Refractor vs SCT

[MjrTom]

Hello,

I am just wondering if the more knowledgeable than me could shed some light on the differences I would see between an 80mm Refractor and a C5 SCT (Omni XLT 127) for lunar and planetary viewing?

Is there going to be a noticable improvement in resolution and contrast compared to an ED80 refractor? I appreciate I should be able to push the magnification higher but by how much? Obviously with all the caveats regarding seeing conditions.

Both would be Alt/az mounted.

I dont have the space for a big dob at the moment so looking for longer focal length but still in a fairly small package.

Thanks

[Rob]

The C5 will give you greater image scale due the C5's focal length. As a question to surface details & contrast - not much. I've owned both, and i have to be honest hated my C5, but learnt that I had one that was created in the nexstar range when it was running. Gold beige colour tube.. just awful. Also my scond C8 was the Nexstar range and was soft on contrast.

I understand the black & also the white tube versions are much better.. but a 5" SCT has a central obstrcution, this reduces contrast by default.

Your equinox 80 is going to be the winner here with a quality barlow.

Rob

[MjrTom]

Thanks Rob, I had my suspicions that this might be the case. Comparing a £400 refractor to £180 C5 is not going to be a marked improvement.

As it stands my ED80 gives amazingly crisp results at 167x magnification with a 3mm eyepiece. However I feel this is about as higher magnification that is possible with this scope. Must admit I have not tried a 2.5 or 2mm ep but then the exit pupil is getting rather small.

[Rob]

No,No you can push to higher than that, no problem. A good Barlow will see you ok here. I have the Orion Shorty Plus which is a 2x APO Barlow.. its an amazing piece of kit. Your FPL 53 lens should allow up to (skies permitting) x250!!. Iused to own a C80ED R and pushed 200x with ease!.

Go for it

Rob

[MjrTom]

Thanks again Rob,

That is very encouraging news, I suppose I should invest in a good barlow or even a powermate.

The specs for this scope say 250x max but I was expecting this to be a bit of marketing hype than actually useful magnification.

Using the age old 50x per inch rule only takes me to about 160x

Hmm might have to reconsider my options then

[Rob]

As you can see with the attached.. beware, there are 2 flavours. Go for the 'Plus' edition as thats the APO version. Sure its pricy, but shop around.

Barlow Lenses - Telescopes UK: Telescopes & Telescope Accessories in your only London shop

Rob

[GazOC]

Whether it was the scope, the conditions or the operator my ED80 seemed happiest below x180 on planets.

(BTW £400 would get a secondhand ED100 and a few extras, might be worth a look around?)

[MjrTom]

Whether it was the scope, the conditions or the operator my ED80 seemed happiest below x180 on planets.

(BTW £400 would get a secondhand ED100 and a few extras, might be worth a look around?)

Thanks for the info Gaz

just to clarify I have the 80mm refractor already. (see below)

I suspect that I will be able to push the magnification higher on lunar targets as opposed to planets due to the inerrant surface brightness of the moon.

[russ]

Thanks for the info Gaz

just to clarify I have the 80mm refractor already. (see below)

I suspect that I will be able to push the magnification higher on lunar targets as opposed to planets due to the inerrant surface brightness of the moon.

Yes that's exactly right. My last ED80 (Celestron C80EDR) really suffered with lack of aperture above 166x when viewing Mars and Saturn. The image was so dim at 200x it was pointless in the end.

But on lunar you can push a lot higher if the seeing allows.

[MjrTom]

Hmm that's good to know Russ, thanks

For planetary viewing I shall just have to make do I suppose and get a barlow/powermate for lunar observing when conditions allow.

To see a noticeable and beneficial improvement would I need a C6 or C8 then?

[John]

If you get a good C5 and if you get it's collimation spot on I reckon it will outperform an ED80. Two "if's" in there though - I'm on my 3rd C5 and this one is much better than the previous two - really excellent in fact. The other two were "OK" but it's taken hindsight and finding a good one to realise this

[Rob]

Please don't get me wrong here. Image scale will still be on the small size in the 80. But 200x is do-able, and certainly on the moon.. but on those rare nights you can/should be able to push the 80ED that high, and certainly against Jupiter & Saturn.

You could as a side line look at a 127 Mak?. That would give you higher contrast and image scale for planets?.

[MjrTom]

Thanks John & Rob,

Hmm maybe I should just reconsider my options for now and just make do

The Equinox 80 with its perfect collimation and sharp contrasty optics I suspect i am going to have to throw a lot of money in the direction of FLO before I can be truly happy with the proposed upgrade.

Time to clear out the shed so I can get something bigger and have somewhere to store it...

EDIT: A mak could be suitable I guess. Although is there not still the issue of the central obstruction?

[coopman827]

You can often use "stupid" high power on the moon. Don't be afraid to push the envelope!

[Rob]

No.. not with the mak. It has a small central obstruction and long focal length, so in turn that does not impact the image.

Its a thought.. 127 can be picked up for as little as £160?

Best to do what you doing really.. give it lots of thought, visit a local group or star party and road test without spending the cash!!

Best to you

Rob

[GazOC]

The 127mm Mak is a very good call, esp. for planets/moon. They are very popular and most users seem very pleased with theirs, they'd give a ED100 a run for its money, never mind a ED80.

[MjrTom]

Thanks Gaz,

That Mak looks very promising, I suppose I best go off and do some more research

Ultimately, I suspect I shall end up with a Newtonian when I have the space to store it, however my options are open at the moment.

[Donkeiller]

I have a 6SE and an ED80 and have had them out simultaneously.

Planets very little difference, except the 6SE gives a brighter image, but I can't see any further detail.

DSOs is a different matter here the 6SE is miles better than the ED80.

Double stars- here the ED80 will produce more "pin-point" stars and split at lower powers, however the 6SE shows contrasting colours better

Lastly the 6SE takes at least 45 mins to cool down sufficiently to give decent high power views and cannot be used as a "grab and go", like the ED80

In short get/keep both, they complement each other very nicely

[MjrTom]

Hello again.

Still toying with ideas, I have still been looking at catadioptric telescopes in particular the C6 and C8 SCT's from celestron.

I even toyed with the idea of the C9.25 in the for sale section of this forum. How big an aperture should I go as a compromise between performance and portability?

The smaller SCTs should be ok on an alt/az mount but how big can you go? I'm thinking along the lines of the giro or sky-T mounts

This will be mainly for lunar and planetary but also small sized DSO's which my ED80 is less suited to.

Thanks in advance for entertaining my ideas

[MjrTom]

Nobody got any thoughts?

[Moonshane]

I know you have said you don't have room for a dob but to my eyes a larger SCT (6"+) plus tripod and other gubbins will take up more space than even a 10" dob. OK the tube is bigger but the actual floor space is less than the average tripod.

I am biased as I am a dob man (and in honesty have never looked through a good refractor) but I am routinely getting excellent magnifications (400x+ on doubles and luna) and superbly sharp planetary images (at 150-200x sometimes higher) with my 6" f11 dob. if you consider the floor area of this, it is less than a foot square.

if you bought e.g. a 6" f8 or even an 8" f6 newt, you could make a dob base for about £30 and then you'd have a superb all rounder set-up for well under £200 I'd think. you could even buy a 6" f8 dob new for £200 and an 8" for £280.

I'll go back in my cage now

by the way, sorry if this is a daft question but what's the difference between image scale and magnification?

[MjrTom]

Many thanks for your input Shane, its much appreciated.

It sounds like a Dob should not be ruled out.

My main reason for thinking about SCT's and also Mak's as well now was the short tube but long focal length. This being important for the reason of transportation to dark sites and also storage at home.

The wife does not see a dob as a piece of furniture for the living/dining room and been told under no circumstances that any big scope is to be kept in the house... to be honest i can see her point and she is very understanding with all my hobbies and the collection of optical instruments I have amassed.

Astronomy is only one of three hobbies I have the other two being nature photography and bird watching hence loads of lenses, bins and field scopes.... The line has to be drawn somewhere lol

I currently have a heavy duty photo tripod which I use as an alt/az mount for the current 80mm ED refractor I was thinking along the lines of a SW Sky-Tee with the ED80 on one side and an SCT/Mak on the other side there by having the option to use two differing scopes with complimenting focal lengths.

I have not ruled out a dob completely as that would be the best all rounder however the weight is a bit off putting plus collimating an SCT is far easier and is a one person affair which is not the case with dobs I believe. As I sustained a spinal injury a few years back I find heavy lifting to be a bit of a problem hence my thoughts on short tube Catadioptrics as more suited.

The dob I had in mind in particular was the SW Skyliner 200p flextube auto which seems a relatively compact and competent scope which is an 8 inch f/6 scope which you have already suggested but being a collapse able tube will fit upright in the car leaving space for the wife and child.

Urh these decisions are tricky

by the way, sorry if this is a daft question but what's the difference between image scale and magnification?

I have wondered this myself, I could not see a definitive answer via google. I suppose image scale is referring to actual sizes where as magnification is referring to the amount that the images has been increased by it has no relation to the measured angluar size of the object. Dunno not sure really

[Moonshane]

ahhh. given all you say, I can totally understand what your are saying and agree that in the circumstances a SCT/Mak may be a better choice at the minute.

if you went the Skywatcher route, their bases are quite large and heavy - the 8" base is bigger than the footprint of my old 12" OOUK dob base.

I collimate my 1600mm focal length scopes solo and it's not a problem but if you have a bad back then maybe not.

[MjrTom]

Thanks again Shane, Yeah I noticed that the bases of the OOUK dobs are very compact since they are of an open truss design as opposed to the ikea styling MDF/chipboard used on the skywatcher ones.

I suppose my heart is set on something like the following:

Celestron C6 XLT SCT

f/10 1500mm focal length

~£580

First Light Optics - Celestron C6 XLT Optical Tube Assembly

or

Skywatcher Skymax-150 PRO

f/12 1800mm focal length

~£550

First Light Optics - Skywatcher Skymax 150 PRO

One of these scopes would be mounted in tandem with my ED80 on a Skywatcher Skytee-2

First Light Optics - Skywatcher SkyTee-2 Alt-azimuth mount

Not as cost effective as a Dob but far more manageable in the long run.

Plus also all my Astro friends seem to have dobs (and transit vans...) so its good complimentary scopes when out star gazing with the lads

[Moonshane]

given both of those scopes have almost identical characteristics to my 6" f11, I bet they will be excellent.

if the tripod is stable enough then you'll have a fine set up and as you say it compliments your mates' scopes and also keeps SWMBO happy! (we all need to ensure domestic harmony/bliss wherever possible).