upgrade advice from celestron 130 slt

[richard2600 (Richard)]

Hi at the moment I have a celestron 130 slt goto telescope which is not too bad. I have seen a few seconehand celestron se8 sct type and meade 8 inch cassegrain type scopes. I was wondering would these type of scopess be a good upgrade over what I have now

[multiweb (Marc)]

The Celestron 130SLT is a very capable imaging platform if you do a few mods to stiffen it up. An SCT 8" will be longer FL and softer with a bigger field curvature. I think it's a step backwards. A quality refractor might be the go.

[PSALM19.1 (Shaun)]

Hi Richard,

Are you wanting to upgrade for imaging purposes only? You could use your 130 SLT for that and invest in an 8" Dob for visual astronomy...? It really depends on your budget because there's some great imaging scopes out there but they cost big bikkies.....I am by no means an imaging expert by the way, but if you want the best of both worlds, having a Dob and the SLT would be a great combo I reckon

[richard2600 (Richard)]

Originally Posted by PSALM19.1

Hi Richard,

Are you wanting to upgrade for imaging purposes only? You could use your 130 SLT for that and invest in an 8" Dob for visual astronomy...? It really depends on your budget because there's some great imaging scopes out there but they cost big bikkies.....I am by no means an imaging expert by the way, but if you want the best of both worlds, having a Dob and the SLT would be a great combo I reckon

Hi no just for visual just wont bit more detail or power I can only just make out the rings on Jupiter would like to get better view and budget is $2000 max under would be better. Its just ive seen lot 8 inch and even bigger seconehand scopes and thought it might be an upgrade

[mental4astro (Alexander)]

Marc, you make SCT's sound like poison !

SCT's are fine visual instruments. Yes, there is some field curvature, but this is really only a photographic concern, not for visual if you use the right eyepieces for an SCT (optical matching of EPs and scope design is waaayyy under explained and under-understood, and sees many eyepieces trashed in many reviews from lack of understanding by the reviewers). An 8" SCT packs plenty of aperture punch. Plus, the mirror and lens coatings have improved a lot over the years, and today's SCTs give visually noticeably brighter images than older SCTs. Not hear-say on my part, but from experience in using an 8SE and an 8" Meade Coma Free SCT. Still, even in my 30 year old C8, the image is lovely. SCT get too much of a bad rap by people who don't see the place in the amateur arsenal. If space was at a premium for you, an SCT, and Mak's too, are an excellent option.

By the way, the rings of Saturn are certainly very easy to make out detail in a 130mm scope. My old C5, a 5" SCT, gave me extraordinary images of Saturn's rings, A, B & C rings, plus Cassini's Division. Here's something to give you a sense of scale with the rings: The entire north-south length of the continent of South America would fit inside the width of the Cassini Division, and still have several hundred km's to spare!

[Amaranthus (Barry)]

The 130 SLT has quite decent optics. I think the first step to improving your views would be to invest in a quality medium-power eyepiece that won't blow your budget, e.g. an ES 82D 14 mm for about $175. I think you'll be stunned by the transformation in what you see with your telescope!

[Camelopardalis (Dunk)]

I love my little C8! It has shown me plenty - and wish it continues to do so

Are SCTs for everyone? No. Is a big Dob for everyone? No also.

For visual, a 4 or 5" apo isn't going to show much/any more than the 130 already is.


A couple of things I'd suggest... watch out for nights of good seeing and transparency, and also get out to a dark site sometime

[acropolite (Phil)]

+ 1 for favourable comments re the 8 inch SCT. That size is very portable, easy to lug about and with GoTo easy to use and find targets.

I've compared the views between my 8 inch meade and much larger dobs, the difference isn't as great as the numbers would indicate. If your need is visual certainly an 8 inch SCT.

That said, if I were buying a new scope for visual with a budget of $2000, I'd be tempted to go for a 16 inch dob, perhaps second hand then fit an Argo Navis.

[brian nordstrom (As avatar)]

+2 on the SCT,s I received a nice C9.25 carbon fiber OTA from the classifieds here for $1200 and its a beautiful scope that performs very,very well.
I will be keeping it for a long time I think.
You never know your luck if you keep your eyes on the classified section here at IIS.
Brian.

[richard2600 (Richard)]

Yes im keeping an eye out you never know what comes up. There a meade 10 inch on ebay at the moment looks like a bargain but is in Melbourne dam

[ausastronomer (John Bambury)]

Originally Posted by mental4astro

Marc, you make SCT's sound like poison !

SCT's are fine visual instruments.

Hi Alex,

I would say they are "reasonable" visual instruments. Pure physics determines that a SCT cannot equal the optical "visual" performance of an equivalent aperture newtonian, or refactor. Nor can a MCT for that matter, although a high quality MCT like a Questar, Quantum, Astrophysics or Intes and Intes Micro will put up a very good show.The 7" Meade MCT is an excellent scope, but it won't outdo a good 7" newt or refractor. Keep in mind also that these Maks are premium scopes and not typical of the mass produced SCT's. In 40 years of searching for the ultimate SCT that can prove the physics wrong, I am yet to find one, despite a heck of a lot of trying. That doesn't mean that some with good optics can't give nice views, they can't match the performance of an equal aperture newt or refractor, although the good ones with decent optics and properly cooled can put up a 1/2 decent fight. Unfortunately a lot of SCT's have marginal optics. Rod Berry (Rodstar) sold his rather expensive 10" Meade LX200 after 2 years of frustration. Every time we observed together, which was quite often back then, Rod would be continually dissappointed with the high power views in his scope compared to my 10" newt. I felt the performance of Rod's scope was typical of the performance of a lot of other SCT's I had looked through over the years from both Meade and Celestron, ranging in aperture from 5" to 16". Rod subsequently sold the SCT and bought the Mary Rose (20"/f5 SDM) and hasn't regretted his decision one bit. The performance of Rod's scope was partly due to mediocre optics and partly due to the cooling issues of a closed tube design. My 10"/f5 GSO newt, which has a great mirror in it for an $800 scope will pull 500x on night of good seeing. My 10"/f5.3 SDM with Suchting primary will pull 675x (5mm Pentax XW + 2.5x TV Powermate) on a night of good seeing. How many mass produced SCT's can do that?

That's not to say SCT's don't have their place. They make an excellent all rounder if you plan to do both visual and imaging with the one telescope and they offer a portable, transportable package. The quality of lunar planetary images Damian Peach takes with 11" and 14" SCT's is testament to that. However, if you want ultimate visual performance from a telescope then a SCT is not the best choice IMO.

Richard has indicated he is only interested in visual astronomy. If storage and transport are not a concern then a 10" GOTO dob ($1,499) is a way better choice than an 8" SCT IMO. He can buy one of these new for what an 8" SCT will cost him 2nd hand if he's lucky; and will see a whole lot more tnan the 8" SCT can show him.

http://www.bintel.com.au/Telescopes/...oductview.aspx

If transport and storage are issues then a 10" collapsible tube dob may be a better option.

http://www.ozscopes.com.au/skywatche...telescope.html

If Richard wants to try imaging at a later time he could start with a small refractor for imaging and keep the newtonian for visual.

Cheers
John B

[brian nordstrom (As avatar)]

That's all and good John , I had and loved a nice Takahashi Mewlon210 ( but hated the diffraction spikes on the brighter objects ) and yes quality optics rule , I had taken mine to 806x ( 3mm TV Radian) on Saturn and jupter with perfect views on those rare perfect nights .

I took my C9.25 to 470x ( seeing was average ) the other night ( 5mm TV Radian) and the views were picture perfect and a better night would have allowed me to go higher , I might have a good one here but its views are almost as good as the Tak , ( and NO spikes ).
The Tak had better contrast , but again that's suggestive if you have not looked thru both .

No-body expects a garden viarity SCT to equal the beautiful SDM , and not all of us can afford one anyway , I would love one but cant justify the cost so I will stay with my 2 refractors and C9.25 .

Hand nudging a 10 inch GSO DOB at 500x is NOT EASY ! for the newby and might cause it to scare them away , so those expectations are a little high , keep it below 200x and its still an art but definatly do-able .

Another + with a SCT is that its a fully usuable set up from the word go , OTA , mount and tripod with drives and maybe GOTO as well if lucky but most today have that anyway .

Richard don't write off a SCT as these are the longest running production scope ever ( VW beetle of scopes ) , so they must be doing something right ? , if you get a good one you wont look back I can assure you and todays optics and coatings are top notch ! .
My 5c .

Brian.

[Amaranthus (Barry)]

For the record, I agree with most of what has been said above - I absolutely love my Celestron Nexstar 8SE as a 'jack-of-all-trades' workhorse.

If you're only going to have 1 telescope, and you want to do a bit of everything (planetary, DSO, solar, lunar), then it's fantastic. Put some good eyepieces in it, and it really shines. Not perfect nor best at everything (best at nothing, perhaps), but as an all rounder, hard to go past. I doubt I'll ever sell mine.

[Astro_Bot]

Originally Posted by ausastronomer

Pure physics determines that a SCT cannot equal the optical "visual" performance of an equivalent aperture newtonian, or refactor.

Pure physics doesn't build affordable telescopes. Most people are limited by more practical concerns like size, weight and cost.

The performance of Rod's scope was partly due to mediocre optics and partly due to the cooling issues of a closed tube design.

Sure, an SCT takes longer to cool. I put mine out at sunset and by the time I've setup and finished alignment, it's had about 2 hours of cooling under a clear sky - quite enough. It's no impediment at all in a typical evening's viewing.

My 10"/f5 GSO newt, which has a great mirror in it for an $800 scope will pull 500x on night of good seeing. My 10"/f5.3 SDM with Suchting primary will pull 675x (5mm Pentax XW + 2.5x TV Powermate) on a night of good seeing. How many mass produced SCT's can do that?

In my 10" LX200-ACF, I've had 420x on a night of good seeing and could have gone higher if I had a shorter focal length eyepiece (the image was crystal clear and steady at that power). We all know that these magnification limits are primarily determined by the seeing, not the 'scope, in any largish aperture used near sea level.

I've looked through 10" and 12" dobs of GSO and Skywatcher origin at many a star party and really can't say I'm impressed, even with the larger 12" aperture - I'm not saying they're bad, but they're not as good as you claim them to be. On the other hand, at every star party I've been to, I receive complements on the relative quality of the view through my SCT and frequently hear things like, "Wow, you've got the best view here". I don't call that "mediocre optics".

That's not to say SCT's don't have their place. They make an excellent all rounder if you plan to do both visual and imaging with the one telescope and they offer a portable, transportable package.

They do make an excellent all-rounder, but you don't need to do imaging to make that the case - they're a good all-rounder just for visual. And some people hate the diffraction spikes of a Newtonian with a vengeance.

If storage and transport are not a concern ...

There's the rub. Storage and transport are a concern for most people - not so much for many of the posters on this site, who can afford their own backyard or remote observatories, but for everyday amateurs who have, at best, a spare room or corner of a garage and an average car to get around. And heaven forbid if you have to negotiate stairs!

Horses for courses. A Dob is cheaper than an SCT (with mount) of the same aperture. If you can transport it (or don't need to); if that's all you want and you don't mind diffraction spikes, then fine. But don't talk down SCTs on some theory that they will be always be inferior - in mass-produced examples, they are not.

[richard2600 (Richard)]

Thanks for all info from everyone plenty to think about. I think I will look at goto dobs and celestron se8 type scopes

[SkyWatch (Dean)]

I have to agree with Astro_Bot.
I assume that when John talks "physics", he is referring to the large secondary of the SCT's that causes a loss of contrast and therefore slightly poorer view to an equivalent newt or refractor. This however would only be if the newt had an optimised (minimum size) secondary- and of course a top refractor will always "beat" (read: "have better contrast than") the equivalent size SCT, MCT or whatever because there are no restrictions on the optical path (assuming all other things are equal).
I have owned and used all of these types, and I have to say that my Celestron 800 CPC has excellent optics (as did my little MCT). I have never had problems with cool-down, and on a good night it has given terrific high-power planetary views, as well as very nice contrasty views of all types of nebulae.
- BTW, its optics improved considerably when I put a 2" GSO diagonal on it instead of the crappy 1 1/4 one that was supplied (why does Celestron do that?!).
If you put my SCT up against an equivalent, top quality, optimised 8" newt I suspect only very experienced eyes would tell the difference, and even then it would be only on nights of top viewing. (I also use a 4" Tak refractor and a good 12" Newt, so I am aware of the differences).
While the 130 is no doubt a very nice scope, it will still be a quantum jump to a good quality 8" SCT which has more than double the light-gathering power.
A good quality 10" newt or above will be another jump, but you have to take into account portability, ease of set-up etc.
A very experienced friend has used an 8"LX90 for over 15 years, he has happily logged many 1000's of objects, taken it all over outback SA- and he has only just recently upgraded; to a 16" Suchting scope... I never heard any complaints from him about the optics of the SCT, and still don't even when he has the 16" to compare.
So back to Richard's original question: yes, a reasonable quality 8" SCT will an upgrade over the 130. But I would definitely go to a few star parties and look at a range of scopes before making the decision.
A great problem to have too!
All the best,

- Dean

[ausastronomer (John Bambury)]

Originally Posted by Astro_Bot

In my 10" LX200-ACF, I've had 420x on a night of good seeing and could have gone higher if I had a shorter focal length eyepiece (the image was crystal clear and steady at that power).

That's a $5,000 telescope you have there. That's a bit more than the 10" collapsible tube Skywatcher I recommended which has GOTO and tracking for $1,700

Originally Posted by Astro_Bot

I've looked through 10" and 12" dobs of GSO and Skywatcher origin at many a star party and really can't say I'm impressed, even with the larger 12" aperture - I'm not saying they're bad, but they're not as good as you claim them to be.

There's the rub. It takes a lot more effort and know how to set up a Newtonian telescope properly, so that it can perform at it's maximum capability. I would guess probably 75% of the 10" and 12" Newtonians you have looked through at star parties haven't been properly set up. In addition a lot of people have the know how but just can't be bothered to set their newtonian telescope up properly. I assumed Rob would know how to set the scope up properly on the basis that he already owns a Newtonian. If he doesn't he lives in Sydney with ready access to plenty of people who would be happy to help him. What I can tell you is that a lot of people with much larger Newtonians (ie well over 20") don't have the know how or make the effort to set their telescope up properly. 3 seconds at the eyepiece of their telescope quickly tells you that. I don't say anything, I just don't waste my time looking through it again. I know of one person who owns a premium 18" telescope who has nfi how to set it up and collimate it, despite being told it wasn't right. Consequently his scope will badly underperform every single time he uses it.

There is no argument from me that its a lot easier to get a SCT performing at its best than a Newtonian. Generally a SCT comes collimated out of the box and it will rarely move.

Originally Posted by Astro_Bot

"On the other hand, at every star party I've been to, I receive complements on the relative quality of the view through my SCT and frequently hear things like, "Wow, you've got the best view here". I don't call that "mediocre optics".

Unless you want to go home disappointed I suggest you never set it up anywhere near my 10" or 14" SDM's, Rick Petrie's 14" Skywatcher, or Rod Berry's 20" SDM

Originally Posted by SkyWatch

I assume that when John talks "physics", he is referring to the large secondary of the SCT's that causes a loss of contrast and therefore slightly poorer view to an equivalent newt or refractor. This however would only be if the newt had an optimised (minimum size) secondary

Hi Dean,

There's a bit more to it than just the size of the secondary central obstruction (CO), although that in itself is important enough. In addition the SCT, ACF or MCT have an additional 2 refractive surfaces, as well as the 2 similar reflective surfaces of a Newtonian.

I have attached a table which shows the light loss comparing an unobstructed 10" aperture, to an optimised Newtonian with an 18% CO, to a mass produced Newtonian with a 25% CO and a SCT design with a 34% CO. Some SCT models have the CO down as low as 32%, some are as high as 38%. Some Newtonians have a lower CO as well. As you can see from the table the Optimised Newtonian has an 11.5% light gathering advantage over the SCT design and the mass produced Newtonian has an 8.2% light gathering advantage over the SCT design. The experts who have conducted laboratory tests on human visual perception will tell you that it takes a 5% change in light intensity to be perceptible to the human eye. In both cases the numbers are well in excess of this. Notwithstanding any perceptible difference in the view on bright targets it can make a significant difference in the observers ability to detect targets on the verge of visibility for a given aperture. In essence a 10" Newtonian will see "deeper" on threshold targets than a 10" SCT.

In addition the larger central obstruction and additional refractive and reflective surfaces of the SCT design cause further degradation of the image quality and reduce contrast. Each optical surface in a system introduces a small degree of light scatter and diffraction. The less of these surfaces the better.

If you have a look at Damien Peach's Optical Simulation you will see the effect of a change in Central Obstruction from 20% to 30% in part 2 of the simulation examples. It's not huge but noticeable.

http://www.damianpeach.com/simulation.htm

This website explains the effects of central obstruction on the MTF curves
http://www.telescope-optics.net/obstruction.htm

In addition to the central obstruction effect the additional refractive surfaces cause further image degradation.

Cheers,
John B

[Astro_Bot]

Originally Posted by ausastronomer

That's a $5,000 telescope you have there. That's a bit more than the 10" collapsible tube Skywatcher I recommended which has GOTO and tracking for $1,700

Around $4k, given it's on an EQ6 Pro. And that "tracking" on the Dob isn't in the same class, even for visual, so I'm getting much more for the money. I've never much liked the "step stair" tracking of a Skywatcher Dob.

There's the rub. It takes a lot more effort and know how to set up a Newtonian telescope properly, so that it can perform at it's maximum capability. I would guess probably 75% of the 10" and 12" Newtonians you have looked through at star parties haven't been properly set up.

You've not been at those star parties, and don't know the people involved. Plenty of them are diligent astronomers well aware of how to collimate a Newtonian and who invest significant effort in the process. But they are all stock-standard, mass-produced Dobs.

In addition a lot of people have the know how but just can't be bothered to set their newtonian telescope up properly. I assumed Rob would know how to set the scope up properly on the basis that he already owns a Newtonian. If he doesn't he lives in Sydney with ready access to plenty of people who would be happy to help him.

You mean Richard? Anyway, I've been talking about well collimated Newtonians (mass-produced) compared to well-collimated SCTs (mass-produced). Comparison to an SDM (or anything else with premium optics) is irrelevant.

Unless you want to go home disappointed I suggest you never set it up anywhere near my 10" or 14" SDM's, Rick Petrie's 14" Skywatcher, or Rod Berry's 20" SDM

Comparing SDM scopes to a 10" mass-produced SCT .... yup, that's a fair comparison. SDMs start at $10k and up not including the primary mirror. $4k for a 10" SCT looks pretty good in that comparison.

Like I said:

Originally Posted by Astro_Bot

Horses for courses. A Dob is cheaper than an SCT (with mount) of the same aperture. If you can transport it (or don't need to); if that's all you want and you don't mind diffraction spikes, then fine. But don't talk down SCTs on some theory that they will be always be inferior - in mass-produced examples, they are not.

[Camelopardalis (Dunk)]

Interesting aside that Damian Peach often chooses to image with a C9.25...the Celestron SCT with the largest diameter secondary obstruction

[Rodstar (Rod)]

Hi all,

Interesting thread, my ears were burning.....

Without wanting to step into the cross-fire, I will just speak from personal experience. I have been observing for over 9 years.

I had a 10" LX200 for several years early on, and would have observed through it 2-3 per week over that entire period. I observed under suburban, semi-rural, and dark skies, including at Ilford. I got to know its optical performance pretty well. I have also observed through numerous other SCTs, particularly an 8" SCT and an LXR200, also under a range of conditions including dark skies.

Whilst I had the LX200, I often observed with other amateurs on the central coast. I spent many nights observing with Mike Salway, comparing views side by side with his 10" basic level dob (Bintel?), and John Bambury's 10" dob (modified, with an outstanding mirror), along with several 8" and even a 6 inch dob.

I have never observed for any length of time through a Celestron SCT, so my comments about SCTs are limited to Meade instruments.

BOTTOM LINE - The views through the SCTs I used were quite obviously inferior to the views through the same or similar sized dob. Some nights I ended up pretty upset, wondering why I had spent $5.2K for my LX200 when Mike had spent only $900 for his, and the views through his scope were quite obviously superior. Galaxies that were a mere blob in my scope were more clearly defined in his scope. Doubles were more clearly split in his scope etc. etc.

An SCT has a longer focal length than a dob of similar aperture. This will result in the SCT showing a higher power view (smaller portion of sky) than a dob with the same eyepiece. The problem is that the quality of that higher power view is inferior.

SCT's suffer badly from dew, as their design involves glass against the open air. This can be reduced with a dew shield, but even still, this is far more vulnerable than having the primary mirror at the base of a dob.

There are some benefits to an SCT, and I learnt a lot and enjoyed my LX200. I loved the database and tracking. I enjoyed being able to observe in a seat. On the downside, though, I found the mirror shift from using the focuser very irritating, requiring constant refocusing. My solution was a crayford focuser, but this would hit the bottom of the fork mount near zenith, which brought its own frustrations!

If I was looking to purchase a 10" for visual use again, and was planning to keep the scope I purchase for more than a year or two, I would (without hesitation) go for a dob over an SCT.