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What do people see in Schmidt-Cassegrains?

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#76 tim53

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Posted 04 October 2014 - 01:05 PM

The observing experience with a SCT is second to none in terms of comfort. 

 

Well, second to one, at least:  Russell Porter's Springfield mount.

 

-Tim.



#77 Jon Isaacs

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Posted 04 October 2014 - 01:26 PM

 

 

 

If I'm dating myself on imaging in the late 90's, I should be ridiculed for not having kept up. But, back then, it was quite a chore...the scope could easily cold down during set up time alone. Add to that winding all those cords, especially the long power cord, around your elbow took a lot of time to tear down and pack up in the wee hours.

 

These says, I don't even bother with GOTO. Keep it simple with a GEM mounted CAT. Plop and drop. It's amazing how quickly it's ready to go. A half hour under an ice pack and the scope is cooled to initial ambient. Soooo much easier, almost grab and go.

 

 

I'm not going to point out that you aren't keeping up.  But during the summer I can literally have my imaging system, set up and up and running in 3 minutes. And about 2 of those are the time it takes the old XP laptop to boot.  Flip the power switch, wake the mount up from hibernation, attach the Canon Camera to the T-ring, focus on a bright star a mouse click (or three) on TheSkyX and I'm good too go.

 

In the winter (when I don't leave the scope on the pier), it take about 5 minutes, but most of that is doing the ASPA and the 2+4 alignment to the stars.  Which I like to do at dusk so as to let the twilight highlight the brightest stars.  I go in, eat dinner and when it is nice and dark the scope is ready to go

 

Not that it is a race, but I almost always have my GEM and SCT up and running faster than most DOBs at star parties I attend. Tripod, head, OTA, HC, power, one cord. Easy.  If I create images at a star party they are short 60-90 second unguided efforts with an IR remote for the camera. Simple, fast, no extra wires.

 

For me, every time I look at (or though) a big DOB I'm left with the feeling, what a nice scope, if there was only some way to put it on an equatorial mount. so we could capture some of those awesome views :undecided:

 

As someone who has been apart of (on and off and on ) this hobby since the early 70's.  It took me years of pain, expense and frustration to discover the best telescope tends to be not the largest aperture, but tends to be the one on the best mount.  That is where the compact size of the SCT really shines. It helps the mount work better, since it is easier to balance, puts lower torque loads on the mount and is less of a wind sail.

 

 

Are you comparing the setup time for your GEM and SCT to similar aperture Dobsonians?  It takes me 10 minutes to setup my 16 inch Dob if it's inside the motor home. But that's a 16 inch scope, setting up a 16 inch SCT takes far longer than 10 minutes and setting up a 10 inch Dob far less.. 

 

Some years back I was interested in the time it took for me to setup my 12.5 inch inch tube Dob. At the time I had a Ford Escort hatch back and the scope was stuffed into the back.  So, that evening when I pulled into my spot, I turned off the motor, started the stop watch, opened the door, and went for it. Now I admit I was working as fast as I could.  Still, I had to open the rear trunks, remove the base, remove the OTA, put it in the base, get out the chair, install the finders... It took me 41 seconds.. In those years, I often setup alongside guys with 12 inch LX-200s, similar aperture.. Never saw anyone setup a 12 inch LX-200 in 41 seconds.. I did help sometimes and when all was said and done, most often it took more than 41 minutes.. 

 

As far as putting a Dob on an equatorial mount, as mentioned previously, it is rather easily done and retains the ergonomic virtues of the Dob design, easy setup, etc. As far as capturing the awesome views, photographs and images can never really capture the awesomeness of the view through the eyepiece.  Every so often, I drag out the camera and lap top and spend the evenning photographing the night sky.  When I am done, I am left with the feeling of dissatisfaction, I have spent the evening communing with my laptop and telescope rather than communing with the night sky.. 

 

Jon



#78 Jon Isaacs

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Posted 04 October 2014 - 01:50 PM

 

A good Newtonian on a GEM with rotating tube rings might provide larger aperture with similar comfort, but setup and breakdown have to be considered more difficult for that scope.

 

Gil:

 

I see you have a 10 inch Starfinder on a GEM.  Have you spent much time observing time with a 10 inch Dobsonian?  Back when I had both my 12.5 inch Meade RG with the rotating rings and all and my 12.5 inch Dob, the comfort difference was big.  In my exprience, larger equatorially mounted Newtonians are just awkward to use.. 

 

Jon



#79 dpippel

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Posted 04 October 2014 - 03:13 PM

Any scope that requires a ladder or standing a good amount of the time is awkward to use IMO. :)



#80 ur7x

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Posted 04 October 2014 - 11:04 PM

 

For me, every time I look at (or though) a big DOB I'm left with the feeling, what a nice scope, if there was only some way to put it on an equatorial mount. so we could capture some of those awesome views :undecided:

 

That's funny ... I've always looked for ways to get all my scopes OFF the equatorial mounts!  Anybody want to buy a GEM?

 

But then I'm strictly visual.  Mark Twain said, "Golf is a good walk ruined."  I agree with that.  But I'll do Twain one better.  "AP is a good view ruined."

 

:grin:

Mike

 

 

 

For me the hobby is about 90% visual and 10% AP.  I prefer a equatorial mount for both.  An alt az mount is like a heavy pair of binoculars,  great for a quick glance, but a real pain if you want to look for any amount of time. I've done my stint with Alt Az mounts, I have no desire to go back...

 

A gem mount models and works with the nights sky, an alt az mount fights it.  Alt az mounts are great for terrestrial viewing but they are a pain when looking at the sky.



#81 Astrojensen

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Posted 05 October 2014 - 03:50 AM

"Dobsonians can't be used for long-exposure astrophotography".

 

Really? Maybe so, but the results when you stack a lot of short-exposure images can be extremely impressive:

 

 

https://www.flickr.c...157625885306790

 

https://www.flickr.c...157625885306790

 

http://www.dansdobim...C2419_image.htm

 

http://www.equatoria...om/images.shtml

 

 

Clear skies!

Thomas, Denmark



#82 Astrojensen

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Posted 05 October 2014 - 03:52 AM

 

Well, second to one, at least:  Russell Porter's Springfield mount.

Or an equatorial coudé refractor. 

 

 

Clear skies!
Thomas, Denmark



#83 Atjous

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Posted 05 October 2014 - 03:53 AM

[…]  An alt az mount is like a heavy pair of binoculars,  great for a quick glance, but a real pain if you want to look for any amount of time. I've done my stint with Alt Az mounts, I have no desire to go back...

 

 

 

 

 

A gem mount models and works with the nights sky, an alt az mount fights it.  Alt az mounts are great for terrestrial viewing but they are a pain when looking at the sky.

 

I am here to learn, but in this case: how can this be true? For example: last August, I had my alt-az mounted stock LX90 out in the German Eifel darkness, and enjoyed the cloudless nights for hours and hours, seated comfortably, go-toing from M13 to M57 and back again. No pain, just gain. Was this just an exception? My scope wasn't fighting the sky either, it was showing it to me.  :confused:



#84 dotnet

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Posted 05 October 2014 - 04:13 AM

I'd say the original poster's purpose has been fulfilled. :lol:

 

Why people are so emotionally bound to convert people to their case for and against I will never understand.

 

If you like SCTs, continue using them happily.

 

If you don't like 'em, don't use 'em.

 

:lol:

 

All scopes are compromises, and there is nothing wrong with revisiting (perhaps even revising) one's choices as circumstances and observing preferences change. And what better place to do so than on an Internet forum  :lol:

 

I currently own three scopes as per my signature, but the one I've been using almost exclusively for over a year has been the MCT. It also happens to be the one I've owned the longest by far. Everyone else's mileage varies. 

 

Cheers

Steffen. 



#85 John J

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Posted 05 October 2014 - 08:35 AM

What I don't understand is how everybody "bashes" the Bird Jones telescopes for their optical design for having a spherical primary with correction through the use of a barlow type corrector after the primary? An SCT has a spherical primary (F2 for that matter) but corrects prior to the primary? ??????? Certainly they are both achieving the same goal of correcting spherical aberration and providing a compact optical tube. Usually when spherical optics are mentioned people spit on the ground but with SCT's "we just don't talk about that".

 

JJ



#86 rmollise

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Posted 05 October 2014 - 09:24 AM

The problem is execution. Most available Bird-Jones telescopes are inexpensive ones that use a simple Barlow in the focuser in hopes of ameliorating spherical aberration. That just don't get it. :lol:

 

I have never heard anybody "not want to talk about" an SCT's (or MCT's or MNT's or SNT's) spherical primary. ;)



#87 Jon Isaacs

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Posted 05 October 2014 - 10:05 AM

What I don't understand is how everybody "bashes" the Bird Jones telescopes for their optical design for having a spherical primary with correction through the use of a barlow type corrector after the primary? An SCT has a spherical primary (F2 for that matter) but corrects prior to the primary? ??????? Certainly they are both achieving the same goal of correcting spherical aberration and providing a compact optical tube. Usually when spherical optics are mentioned people spit on the ground but with SCT's "we just don't talk about that".

 

JJ

 

It is all about the design and execution.  In other words, does it work?  

 

The Jones-Bird designs attempts to correct the spherical aberration of the primary with an pair of lenses.  As we all know, this just does not seem to be a very good design.  I am quite sure that with a sophisticated sub-aperture corrector, such a scheme could work but collimation would likely be very critical.   This is one reason why such designs are typically Cassegrains, any time there are multiple curved elements that require precise optical alignment, having a single optical axis rather than the two optical axes of the Newtonian makes it doable.  

 

The Schmidt corrector is a sophisticated full aperture corrector with a complex shape.  When the corrector is placed at center of curvature of the primary mirror, it is fully corrects for spherical aberration and the camera/telescope is corrected for coma and astigmatism, leaving only field curvature as an aberration.  The description of the Schmidt Camera and of Bernard Schmidt's life in Henry Kings "The History of the Telescope" is a good read.  The Maksutov design is also all spherical and like the Schmidt, can be constructed as a Newtonian.  

 

If one fears all spherical optics, it's best to avoid refractors because there are essentially zero refractors that use aspherics.  But a bigger problem awaits an observer with spheri-phobia: the eyepiece.  There are a few examples of aspherical designs but the vast majority of the eyepieces available use all spherical optics. 

 

Jon



#88 ur7x

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Posted 05 October 2014 - 10:08 AM

 

[…]  An alt az mount is like a heavy pair of binoculars,  great for a quick glance, but a real pain if you want to look for any amount of time. I've done my stint with Alt Az mounts, I have no desire to go back...

 

 

 

 

 

A gem mount models and works with the nights sky, an alt az mount fights it.  Alt az mounts are great for terrestrial viewing but they are a pain when looking at the sky.

 

I am here to learn, but in this case: how can this be true? For example: last August, I had my alt-az mounted stock LX90 out in the German Eifel darkness, and enjoyed the cloudless nights for hours and hours, seated comfortably, go-toing from M13 to M57 and back again. No pain, just gain. Was this just an exception? My scope wasn't fighting the sky either, it was showing it to me.  :confused:

 

 

We are all here to learn.

 

So with that, yes your scope was fighting the nights sky.  The stars (and to a degree the moon and planets too) appear to turn around a central point in the sky.  That point is called the celestial pole.  In the northern hemisphere it is right close to Polaris the north star.  Mounts that model that behavior out of the box generally work better. Since once set up they only need to compensate on one axis to keep the scope pointed to the desired object. 

 

Keep in mid that not all Alt Ax mounts are created equal.  Your LX90 (or Celestrons CPC) when in Alt Az mod "understands" and compensates for this problem and performs somewhat complicated calculations and drives the scope in both axis to keep objects in the field of view.  This works, but it does not eliminate the "fighting" problem since as object turn around the celestial pole they also appear to rotate in the sky.  Again an equatorial mount does this by design, even a full automatic Alt Az mount can not do this and objects will slowly over the night rotate.

 

Of course the real problem with Alt Az can be really seen in the original "manual" DOB design.  The celestial polar rotation of objects means that they are always try to drift out of view.  Using anything but the widest field of view means that you are constantly bumping the scope to keep up with the nights sky.  This is a particular problem when you look at smaller objects, like planetary nebula or double stars.

 

Most Old timers here  "graduated" to equatorial mounts since any sky chart worth its salt is laid out that way.  The equatorial mount not only works with the sky, but it works  with the charts and it works with the science of astronomy.


Edited by ur7x, 05 October 2014 - 10:17 AM.


#89 Glen A W

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Posted 05 October 2014 - 10:16 AM

I have a Vixen Polaris mount which my mother bought for me in the 1980s.  It had never been set at anything but 39.5 degrees.  Currently, it has on it a 100ED refractor.

 

At a star party in August, I decided to try standing it straight up to use it as an alt-az.  It is never going back.  This alt-az setup is so much fun to use, and it is very smooth and easy to deal with.  The counterweight has become a handle.

 

It is just the same with a Dobsonian.  If you have one like my xx12g, which is very smooth and pleasant to use - I don't use the electronics - then it is a beautiful experience to use it.

 

The main thing with these setups is that the scope and eyepiece never get into weird positions.  Also, I can use the 100ED at star parties and let complete beginners use it.  Sometimes, they spend an hour or more on it, they have so much fun.

 

I have spent so many thousands of hours under the sky that I never even think about the fact that I am moving the scope to re-center objects, so long as it has smooth motions without backlash or play.

 

Glen

Attached Thumbnails

  • 100EDonaltaz.jpg

Edited by Glen A W, 05 October 2014 - 10:25 AM.


#90 Sarkikos

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Posted 05 October 2014 - 10:31 AM

Alt-az's might fight against the night sky but GEMs fight against the observer.  Moving a mount along DEC and RA axes is not natural for a human being. Also, there is the extra weight of the GEM and the uncomfortable positions.  In my experience, GEMs are not user-friendly or ergonomically sound. Unless the user is into AP or the GEM is permanently mounted in an observatory, I would avoid them.

 

Mike



#91 tim53

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Posted 05 October 2014 - 10:35 AM

The problem is execution. Most available Bird-Jones telescopes are inexpensive ones that use a simple Barlow in the focuser in hopes of ameliorating spherical aberration. That just don't get it. :lol:

 

I have never heard anybody "not want to talk about" an SCT's (or MCT's or MNT's or SNT's) spherical primary. ;)

Absolutely.  My wife brought home a Meade Bird-Jones from a swap meet for a very good price ($20, only missing the counterweight shaft, and including a nice star atlas that retails for $20).  

 

I collimated it on a bright star, and found the views very nice.  But changing focus (sloppy plastic focuser) threw the collimation out by a lot, destroying the views.  Someday, I'l put a metal focuser on it and adapt the corrector to fit.

 

...after that long list of other projects...!

 

-Tim.



#92 Sarkikos

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Posted 05 October 2014 - 10:43 AM

Years ago some observers setup their spherical mirrors so that there was a constant pull on the middle of the back of the mirror.  This deformed the figure into a parabola.  Usually it was done by gluing a screw to the back of the mirror and then tightening the screwing a little, which pulled slightly on the mirror.

 

I always thought this was a project for folks who had too much time on their hands ... or not enough money in them.  If at all possible, just buy a nice parabolic mirror in the first place! :shrug:

 

On the other hand, a spherical mirror at a slow f number is not too bad.  My old Edmund 4.25" f/10 gave nice images of the planets.

 

:grin:

Mike


Edited by Sarkikos, 05 October 2014 - 10:45 AM.


#93 dpippel

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Posted 05 October 2014 - 11:08 AM

Alt-az's might fight against the night sky but GEMs fight against the observer.  Moving a mount along DEC and RA axes is not natural for a human being. Also, there is the extra weight of the GEM and the uncomfortable positions.  In my experience, GEMs are not user-friendly or ergonomically sound. Unless the user is into AP or the GEM is permanently mounted in an observatory, I would avoid them.

 

Mike

 

 If you're talking about using a GEM manually then you have some valid points, but a modern equatorial mount with tracking and goto suffers from none of these limitations IMO. The motors do the work, not the observer, and with a rotating diagonal and adjustable observing chair I find the "uncomfortable positions" to be a non-issue.



#94 Sarkikos

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Posted 05 October 2014 - 11:21 AM

To each their own.  De gustibus non est disputandum.

 

Mike



#95 dpippel

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Posted 05 October 2014 - 11:22 AM

Choices are one of the things that make this such a great pastime. :)



#96 Ed Holland

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Posted 05 October 2014 - 12:35 PM

I like my GEM (With SCT or refractor) but it can play merry hell with the position of the finder :)

 

Luckily, I do this in the dark, so the neighbours never get to see me sprawled across the driveway in that peculiar variety of postures that we all know and love :lol:



#97 ColoHank

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Posted 05 October 2014 - 01:10 PM

 

 

[…]  An alt az mount is like a heavy pair of binoculars,  great for a quick glance, but a real pain if you want to look for any amount of time. I've done my stint with Alt Az mounts, I have no desire to go back...

 

 

 

 

 

A gem mount models and works with the nights sky, an alt az mount fights it.  Alt az mounts are great for terrestrial viewing but they are a pain when looking at the sky.

 

I am here to learn, but in this case: how can this be true? For example: last August, I had my alt-az mounted stock LX90 out in the German Eifel darkness, and enjoyed the cloudless nights for hours and hours, seated comfortably, go-toing from M13 to M57 and back again. No pain, just gain. Was this just an exception? My scope wasn't fighting the sky either, it was showing it to me.  :confused:

 

 

We are all here to learn.

 

So with that, yes your scope was fighting the nights sky.  The stars (and to a degree the moon and planets too) appear to turn around a central point in the sky.  That point is called the celestial pole.  In the northern hemisphere it is right close to Polaris the north star.  Mounts that model that behavior out of the box generally work better. Since once set up they only need to compensate on one axis to keep the scope pointed to the desired object. 

 

Keep in mid that not all Alt Ax mounts are created equal.  Your LX90 (or Celestrons CPC) when in Alt Az mod "understands" and compensates for this problem and performs somewhat complicated calculations and drives the scope in both axis to keep objects in the field of view.  This works, but it does not eliminate the "fighting" problem since as object turn around the celestial pole they also appear to rotate in the sky.  Again an equatorial mount does this by design, even a full automatic Alt Az mount can not do this and objects will slowly over the night rotate.

 

Of course the real problem with Alt Az can be really seen in the original "manual" DOB design.  The celestial polar rotation of objects means that they are always try to drift out of view.  Using anything but the widest field of view means that you are constantly bumping the scope to keep up with the nights sky.  This is a particular problem when you look at smaller objects, like planetary nebula or double stars.

 

Most Old timers here  "graduated" to equatorial mounts since any sky chart worth its salt is laid out that way.  The equatorial mount not only works with the sky, but it works  with the charts and it works with the science of astronomy.

 

It's no big deal to mount an alt-az scope on an equatorial wedge, is it?



#98 Mark Costello

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Posted 05 October 2014 - 03:28 PM

That depends on the size of the scope and alt-az mount.



#99 jrbarnett

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Posted 05 October 2014 - 03:54 PM

Something that I have wondered about for quite a while - why are Schmidt-Cassegrains so popular?

 

They are much more expensive than Newtonians of a given aperture - and never seem to perform quite as well.

Portability is commonly cited - but when factoring in a beefy mount and tripod, they will not be as portable as a dobsonian of equivalent aperture.

The goto and tracking popular on SCTs can be had on dobsonians - and often work extremely well with features such as dual encoders.

 

For a comparison, take the orion xt8g and the celestron cpc 800. The dobsonian performs slightly better, has an equally good goto system, costs half as much, and is more portable.

 

So what gives?

That's an easy one.  Put an 8" f/6 Newtonian on a CG5 EQ mount, and then think about your question.

 

SCTs are all about compromise; the balancing of virtues and vices.  While not quite as good as a scope with a smaller CO and a more thermally sensible design in image quality, they are compact and easily mountable on driven, tracking mounts, and are relatively cheap to boot.  Maybe not entry level Dob/Newt cheap, but close.

 

Also, a properly collimated and cooled C8 will give most mass-produced 8" Dob/Newts a run for the money.  The "properly collimated and cooled" part is the hard part, and 5 or 6 out of 10 times the reason an SCT under perform other designs.

 

- Jim



#100 ur7x

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Posted 05 October 2014 - 04:47 PM

It's no big deal to mount an alt-az scope on an equatorial wedge, is it?

 

 

 

In most cases, of course not.  Of course, by definition and practice, it then becomes an equatorial mount.

 

Not all equatorial mounts are GEMs, there are lots of flavors of equatorial mounts.


Edited by ur7x, 05 October 2014 - 04:50 PM.



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