your SCT.  

[ollypenrice]

16 hours ago, mikeDnight said:

For visual planetary observing I would say "yes"! From personal experience, and I've literally used hundreds of SCT's, they are the absolute worst visual scope in existence for planetary. In forty years I've only once seen an SCT give an acceptable planetary view. Everything else is a better option!  As an imaging scope SCT's can be great, but visually they give the softest, definitionless view imaginable.  For visual planetary definition is everything, and a Newtonian will outstrip the same aperture SCT, but so will a smaller aperture Maksutov Cassegrain. So it's not the Cassegrain design that's the problem, its the fourth order curve of the Schmidt corrector plate. They are a visual observers nightmare! If you're in any doubt, try comparing star images between the SCT and the Newtonian, Maksutov Cassegrain, classical Cassegrain and refractor. 

As for changing the SCT for a 102mm ED refractor, the view will be sharper but dimmer. Image scale can be increased but local seeing conditions may not always allow the use of very high powers. 

I have to say that I've felt this way about SCTs in the past, myself.  The first good one I tried was hand made by Ralf Ottow and the second was the 14 inch Meade I inherited from Alan Longstaff. I must say that I do like the 14 inch. It gives far better stars and finer planetary detail than I was expecting. So I think SCTs are very variable.

Olly

Edited October 13, 2020 by ollypenrice

[Alkaid]

19 hours ago, mikeDnight said:

From personal experience, and I've literally used hundreds of SCT's, they are the absolute worst visual scope in existence for planetary. In forty years I've only once seen an SCT give an acceptable planetary view. Everything else is a better option!  As an imaging scope SCT's can be great, but visually they give the softest, definitionless view imaginable. 

Aw...c'mon Mike....my C8 heard that and is now crying in the corner 😜

Kidding aside, the worst enemy for me is UK seeing.  Generally speaking, it is terrible for 99% of the year.  Refractors are just better at dealing with it.  I'm sure that a good SCT, collimated and away from the jet-stream would deliver.