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Always wanted to own an SCT but........

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#1 Guest_**DONOTDELETE**_*

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Posted 27 May 2004 - 09:16 PM

All of the sct's I have looked through always offer soft images, never seem to be able to bring them to a perfect focus. Is this inherent in the design with such a large central obstruction or were they all out of collimation or some other problem. I have viewed through an 8", 10" and 14" Sct and still my 8" reflector has sharper images and more contrast. This has been verified by others present also.
All opinons welcome :)

#2 erik

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Posted 27 May 2004 - 09:31 PM

may have been a cooling issue as they can take longer to cool down than newts because of the closed tube. i haven't noticed the images necessarily being "soft" in the sct's i've looked through, but the planets do appear "washed out" looking compared to some newts, due to the larger obstruction. sct's usually hold their collimation well, but are harder to collimate for a beginner than a newt.

#3 jrcrilly

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Posted 27 May 2004 - 09:53 PM

I have viewed through an 8", 10" and 14" Sct and still my 8" reflector has sharper images and more contrast. This has been verified by others present also.
All opinons welcome :)


It's hard to say; collimation is sometimes an issue because at F/2 the SCT requires more precision than even an F/4 Newt. Cooldown is also more of an issue with the SCT. What I have noticed many times when folks are comparing scopes side by side is that they aren't careful to match magnifications. The SCT's, with their longer focal lengths, tend to be used at higher mags and of course another telescope at lower magnification will give a sharper image - just as the SCT would if its magnification were reduced. The only Newt I've used that has noticeably higher contrast than my SCT's is my F/7.5, which has a tiny secondary obstruction. Faster Newts (like my F/4 Dob) have obstructions not that much smaller than SCT's.

#4 Guest_**DONOTDELETE**_*

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Posted 27 May 2004 - 10:06 PM

Hmmmmm...From what I can tell, an SCT has about twice the central obstruction that my newt has, percentage wise. Cooldown was not the issue since it was the same all night long. Many different eyepieces were used, so I wouldnt blame it on that. So, im thinking, CO or collimation. I sure would love to have an SCT, they have alot of pluses, but image quality has to be at the top, they have to throw up an image better than my homemade job..
Also, does the image always shift when focusing? I hate that ...
Im not trying to start a fight, just wondering if the ones I viewed through are not a good indication of SCT's in general.
Thanks for replying..

#5 wilash

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Posted 27 May 2004 - 11:41 PM

Why don't you buy a MCT?

#6 David Knisely

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Posted 28 May 2004 - 12:34 AM

All of the sct's I have looked through always offer soft images, never seem to be able to bring them to a perfect focus. Is this inherent in the design with such a large central obstruction or were they all out of collimation or some other problem. I have viewed through an 8", 10" and 14" Sct and still my 8" reflector has sharper images and more contrast. This has been verified by others present also.
All opinons welcome :)


Probably, your 8 inch reflector has a shorter f/ratio than an equivalent SCT. Thus, you approach focus faster on the reflector than you would the SCT and this can make the view seem to not "snap" as quickly. However, you really need to compare scopes which are nearly of equivalent aperture, and you need to see a lot of them under a variety of conditions before you make a final judgement. If you want some of the better SCT's, the Celestron NexStar GPS series has a lot going for it. I got a NexStar 9.25 GPS for a Cloudynights review and ended up buying it. It has about the sharpest optics in an SCT I have ever seen. My 10 inch f/5.6 provides a slightly better image at high power due to its larger aperture and smaller central obstruction, but quite frankly, the NexStar 9.25GPS was remarkably close in its performance. Right now, its getting most of the use, while my Newtonian tends to be sitting downstairs much of the time. Clear skies to you.

#7 BluewaterObserva

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Posted 28 May 2004 - 09:10 AM

SCT focusing is an aquired skill... I don't think many people especially if you have to adjust focus ever get them to sharp focus. JMI makes products that micro fine focus, SCT's with these always seem razor sharp to me.

#8 jrcrilly

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Posted 28 May 2004 - 09:44 AM

SCT focusing is an aquired skill... I don't think many people especially if you have to adjust focus ever get them to sharp focus.


I suppose that's another factor - since the primary mirror is F/2 the physical range of critical focus is smaller than on pretty much any non-catadioptric telescope.

#9 wilash

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Posted 28 May 2004 - 10:02 AM

SCT focusing is an aquired skill... I don't think many people especially if you have to adjust focus ever get them to sharp focus.


I suppose that's another factor - since the primary mirror is F/2 the physical range of critical focus is smaller than on pretty much any non-catadioptric telescope.


John, you have me confused. The effective focal length of an SCT is f/10. So the depth of focus is equal to an f/10 scope. Any intermediate focal length of the optical componants have nothing to do with the depth of focus for the system.

Actually, slow systems are difficult is focus because of the large depth of focus.

#10 Starman1

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Posted 28 May 2004 - 10:59 AM

Well, with typically 5 optical surfaces before the eyepiece (don't forget the diagonal), it's a wonder we get good images at all. However, I have noticed with my 8" (and I'm a stickler for perfect collimation) that seeing is critical to evaluating the image. Smaller scopes tend to be less affected by seeing (6" and down), while larger scopes are more affected.
In my scope, I have seen, on a night of 10/10 for seeing, perfect airy discs with a single diffraction ring around them at about 300X magnification. Most of the time the image is fuzzier, and 150X is about maximum for the scope (unless you enjoy blobby star images). So, I KNOW the scope has good optics, yet I don't see refractor-like images most of the time. And I usually pay attention to images after several hours at the nighttime temperatures, so the scope is thermally settled.
If the night requires a constant readjustment of focus, I face facts--the seeing is not good.
In Meades, the focusing image-shift can be mitigated by a careful adjustment of the focus tension (I believe the technique is on a tech paper from Meade from back in the '90s), but the image shift is due to the tolerances between the inner and outer baffle tubes for the primary mirror. Sometimes a change of grease or a redistribution of grease will help. Visually, the image shift is rarely severe, and just a character of the beast--you put up with it. Or, add a Crayford focuser to the visual back, do rough focusing with the usual knob and fine focusing with the Crayford. This, in essence, is what Meade does with the new LX200, and adds the step of mirror lockdown before final focusing. All this is totally unnecessary visually, but important for CCD's.
I installed a 2-1/2" focus knob from EZTelescopes. It did not eliminate image shift , but makes fine focus super-easy due to the large diameter.
Also, the bearings are so smooth the knob feels detached most of the time, yet focuses easily. Recommended.
On an SCT, you move the mirror to focus (normally), so the movement is magnified 5X (4.3 for David's 9.5") by the secondary. It does not focus as slowly as an f/10 reflector/refractor because of that. It really does focus like an f/2. Fortunately, the focuser screw has very fine threads and moves the primary very slowly.
An outboard Crayford focuser at the visual back focuses like an f/10 refractor, so the focus movement is slower. That's why Meade calls it "fine focus".
There is just one distance between the primary and corrector plate that results in diffraction-limited images. One of these days, I'd like to have an SCT with the mirror unmoveable, set at this distance, with a focuser on the visual back. It'd not be able to be used for anything except astronomy. And it'd be cheaper to make. But it would have the best star images possible in the design.
Also remember that the focal plane of an SCT is curved--the wider the field of the eyepiece, the more difficult to focus all the star images in the field. That's why I use a field flattener--not for the focal reduction (which does give wider fields), but for the image correction in the outside of my wide-field eyepieces.
So, with diagonal, I have 9 optical surfaces in my telescope. And the usual comments, from people who look through it, are along the lines of "boy, you have nice star images!" .
So, does the design inherently produce bad star images? No.
Is it difficult to produce a scope with good star images? No.
Is the design less likely, given normal circumstances, to produce poorer star images than, say, a refractor? Probably.
Most of the people I know who complain about the star images in an SCT don't stay up late enough to allow their scopes to reach thermal equilibrium. I don't think the design is a "grab-and-go" design.
Very little can beat the small refractor for that.
Don

#11 Guest_**DONOTDELETE**_*

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Posted 28 May 2004 - 04:06 PM

SCT focusing is an aquired skill... I don't think many people especially if you have to adjust focus ever get them to sharp focus. JMI makes products that micro fine focus, SCT's with these always seem razor sharp to me.


Can anyone explain this JMI forcuser and how it works? I've heard a few others here talk about ordering one. I am wondering if this is something that I might want / need for my SCT. Focusing is not an easy task to get the clearest view and it takes a little time and "tweeking" to get it at it's best. If there is an easier method out there, then I'm all for it! :D

#12 gazerjim

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Posted 28 May 2004 - 07:04 PM

Keith: I used my 9.25 for almost a year before seeing the fine images of which it is capable. All of the above mentioned factors are true, esp. cooling and collimation. I have also found that the corrector plate is susceptible to the formation of a light "haze" which is not readily noticeable, but reduces planetary contrast. IMHO, SCT's are quirky, but capable of excellent images. Contrast will probably never be quite as good as a long focus reflector or apo.

Jim

#13 werewolf6977

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Posted 29 May 2004 - 06:16 AM

Gazerjim, yeah, but they sure have some advantages, don't they? I'd need to go to Home Depot to buy a step-ladder, if my Nexstar 8 were an F/10 Newt, wouldn't I? I love the fact that my setup weighs 35 lbs. Heck, my Sp130EQ weighs 30 lbs! Without drives, and about 50% of the light gathering power! Pete

#14 Don W

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Posted 29 May 2004 - 10:50 AM

Angel, JMI sells a couple models of electric focusers. One replaces the stock unit with a motor and the other replaces the visual back with a motorized crayford type focuser. The biggest advantage with either type is that you eliminate vibrations caused by your hand when focusing. It's all done with a handcontroller with buttons. The second reason to get such a thing is for imaging, again to eliminate vibrations and give accurate focus control.

#15 conus

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Posted 29 May 2004 - 03:48 PM

Can anyone explain this JMI forcuser and how it works?


I know what you mean. When observing Jupiter or Saturn I spend a lot of time playing with the focus. I've decided to get the Feathertouch. Check out these reviews.

http://www.cloudynig...eathertouch.htm

http://www.cloudynig...ries/JMINGF.htm

#16 Don W

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Posted 29 May 2004 - 03:51 PM

Feathertouch also makes a dual speed knob that replaces the stock knob. It's very highly regarded as well as being much cheaper.

#17 Guest_**DONOTDELETE**_*

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Posted 29 May 2004 - 05:17 PM

300 dollars! Is this thing made out of Platinum? I want one! Will it never end? So if a guy was crazy enough to order one of these and this nut (me) has a Nexstar 8 GPS, I would need the adapter and which lenght focuser? What about the focuser bottoming out when you aim the scope striaght up?

#18 Guest_**DONOTDELETE**_*

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Posted 29 May 2004 - 05:43 PM

I like the JMI electric focuser, it's only $114, I wonder how accurate it is? Also, what about it bottoming out?

#19 LivingNDixie

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Posted 29 May 2004 - 10:12 PM

I have never been very fond of the electric focusers for SCTs, I prefer the manual version, that hooks up to the back of the scope and has regular focus knobs like a reflector


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