SCT and the moon

[ngc2403]

thank you for taking the time to read this.

i will be honest i don't like SCT or the moon much but i understand that they do give a long focal lenght for detailed lunar work.

However why do so many people have them? do they all like the moon more than DSO? a know about the flatteners that come with them but only the f/6.3 reducer is good, i found the f/3.3 rubbish the field was not flat even for a webcam sized chip.

on lifting one 10'' SCT yesterday i found that it was very heavy too.

so please can people with sct's post our thoughts and feeling about them

thank you

[GazOC]

A major attraction with SCTs is that you get a lot of aperture in a relatively small package, Newts of same size as 8"-14" f10 SCTs would be huge. They arn't 'just' lunar scopes, they are decent at deep sky objects as well.

[Whippy]

I'd contend that Mak-Casses would be better for Lunar because of their longer focal ratio...

Tony..

[GazOC]

I agree Tony, but once you get over 8" they get far too expensive for us mere mortals and I'm not sure the difference on the Moon would be that dramatic TBH.

[Whippy]

Indeed Gaz, but you know you'd love to look through those AP 10" Maks . The weight of the Meniscus alone would kill most mounts!

Tony..

[GazOC]

Yep mate, thats one of my "Lottery Scopes", realistically I hope to have a Russian 8" Mak at some point in the future though (maybe a retirement pressie to myself?)

[Whippy]

I know someone who's got a 6" for sale... .

[MikeP]

I started out with a 10" Newtonian on an EQ6. For someone who is not very tall and has a bad back / neck that combination was a nightmare of using steps and continually rotating the OTA in the rings to get to the eyepiece and / or finder.

The Newtonian was sold and replaced with a C11. I wanted aperture, goto and something easier to observe with. The C11 met all three for me. Whether the C11 is heavier or lighter than the 10" Newtonian I'm not sure, but is a heck of a lot easier to carry and put onto the mount.

I rarely look at the moon or planets because of problems with trees, but when I do the views are superb. So despite the long focal length the C11 is mostly used for DSOs. It works OK for me even though the maximum FOV I can get is not as big as I would like.

To answer your question, compactness and convenience are the main benefits for me.

However, by way of a postscript, the time spent imaging with my ED80 is tying up the EQ6, so ironically I will be buying a 12" dob so that I can observe while the subs roll in (if we ever get a clear night). The dob can be moved around to dodge the trees and I'll be forced to find my way round the sky.

Mike

[davekermito]

I have a 6" SCT and use it for various targets (ie neither specifically DSO or solar system/lunar). For me it offers the best aperture size to be snugly housed in my garage when I'm not using it! I agree there isn't much of a weight saving, but the compact nature of the SCT means I'm much more likely to use it due to practicalities.

Of course there are definite cons to having an SCT though especially the narrow FOV, but hey at least it's not dented due to whacking it off door ways etc. !

[ngc2403]

ok,

yes i can see the compacked nature of the telescope is a good thing i understand how that would help.

gaz, i would like to ask would DSO can be seen through an f/10 scope. All deep sky object would be faint as, and the price of the very good mak's are good but then they are pricey. however a Newt on a sw mount is cheaper than a SCT off the same aperture?

i know that the eyepiece being in a good place all the time is also a benefit but if you are taking images then that is not a problem?

is the long focal lenght a big thing?

[mike bacanin]

SCT's are capable of surprisingly good planetary and lunar performance when perfectly collimated, despite the central obstruction. even a small degree of miscollimation can soften the details.

they are very compact for quite large apertures, and some used examples are a good buy for the money.

occasionally you find one with excellent optics.

contrast is not as good as some other designs (my Tak sct is an exception!) but i have had some wonderful views of jupiter with a very fine C8.

the closed tube is for me a nice thing, no need to worry about cleaning the primary! you do need to allow sufficient cooldown for optimal performance.

they are also rigid on a suitable mount due to the very short tube, hence no real moment arm effect.

i also had a fantastic view several years ago with a c9.25 using denk binoviewers of M42.

very versatile scopes,and if you are lucky to come across an example with better than average optics, you will be happy with the views!

mike

[GazOC]

ok,

yes i can see the compacked nature of the telescope is a good thing i understand how that would help.

gaz, i would like to ask would DSO can be seen through an f/10 scope. All deep sky object would be faint as, and the price of the very good mak's are good but then they are pricey. however a Newt on a sw mount is cheaper than a SCT off the same aperture?

i know that the eyepiece being in a good place all the time is also a benefit but if you are taking images then that is not a problem?

is the long focal lenght a big thing?

Globs, PNs etc are bright enough at f10 (f15 is fine on my Mak), with an SCT you also have the choice of going down to f6 with a reducer. Yep, Newts are cheaper but they are harder to mount, the eyepiece can end up in some awkward positions when on a GEM and the range of focus usually precludes using a reducer of any kind.

Having said that, up to 8" I prefer Newts, anything larger than that and the compact nature of the SCT makes that more atttactive, but thats just a personal opinion. Newts are cheaper because they are a easier design to implement, its a simple as that.

[Helen]

Every scope is a compromise. But for an all-rounder I think my C8 is great. Its convenient to carry and store, eyepiece accessible, works with FR and then there is fastar

Helen

[ngc2403]

yeah the larger sct are more practical then the large newts but the large sct's that i used had a problem with focus. i didn't like moving the optical compents to find focus because you start to lose collimation unless it is perfect! plus moving the main mirror is heavy work and i found that i would slope around when slewing across the sky thus collimation lost and focus?

hello mike,

what do you mean by excellent optics?

ally

[ngc2403]

hey helen,

i would say that a C8 would be a nice scope but i think that i would have it as to complete a collection not as the ultimate scope. even at that i think it would be a mak or Klevzov cassigrain, something where the main mirror didn't move to focus

[GazOC]

Theres nothing in the SCT design that means the mirror has to move to focus, thats just the way that Meade and Celestron have decided to do it. Plenty of Maks have moving mirrors, its a double edged sword, you get a larger range of focus at the expense of mirror flop and other (slight) optical aberrations.

[Whippy]

My understanding is that the feathertouch focusers cut the image shift right down on SCT's. My Intes to all intents and purposes has no image shift either and that uses a moving mirror focusing system. You can always lock the mirror and use a crayford on the back too.

Have to agree with Gaz here, up to 8", Newts aren't an issue size wise but you can't argue with compound scopes if you want to go bigger. The lack of size just makes them more practical. Last year at Kelling, I saw M13 through FLO's CPC1100 and a Tak Mewlon 300 (I think, might have been a 250) and both were memorable sights!

Like all scopes, they have their pros and cons.

Tony..

[mike bacanin]

Hi Ally,

most mass produced SCT's have a final wavefront typically 1/4 wave, and sometimes slightly worse,

but there are examples which have shown 1/6 wave error. in a mass produced situation you will always get a variation, but meade and celestron state the optics are diffraction limited, which should resolve to the theoretical limit for the given aperture.

just by example, my tak sct has a final wavefront of at least 1/8 wave, but it is essentially a handbuilt scope, only 100 were ever made by takahashi.

the downside to this particular sct is that it is very expensive used!

regards

mike

[Talitha]

When deciding to step up in aperture from my first scope (ETX 90RA), the 8" SCT seemed ideal because it was advertised as giving good views of both DSOs and solar system objects. At the time, I wasn't quite sure what would turn out to be my preference.. turns out I love them both.

To be perfectly honest, the 8" SCT is the favorite of all my scopes. Being an f/10, it brings me in nice and close to my targets, and this has allowed me to study Luna and get to know her face as well as I know the back of my hand.

The 8" aperture was adequate enough to show me all 110 Messiers and almost all of both Herschel 400 lists. (I completed the first list but missed some targets on the second one because of their low surface brightness.)

As already mentioned, a focal reducer can be used to speed the scope up to an f/6.3.. ideal for cruising around or sketching larger open clusters.

In hindsight I should have purchased a 7"-8" Mak for lunar work and a 20" Dob for deep sky, but would have had to sell a lung to buy them, and wouldn't be able to physically handle the Dob, so I'm more than happy with the 8" SCT.

Sure, there's always something else that looks bigger and better, but if you push yourself and your equipment to the limit, it's amazing how far you can get.

[narrowbandpaul]

well done on the messier and herschel lists. i am impressed

[dogfish]

I too love my C8 and echo all the comments about portability for the aperture you get - very important if you can't have a permanent set up and especially so if you have stairs. The narrow FOV is only occasionally a pain (M45 springs immediately to mind) but for most objects a low power, widefield EP solves the issue (I've the Rigel 70 degree 38mm EP, which is great). The other benefit is with it being a slower scope, it's much more forgiving of EP quality, which can save you a fortune. In comparison to my little Megrez, the contrast is definitely poorer on planets and luna, but that's more than made up for by the increased image scale and aperture.

HTH, Martin

[ngc2403]

hey all,

well done on getting all those lovely objects through the eyepiece. :hello1: :hello1: :hello1:

From what you say it sounds like you like the focal length of the scope which is fine and the correct scope for you to buy, but if you were able to see all those objects then ur sky must be good enough for all that focal length.

Optical quality and moving mirrors, i find the idea of moving the mirror a bad one for a start the secondary has to be bigger to handle the motion and changing light path and that takes more contrast and puts pressure on the central part that the mirror moves on to be perfectly central and straight. i don't think that i trust the likes of Meade and celestron to make a central post for a 12'' or 14'' mirror perfectly straight. plus those mirrors are heavy too.

they sell telescopes with optics that are diffraction limited? well am yet to find a telescope that is not bound by the laws of diffraction but having used SCT's in the past in Britain i think that they are always stopped well short of diffraction limited by the atmosphere. If you think about it SCT's have more optical surfaces than an disco ball and to believe that they all work together to give 1/6th wave would be suprising. i don't know how they add errors of multiple optical componets but my guess is square the error on each optical component, add them together and then square root it. if you are using a SCT you have at least three optical surfaces. With that even if they all work at 1/6th wave they give worse than 1/4th wave together. :?

the telescopes with focusers fitted to the back like the feather touch ones are good they stop the need to move the main mirror but only if you stay with one setup ( one reducer setup) cause the travel from one reducer focal point to the next is to far

ally

[Talitha]

Thank you Paul, it was a great learning experience.

[Paxo]

Ally,

I have a 12" Meade SCT (LX200R). It has better than 1/6th wave optics and a perfectly aligned central post. It also has an extremely flat coma free field at 35mm sizes (which most people would die for). Meade have gotten themselves a perceived bad reputation and there have been many Meade bashing threads on this forum, but give some credit to an organisation who brought affordable reasonable quality optics to the masses. I have been critical in the past over the finishing touches on some of their scopes, but I, and an awful lot of people, are happy with their sct's It is clear from the thread you are not particularly keen on either sct's or Meade, but we live in a world of you pay your money and you take your choice.

I happen to take my 12" instrument to star parties, yes it's heavy but I am extremely happy with it. On its permanent mount I can image for 20 minutes unguided with no noticeable drift, I get objects bang in the centre of my Atik camera with goto when imaging, it does everything I ask.

The only drawback is for widefield imaging, ut all I do is piggyback a WO72 to it, you would have the same problem with most newts of this aperture (and no flat coma free field by design either).

Steve..

[GazOC]

Steve, given that Meade were the major supplier of scopes for years and the more scopes sold generally means the more the complaints you'll hear, I don't think that we've had too much Meade bashing on SGL?

Gaz