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SCT Mythology

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#1 mloffland



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Posted 21 August 2008 - 02:44 PM

SCT Mythology

By: Rod Mollise

#2 ranger


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Posted 21 August 2008 - 03:15 PM

Nicely done but what about the most fatal SCT flaw of all time -- dew?

#3 BlueRidge


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Posted 21 August 2008 - 03:34 PM

Very entertaining. Great reading.

Bill H.
Syria, VA

#4 Lee Jay

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Posted 21 August 2008 - 03:55 PM

Is there a myth in the article?

"What most folks are actually seeing when they note the nice BLACK BACKGROUND in a Mak (including Sweet Charity) is the naturally smaller exit pupils generated by the scopes’ (usually) slower focal ratios and resulting longer focal lengths—the average MCT is around f/15; the average SCT is an f/10."

My understanding is that, at the same magnification, all scopes of a given aperture (ignore the CO for now) will have the same background brightness in the same conditions. Thus, the reasons for the Mak's reputation in this area come down to:

1) Smaller apertures
2) Using higher magnifications

What you said in the article could be related to number 2 above, if you use the same eye piece on both f10 and f15 scopes of the same aperture, because that doesn't result in the same magnification. But wouldn't folks pick their magnification based on the object they are looking at and the current conditions, not the f-ratio of their scope? In other words, if you'd use a 10mm EP in an f10 5" SCT, wouldn't the same user choose a 15mm EP in an f15 5" Mak?

#5 Bonco



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Posted 21 August 2008 - 04:05 PM

Excellent read. Most of what was written I heard or experienced over the years. Bought my C8 around 1974. Attended an astronomy class offered by the Univ of Texas. The master's degree student instructor told me in a very condescending tone, "You know those corrector's are made from plate window glass"...He lost all cedibility with me with that stupid comment.
Bill N.

#6 bcuddihee



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Posted 21 August 2008 - 06:00 PM

Thanks Rod for demystifying the Cat. I have a C8SE that has very nice optics, star tests well, but....rather than think that I have a singular example of fine optics , I prefer to think that most folks just do not or will not collimate their optics..."Collimphobia".
I routinely see numerous posts decrying the SCT's "soft images" and I'm sure thats exactly what they are seeing. Unfortunately, the same folks say their scope are collimated "out of the box" because the big dark spot is centered in the big donut. We know ..that is not a star test by anyones definition. I feel that I have been on a sort of quest lately trying to inform the uninformed. The remarks that SCT's are terrible planetary scopes, or that cool down times make the scopes unusable in northern climates just a sampling..I've heard them all. Thanks for adding your voice to the mix. It is not one of the most popular designs for no reason and the current crop seems to be very well made.
Thanks again,
bc :cool:

#7 bluedandelion



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Posted 22 August 2008 - 06:57 PM

Very informative and well argued in Uncle Rod's own inimitable style. I chuckled out aloud more than once yet learned a few things.

#8 Mike Rapchak

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Posted 22 August 2008 - 08:23 PM

Very good, Rod! Now, how about some follow-up articles to cover more SCT myths? :grin:

Mike Rapchak Jr.

#9 David Pavlich

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Posted 22 August 2008 - 09:23 PM

Rod, as a confirmed fan of the SC, I raise a toast to you and your article. SCs get a lot of bad press. Your article lays to rest a lot of the bugaboos that keep prospective users from taking the leap. I was at an outreach with my ole' C6R. I was parked next to the club president that had a C11 on a G11. I looked at Jupiter at about 350X and I was hooked. Guess what got sold and guess what got bought? :grin:

Thanks again for a great article.


#10 gamccain



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Posted 23 August 2008 - 12:38 AM

Hello again Rod,

I'm the proud owner of a new Cele 9.25 and can't be happier.

I had avoided SCT's for over 15 years for all the wrong reasons you noted.

After 'discovering' your Urban observing book in the local library and realizing you were correct about size and magnification AND being completely blown-away with the imaging work a club member was doing with his 9.25 AND being frustrated with getting better observing in around Washington, D.C. with my 4" refractor, I ordered the Cele.

Since I got it I've been working at f/6.3 and gobbling up dozens of interesting objects with my Stella Cams; outstanding detail, bigger image scale and shorter integrations that I ever got with the 4", even working at f/5.67.

I can't wait for some steadier air to see how Cele will do at f /20 on planets and la lune.

Sorry I couldn't get up to AHSP and meet you in person. Hope to rectify that some day.

#11 Lamb0



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Posted 23 August 2008 - 06:44 AM

I have never preferred the SCT, I'm a Newtonian type of guy. In SCTs the contrast seemed to be lacking, and the images were rarely as sharp as I thought they should be. However, when David Knisely bought his NexStar 9.25, I was finally convinced they didn't need to be that way. The primary's somewhat longer f/ratio helps, and I also prefer the larger hole through the primary to better exercise the somewhat larger aperture. Of course, Dave keeps his scopes well collimated. However, the 9.25" just seems better optimized for Astronomy - in particular, the baffling isn't compromised for combined terrestrial/astronomical use. When combined with excellent coatings, the package delivers optical performance I like! I doubt I'll ever own a SCT design, it has mechanical compromises I don't appreciate. However, I certainly wouldn't want anything less than the C9.25 - it's definitely a cut above the ho-hum 8" SCTs by a visible margin. ;)

#12 RogerRZ


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Posted 23 August 2008 - 06:57 AM

. However, I certainly wouldn't want anything less than the C9.25 - it's definitely a cut above the ho-hum 8" SCTs by a visible margin. ;)

I would be willing to bet Mr Knisely's C9.25 is likely a cut above a ho-hum 9.25" too...

#13 Robin Lee

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Posted 23 August 2008 - 11:12 AM

Uncle Rod, I like your article! Keep it coming! :)

P/S: I'm going to re-read again to try to absorb every point. :p

#14 J_D_Metzger


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Posted 23 August 2008 - 11:44 AM

Hey Uncle Rod,

I have owned four catadioptrics; an LX-90, an LX200R, a C9.25, and most recently, a C8. Out of the four, my new chinese C8 OTA has, in my opinion, the best optics of the bunch. A couple of nights ago, I had an opportunity to observe Jupiter in excellent skies, and I chose to use the C8 over my 4" APO refractor, and was rewarded with my best views of the summer. If I ever have to sell one of my scopes, it won't be the C8... :)

#15 stevecoe


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Posted 23 August 2008 - 11:58 AM

Howdy all;

Much like many of you, I went for two decades without truly considering an SCT. Mostly driven by several views with poorly made or collimated C8s during those years. Then I attended Riverside and got some time with a Nexstar 11 GPS. I was impressed with the views and the electronics. AND...I was tired of star hopping. I sold off my 13 inch on a Bigfoot mount and eventually got a Nexstar 11 GPS. For over 5 years it was my primary scope. I liked it so much I replaced the Meade 7 inch Maksutov in my observatory with a Nexstar 8 GPS and enjoyed it for several years. It was great having the same handpaddle on both scopes so my aging memory did not have to try and remember two different sets of commands.

I saw lots of the sky with those two scopes and was happy with their performance. Once I was ready to move on I really changed my telescopes. I have sold both Nexstars, got a C6R refractor on a Sirius mount and it provides very different views of the sky and I am having a lot of fun looking at old favorites with this telescope. Early next year I am going to buy an RV and travel for a while, having the smaller and easy to set up scope will be helpful. I am happy with my decisions.

Clear skies;
Steve Coe

#16 Chris Schroeder

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Posted 23 August 2008 - 12:14 PM

Well done Uncle Rod, Astronomies version of the DSC's 'Myth Busters'

#17 bwilson


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Posted 23 August 2008 - 11:52 PM

Ah, Yhe Celestron Pacific Blue and White. We have one in the observatory, a C10 very nice. I suppose a collectors item now. Also a Compustar 14, still going strong from 1989, on its second hand controller. These are getting hard to find.

#18 jake47


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Posted 24 August 2008 - 01:08 AM

Great article. I hope it helps those who are in the same place I was a few years ago.

I was a complete newbie. Bought a NS11GPS for my wife for a Christmas present. She was the one who wanted a telescope and a telescope to her was a big old SCT. She was thinking an 8, but I decided to go bigger. Too big for her to set up, I became the helper. First light was Saturn and M13. That night, I became an astronomer - a newbie amateur - but a totally hooked one.

We loved the scope. Viewed from the drive way. Took it to dark skies. Amazed and happy - every time. We love the deep sky, globs especially. Then I started reading more on CN and some other forums. There I discovered that I had a "second rate" telescope design. That I was looking at blobby and mushy stars. That I couldn't possibly enjoy this scope. That I was too dumb to know the difference between our beloved scope and a real "serious" telescope.

I was sad. I wanted a good scope. So I started going to star parties. I went to some big ones where I would look through other scopes. Hmmmm. Not better than mine. Maybe I just didn't know what to look for. So I read and studied some more. More star parties. I would beg a look though this $10K scope and that $8K scope. Thank the nice man and scurry back to my NS11 to dial in the same object. Not once was I disappointed in what I saw in mine. Aperture differences, yes. But not sharpness. Field of view differences, yes, But not detail.

At one star party, lined up Omega Centauri at it's peak at about 11 degrees above horizon. Nice steady seeing. i was mesmerized. Many there were also taking this rare opportunity to view the giant glob. (Or not glob as we now seem to know). I finally got some to take a look through my 11. At these big parties, you rarely have people asking to look through an SCT. I think many took a look just to be polite. But what I got were several variations on the first two comments - what i call the SCT prayers - First was "Holy Mother of God" and the second was "Sweet Jesus". (I'm sorry if these offend anyone, but I assure you they were said in awe and reverence.)

Since then I have added a few scopes to my arsenal and decked out our beautiful Celeste. I wish I had seen Rod's article three years ago. It would have saved me from a bout of inferiority telescope complex that I no longer have. :D

#19 Matthew Ota

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Posted 24 August 2008 - 02:06 AM

Yes never let anybody knock an SCT. The sales figures prove their worthiness.

#20 RogerRZ


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Posted 24 August 2008 - 06:22 AM

Thanks for the debunk, Unk... Kinda happy to read this--tomorrow, I trade my big refractor for not one, but two, SCTs (a blue one, and a teal green one)...

How do you collimate again? :question: :shocked: :lol: :lol:

#21 ibase



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Posted 24 August 2008 - 12:52 PM

Unk Rod, you're my man!:bow: Thanks for championing the SCT, I'm eagerly awaiting for the release your new book about them - "Choosing and Using a new CAT.." - every CAT owner must get this gem of a book! ..and I don't mean the one that purrs :)


#22 David Knisely

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Posted 25 August 2008 - 12:22 AM

. However, I certainly wouldn't want anything less than the C9.25 - it's definitely a cut above the ho-hum 8" SCTs by a visible margin. ;)

I would be willing to bet Mr Knisely's C9.25 is likely a cut above a ho-hum 9.25" too...

Nope, it was one selected at random for review on Cloudynights. Overall, it is a good basic SCT with performance that was comparable enough to my 10 inch Newtonian that I went out and just bought the review instrument. With the Ronchi test, I can see a faint zonal defect near the 70% radius, but it is fairly minor and the bands are pretty straight. Since then, I have used a few other NexStars in the 9.25 and 11 inch range and found those I used to have equal optical quality (although that does not rule out someone getting a dog now and then). Indeed, that is what made me push Hyde Observatory into getting a NexStar 11 to replace our 12 inch Newtonian/Cassegrain. Our NexStar 11 at Hyde gives wonderful high-power images and has beaten our old 1970's vintage C14, so it is a very nice scope. Clear skies to you.

#23 rmollise



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Posted 25 August 2008 - 07:53 AM

Is there a myth in the article?

Not sure what you're calling a "myth" in the article. Shorter f/l eyepieces produce smaller exit pupils, higher magnifications, and darker backgrounds with longer focal length scopes. That's all I'm a-sayin...


#24 rmollise



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Posted 25 August 2008 - 07:54 AM

Nicely done but what about the most fatal SCT flaw of all time -- dew?

The DewBuster done drove a stake in the heart of that one.

#25 rmollise



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Posted 25 August 2008 - 07:55 AM


LOL! I'm stealin' that one!

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