My First Refractor / Catadioptric

[needleundernail]

Hello Everybody,

As I'm sure, this happened to all of us... All I wanted couple of days ago is to look what telescopes (with absolutely no prior knowledge on the subject) do they sell in my home town... 48 hours later of almost solid reading about everything telescope-wise and astronomy I found myself in a vacuum of hard decisions. Don't get me wrong, I had a HUGE amount of fun reading all the information I can digest on the forums and the web. That's even a little bit frightening.

So, now I'm looking now at a Skywatchers telescopes (don't have access to other brands) primarily in 90-130mm aperture, Refractor or Catadioptric. Don't want Dobs because of the size, don't want Reflectors because of upside-down terrestrial view. Want a not heavy/big good-looking beginners all-rounder sky-terrestrial telescope. No astrophotography in thoughts.

I know, I will be laughing at my post myself in some time, but please bear with my unfamiliarity.

The kicker is that at night I want to primarily look at Deep Space Objects, because I think I'll get tired of a handful of planets pretty quick.

1. So, the question is will I be able to do that with the telescopes I'm looking at. The Focal Ratio of them ranges from 5 to 13.

Read a lot of people saying that they enjoy DSOs with Skywatcher's Skymax's, but their F ratio is at the high end of around 12. 

2. Can this be true?

3. How much different viewing experience with the same telescope but an F ratio of 5? (do you see noticeably more detail and more light?)

4. For terrestrial view does F ratio mean anything?

5. Will I be able to see planets with lower F ratio as good as with higher? (Can I increase F ratio?)

6. Can I lower Skywatcher's Skymax's Focal Ratio somehow?

And of course, I would be all that much happy for any input on Refractor/Catadioptric experiences.

English is not my primary language, so I'm sorry.

Thanks!

[michael.h.f.wilkinson]

Visually aperture is king. My C8 (at F/10) has bagged me more than 400 galaxies, and 800+ DSOs total. An 8" F/15 scope with a 30mm EP, an 8" F/10 scope with a 20mm EP and an 8" F/5 scope with a 10mm EP all give the same view of DSOs (all other things being equal) at the same exit pupil. The only thing that very slow scopes do not manage well is a handful of very wide field objects. These are pften best seen with binocualrs or a rich-field scope.

Fast scopes can give good views of planets, but it can be harder to control aberrations in fast scopes, hence the preference many planetary observers have for slower scopes. A comparatively fast, well-collimated Newton with good EPs will be excellent on planets, however.

[ronin]

A Mak or SCT has a long focal length and these tend to enable higher magnification easier.

An 8mm BST eyepiece in a 1500mm Mak gives 187x, when in a 150P reflector it gives 94x.

As a DSO tends to be dim in the Mak it is really dim, and not quite so dim in the 150P.

It is this that makes the Mak/SCT "unsuitable to DSO observing.

IF yoiu can not go for magnification and buy long focal length eyepieces then you can counter this.

But just about everyone wants more and more magnification.

All astro scope put things upside down and at funny angles, and I would not suggest a right image converter simply as an astro scope is not meant for that application.

Best scope really cannot be answered.

I would say get a 102 SW Evostar refractor, many will say the 150P dobsonian and others the 127 Mak.

If DSO's is what you want then the 200P dobsonian tends to be the suggested item, simply bigger mirror.

Do you want motors or to manually move it yourself all the time?

Motors mean a driven EQ mount but motors make life plesanter, you can stand away from the eyepiece, talk to someone, scratch eyes and have a half chance that the object is still in view. But with motors come the need to power them.

Expect to buy additional items to about the same cost as the scope, 4 eyepieces will be £200 (no idea what that is in your curracncy), a collimator for a reflector add say £30.

[dweller25]

Hello Everybody,

As I'm sure, this happened to all of us... All I wanted couple of days ago is to look what telescopes (with absolutely no prior knowledge on the subject) do they sell in my home town... 48 hours later of almost solid reading about everything telescope-wise and astronomy I found myself in a vacuum of hard decisions. Don't get me wrong, I had a HUGE amount of fun reading all the information I can digest on the forums and the web. That's even a little bit frightening.

So, now I'm looking now at a Skywatchers telescopes (don't have access to other brands) primarily in 90-130mm aperture, Refractor or Catadioptric. Don't want Dobs because of the size, don't want Reflectors because of upside-down terrestrial view. Want a not heavy/big good-looking beginners all-rounder sky-terrestrial telescope. No astrophotography in thoughts.

I know, I will be laughing at my post myself in some time, but please bear with my unfamiliarity.

The kicker is that at night I want to primarily look at Deep Space Objects, because I think I'll get tired of a handful of planets pretty quick.

1. So, the question is will I be able to do that with the telescopes I'm looking at. The Focal Ratio of them ranges from 5 to 13.

Read a lot of people saying that they enjoy DSOs with Skywatcher's Skymax's, but their F ratio is at the high end of around 12. 

2. Can this be true?

3. How much different viewing experience with the same telescope but an F ratio of 5? (do you see noticeably more detail and more light?)

4. For terrestrial view does F ratio mean anything?

5. Will I be able to see planets with lower F ratio as good as with higher? (Can I increase F ratio?)

6. Can I lower Skywatcher's Skymax's Focal Ratio somehow?

And of course, I would be all that much happy for any input on Refractor/Catadioptric experiences.

English is not my primary language, so I'm sorry.

Thanks!

You have ruled out Newtonians so .....

For DSO's aperture counts - An SCT or Mak can be use for DSO's as long as you use a long focal length eyepiece. I have a 7" F/15 Mak and use a 40mm eyepiece giving x67 and a 25mm eyepiece giving x108 which are fine for most DSO's aprt from the very wide ones. I then use a 15mm eyepiece giving x180 which is good for the planets.

Perhaps a 6" SCT or Mak would be a good compromise for you ?

The Skywatcher 6" Mak has a focal length of 1800mm so a 32mm eyepiece will give x56 and an 18mm will give a power of x100, a 10mm will give x180 - good for the planets.

HOWEVER a 6" newtonian will be better suited to DSO's.......

[michael.h.f.wilkinson]

People keep banging on about fast scopes being better for DSOs, slow scopes being better for planets. The former statement holds true only for imaging on DSOs. The latter statement needs lots of qualifications. The three most important features of a scope for visual are:

1. Aperture,
2. Optical quality, and
3. Transmission

At some distance we have

      4. Maximum field of view.

On DSOs aperture and transmission are most important (and for a few max FOV), on planets aperture and optical quality. The type of scope and the focal ratio are only important insofar as they may influence these parameters. Cheaper refractors are almost exclusively achromats, and in faster ones that means that they suffer in terms of optical quality (chromatic aberrations, spherical aberration). These cheap, fast fracs are pretty good for wider DSOs, but less so on planets because the high magnification brings out the optical aberrations.

A 6" Newtonian (I built one, had it for years, loved it) is an excellent all-round visual performer. Despite being faster than my current 8" SCT, the latter took my beloved 6" to the cleaners on both DSOs and planets, despite the 1/10th lambda mirrors fitted in the Newtonian. A 5" SkyMax will beat a 4" Evostar visually by a small margin on both planets and DSOs (actually, on DSOs the advantage may be more pronounced). An 8" Newtonian would essentially give me the same performance, visually.

The reason I like SCTs has little to do with the visual performance, however. The biggest advantage I found in catadioptric scopes (mak-cass and SCT alike) is that they are tremendously compact for their aperture. My 8" scope has an optical tube which is probably the lightest for its aperture (4.7kg) and just 45cm long. At 8" that is just amazing. The compact size and light weight mean that I can take it many places even in a small car.

[needleundernail]

Thank you for all your replies. You were all spot-on and gave me invaluable information.

ronin: Yes, I want to operate the telescope manually

dweller25: It would be a great compromise, but I'm afraind not with my funds

michael.h.f.wilkinson: you gave the best shortest explanation I have read to this point

Where am I now: Considering all, and adding Reflectors to the bunch, I'm down to three telescopes:

1. [Refractor] Skywatcher Startravel 120 (120/600, f/5)

2. [Reflector] Skywatcher Explorer 150PL (150/1200, f/8)

3. [Catadioptric] Skywatcher Skymax 127 (127/1500, f/12)

Once again, I'm not interested in astrophotography (nor motorised mounts), only in the pleasure of viewing

Which of these will give me the best Deep Space / Planetary (and maybe Terrestrial) viewing experience? (Brightness, sharpness, detail, artifacts of sorts, maintenance, demand for quality eyepieces, sturdiness and ageing, dew problem, temperature normalisation and etc.).

[rowan46]

no irony in this question  but what would you use a telescope terrestrially for? spotter scopes I get, binoculars I get but these scopes you have chosen are too big  to take  out  and be used effectively  for nature or plane spotting. The skymax probably comes closest in terms of size to being useful as a terrestrial scope  but that narrow field of view will make it awfully difficult to use in the field.  For  most terrestrial use binoculars or spotting scopes, make best sense. So if you want dual use get binoculars as these are probably the best compromise. if however astronomy is your primary concern get an astronomy instrument, if portability is an issue get the skymax. if you want  better views of deep space you  need apparture and for this reflectors are the most cost effective more specifically a 150mm-200mm dobsonion scope.

[John]

I think you really need to think about getting to an 8" / 200mm aperture scope if you can. The lowest cost way to do this is the dobsonian design. I realise that this might not be your preference but, to me, if photography is not an issue, there is no escaping that they deliver the most viewing potential for the money spent by quite a margin.

By going for other designs and understandably wanting to stay within a budget, you will be compromising viewing potential on the moon and planets and, of importance to you, deep sky objects. I'm just not convinced that a 6" aperture would give you lasting enjoyment / challenge for deep sky viewing.

[swamp thing]

Must admit I find this obsession with FOV and DSO's a bit daft. I guess if you are continuously observing giant objects like the NA nebulae a wide field is a benefit. On 98% of DSO's a wide field is a handicap not a blessing. For a wide field equals small aperture and for DSO's you want image scale and aperture. Not wide fields.

Remember all scopes show all objects at identical surface brightness when the exit pupil is the same, the only difference is image scale. Aperture gives greater image scale and therefore reveals more detail.

For most DSO's you want. Dark sky, and large apertures. FOV is totally irrelevant.

My 20" f/4 Dob has a maximum FOV of around a degree yet it works pretty well on deep sky.

[John]

1 degree is a very wide field for a 20" scope

[swamp thing]

1 degree is a very wide field for a 20" scope

But it's not a wide field and that's my point. I'd be perfectly happy if it only had half a degree if it could keep the exit pupil the same size, sadly that's not possible and would probably require a 30" scope.