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Meade LX600-ACF 12" f/8 or CPC1100

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#76 Starhawk

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Posted 22 September 2013 - 02:26 PM

The point about the mount and scope being combined in a fork shouldn't be soft sold- it's an extremely limiting feature. Owning a fork mounted SCT gets you nowhere for getting a second OTA. Yes, you can piggyback, but the process is deep in Rube Goldberg territory, and if you look at the LX600, you'll see Rube has already been at work, twice.

The GEM lets you get it with one OTA, and if you want to get something else- reflector or refractor, or what have you, if it is within that mount's weight limits, you're in business for just the cost of the OTA.

The weight issue is huge as well. I've always been using fork mounted SCTs at around 30° north latitude. That means when putting it on a wedge, you end up supporting at least 86% of the weight of the fork and scope. And if you're thinking the breakdown arms of the LX600 are a handy solution- surprise, you get to try to alight the two of those while holting the weight of the OTA between them.

The long story short is forks aren't really all that user friendly for setup, transport, takedown, and migrating to other OTAs.

In comparison, the only real drawback to GEMs is the meridian flip. In reality, this isn't as big a deal as it once was with the advent of mounts which will track for over an hour on each side.

In compensation, you get real modularity- whatever you've got will go on there if it has the weight capacity. The mount heads are compact, and even if a bit heavy for larger ones, they are small enough to hug to your body so they are near your own center of gravity, They also don't have bulk and glass- quite an improvement in the pucker factor. The ability to part it out also means the ability to get creative in packing for transport. The pieces are small enough to arrange with other things you are traveling with.

As for OTAs, the Celestron Edge HD currently has quite a bit more flexibility with immediately available f/#s of f/10, f/7, and f/2. If you're starting out in AP, f/2 is where you want to be. But that's another thread.

-Rich

#77 WardyNew

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Posted 22 September 2013 - 03:41 PM

Thanks for all the above information...although I'm not sure whether this has narrowed things down or opened them up (but still appreciated as things for me to consider rather than me charging in ignorantly)!

Am I right in thinking that the issue around central obstruction is just the light gathering area it cuts out or is there a further effect? Working this out based on Spacetravellerx's figures the total light gathering area of the LX600 should still be greater than the CPC1100's due to the extra inch of aperature or am I missing something?

If I went down the GEM route, the LX850 would be well out of my reach so it would probably be the Celestron CGEM 1100HD or same but with DX mount.

Few things I'm questioning with this is:
- Time to balance the scope each time (doesn't look too difficult so not as worried about this)
- Polar aligning even if doing visual...is this difficult / does it take a long time (and does the Starsense module help any with doing this accurately?)
- Is the standard CGEM mount enough to cut the mustard with the 11" SCT or for imaging would I really need to be getting the DX mount? DX, starts getting a bit tight on budget (based on UK prices from shopping around) once start to add on autoguider, guidescope and if its useful, the StarSense module (still not sure if the StarSense module is useful or not).

I can see the advantages of the GEM and makes sense but also want a scope I can use easily from day 1 of starting out as opposed to needing to gain experience before being able to use it effectively. Realise I'm going in at the deep end anyway with going in for bigger scopes when inexperienced as it is but don't really want to spend £1K on a smaller / less capable one and then have to pay out more to upgrade not too long after...really want a scope that (as I say) I can use easily from day 1 but that has the capability to grow with me for 5-10 years (has taken me that long to save up the funds for this step so at limit of my budget and wife's foerbearance).

#78 WardyNew

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Posted 22 September 2013 - 03:59 PM

Should have said that I've been wandering round my garden today looking up at the sky (and probably looking like a nutcase) and I think there is probably too much obstruction for me to go down the route of permanent observatory, also the comments earlier about telescope vs. mount means their isn't a combination of the 2 that would leave me with sufficient budget for the observatory without sacrificing too much (from my POV) in terms of the actual telescope.

Should also just clarify, previous post not questioning validity of Starhawk's points, just looking for further information / clarification on the I points listed.

Thanks again.

#79 Starhawk

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Posted 22 September 2013 - 05:29 PM

The central obstruction/ total area aren't actually helpful for astrophotography. All that matters is the f/#. All images shot at any given f/# put the same light flux on the detector. So, for example, a C5 f/10 telescope produces the same image brightness as a 14" f/10 SCT. However, the f/# brightness varies as an inverse square. I have developed a non-dimensional brightness coefficient to allow different telescopes to be directly compared with each other, and to make it possible to predict limiting magnitudes when migrating from one system to another.

Brightness= 1000/(f/#)^2

So, here are some brightness coefficients for common f/#s found in imaging systems:

f/12 Br= 1000/(12)^2= 1000/144= 6.94444

f/11 Br= 1000/(11)^2= 1000/121= 8.26

F/10: Br= 1000/(10)^2= 1000/100= 10

F/8: Br= 1000/(8)^2= 1000/64= 15.625

F/7: Br= 1000/(7)^2= 1000/49= 20.408

f/6.3: Br= 1000/(6.3)^2= 1000/39.69=25.195

f/5 Br= 1000/(5)^2= 1000/25= 40

f/2 Br= 1000/(2)^2= 1000/4= 250

Now, here's what you can do with it: The brightness numbers are absolute. So, let's say we want to compare an f/10 SCT (plain standard type) with its performance using the f/6.3 focal reducer.

So, the brightness at f/10 is is 10, and at f/6.3 it is 25.19. The image with the reducer is likely losing some energy to the glass, but let's leave that alone for now since it is a small effect. The ratio in brightnesses is 25/10= 2.5. Now, 2.5 happens to be the ratio between two stellar magnitudes, so you can expect in the same exposure time to get one magnitude deeper.

So, if you have done a plate solve on an image with a given camera at f/10 and notice you are getting to 15th magnitude in 30 seconds, then you can expect to get to 16th magnitude in 30 seconds with the reducer.

However, since the reducer also reduces the image scale, the telescope will behave as though the drive is more accurate at shorter focal lengths, meaning if you are anticipating 30 second exposures before you get egg shaped stars at f/10, you can run 10/6.3, or 58% longer (say, 45 seconds) without egging at f/6.3. So, total available brightness change now would be 1.58X25 = 39.68.

So, lets say we decide to cease kidding around and go to f/2, with a hyperstar lens from Starizona. In this case, the brightness is 250, meaning 25 times that at f/10, or 10 magnitudes. Since the standard sky has a theoretical magnitude limit around magnitude 25, if you had been getting magnitude 15 at f/10, this system would now allow you to reach the limits of what is possible from the ground in 30 seconds.

Now, let's say we are going to go in another direction, and move from a C5 with the f/6.3 reducer to an AP 130 EDFGT at f/6.3, which has no central obstruction, so in this case, the central obstruction will change the answer. their focal lengths are going to be so close I am going to say they are the same, though the central obstruction doesn't even exist for one. So, how much does it change it?

The C5 has a central obstruction of 40% by diameter. The light penalty is by area, from A= Pi X r^2, so the brightness factor will be scaled this way:

[ (A full diameter- A central obstruction)/ (A full diameter) ] X Br.

Br* = Pi (ro^2- ri^2)/ (Pi (ro^2) = 0.84 * Br

So, the C5 pays a 16% image brightness penalty compared to the unobstructed system, which is only 6.4% of the difference needed to move one magnitude in image brightness.

So, either you can tolerate the small amount of error in going between obstructed and unobstructed, or you can compensate. If your stable is all SCTs, you can completely ignore this since all of them have about the same size central obstruction, and it is proportional to diameter.

What is far more important is the f/# of the system. So, once you have characterized a camera's image in one case, you will be able to predict your imaging capability across the board with telescopes you haven't bought yet. This is why I have 10 scopes, all with different f/#s and focal lengths.

-Rich

#80 jrcrilly

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Posted 22 September 2013 - 05:48 PM

Am I right in thinking that the issue around central obstruction is just the light gathering area it cuts out or is there a further effect?


The impact on light gathering is minimal and not a major concern. What folks worry about the impact on contrast. That's a real issue BUT once one has decided to live with the obstruction percentage found in a typical SCT I don't think the effect of a small increase is important. The C9.25 has a reputation as a better-than-average SCT - yet it has the highest percentage obstruction of any commonly available SCT in the 8" and above category. The effect of going from 0% to 30% (by diameter) is easily seen but the difference from there to 40% is subtle.

#81 jrcrilly

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Posted 22 September 2013 - 05:52 PM

The C5 has a central obstruction of 40% by diameter. The light penalty is by area, from A= Pi X r^2, so the brightness factor will be scaled this way:

[ (A full diameter- A central obstruction)/ (A full diameter) ] X Br.

Br* = Pi (ro^2- ri^2)/ (Pi (ro^2) = 0.84 * Br


When making such comparisons it is MUCH simpler (and a trifle more precise due to fewer rounding errors) to just square the obstruction percentage by diameter and derive the obstruction percentage by area (and thus light loss).

.4 squared is .16

All done.

#82 WardyNew

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Posted 22 September 2013 - 06:12 PM

IS the central obstruction much of an issue for visual between CPC and LX600? Or is their little impact based on the above between different SCTs?

#83 gavinm

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Posted 22 September 2013 - 06:20 PM

There are fork mounts (or varieties) that allow you to change OTA's - Chronos and Mesu come to mind. Different price range but they are out there.

#84 Starhawk

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Posted 22 September 2013 - 07:30 PM

It's mathematically identical, and there is no additional roundoff error either way, since Pi cancels out (so if you used 3.14 or 3.14159, it cancels). Since the central obstruction gets brought up a lot, I went ahead and showed the complete math solution.

-Rich

>

The C5 has a central obstruction of 40% by diameter. The light penalty is by area, from A= Pi X r^2, so the brightness factor will be scaled this way:

[ (A full diameter- A central obstruction)/ (A full diameter) ] X Br.

Br* = Pi (ro^2- ri^2)/ (Pi (ro^2) = 0.84 * Br


When making such comparisons it is MUCH simpler (and a trifle more precise due to fewer rounding errors) to just square the obstruction percentage by diameter and derive the obstruction percentage by area (and thus light loss).

.4 squared is .16

All done.



#85 Starhawk

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Posted 22 September 2013 - 08:28 PM

The central obstruction is a contrast reducer for visual use, and is approximately equivalent between the two, though the LX 600 has to be larger to operate at f/8. The main differences are things like what has been done for the ACF prescription, which both makes the LX 600 incompatible with the rear accessories previously available as well as front end accessories like Hyperstar the Meade SCTs before ACF could use. The Celestron Edge HD has a back end which rules out all of the old SCT accessories, though more expensive Edge-only ones are available. Meade is supposed to eventually go that route, though the reducers do not exist yet.

The Edge didn't change the front end other than some spacing, so it needs a short standoff to use Hyperstar, but is otherwise completely compatible.

As for why there is a $1400 difference between the CPC 1100 and the LX600, I really don't know how to answer that. I can't find what costs so much on an LX600.

-Rich

IS the central obstruction much of an issue for visual between CPC and LX600? Or is their little impact based on the above between different SCTs?



#86 gmartin02

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Posted 22 September 2013 - 08:43 PM

Should have said that I've been wandering round my garden today looking up at the sky (and probably looking like a nutcase) and I think there is probably too much obstruction for me to go down the route of permanent observatory...

Sounds like it's time to take out the chain saw and do a little "creative tree trimming" (removal) :)

Now, if I could only find a way to get my neighbor's 40 foot Leylandii Cypress tree that blocks my south east imaging zone to have a "chain saw accident". Oh well - not going to happen. The two other Cypress trees that were next to it have died in the last 10 years, and the top 10 feet of the remaining one has already died off...I guess I will have to continue putting voodoo spells on the remaining Cypress and keep waiting.

#87 David Pavlich

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Posted 22 September 2013 - 09:43 PM

Rich...you've given me a headache! :lol:

David

#88 Cotts

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Posted 22 September 2013 - 10:00 PM

Rich, Bravo for posting all that math!!

You have only told half the story, however. Everything you said in your long post applies to extended objects only.

The ability of a telescope to see fainter and fainter stars is entirely to do with aperture and has nothing whatsoever to do with f/ratio.


Dave

#89 Cotts

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Posted 22 September 2013 - 10:13 PM

Am I right in thinking that the issue around central obstruction is just the light gathering area it cuts out or is there a further effect?


The impact on light gathering is minimal and not a major concern. What folks worry about the impact on contrast. That's a real issue BUT once one has decided to live with the obstruction percentage found in a typical SCT I don't think the effect of a small increase is important. The C9.25 has a reputation as a better-than-average SCT - yet it has the highest percentage obstruction of any commonly available SCT in the 8" and above category. The effect of going from 0% to 30% (by diameter) is easily seen but the difference from there to 40% is subtle.


John, the effect of central obstruction, as you well know, is to take light from the central disc of the diffraction pattern and put the light into the first ring (mostly) and a bit more into the subsequent rings. This affects contrast when viewing planets at high power where the magnification actually resolves the diffraction pattern. When taking astrophotos at prime focus of the instrument the diffraction pattern isn't resolved at all - it's not magnified nearly enough, the stars are virtual pinpoints - and so the central obstruction has virtually no effect on prime-focus astrophotography. You can buy some $20 000 high-end RC astrographs with 50% central obstruction and the contrast in images with these scopes is simply stunning. Careful baffling, top notch coatings and smoooth optical surfaces are far more important factors for contrast in prime focus astrophotography.

Dave

#90 jrcrilly

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Posted 22 September 2013 - 10:22 PM

the central obstruction has virtually no effect on prime-focus astrophotography.


Hi, Dave!

No argument here - and I've gone up to 50% obstruction myself with no ill effect. I intended my comments to apply to visual observing but failed to make that clear.

#91 Starhawk

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Posted 22 September 2013 - 11:15 PM

Dave,

It depends on image focus scale versus pixel,size. And there's no way I'm hitting the math for that tonight. Maybe tomorrow- it's interesting though- what Dave pointed out is if a star's energy all hits one pixel, then the f/# effects don't impact it. More math coming. Ye be warned.

-Rich

Rich, Bravo for posting all that math!!

You have only told half the story, however. Everything you said in your long post applies to extended objects only.

The ability of a telescope to see fainter and fainter stars is entirely to do with aperture and has nothing whatsoever to do with f/ratio.


Dave



#92 WardyNew

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Posted 23 September 2013 - 01:45 AM

Thanks for above, light gathering vs central obstruction well and truly answered!

With regards to previous points on ease of set-up and use of a GEM for relatively inexperienced astronomer...what are your views on this...?

#93 jrcrilly

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Posted 23 September 2013 - 02:09 AM

There is nothing simpler to set up than an alt/az forkmount. We spend a certain amount of time here explaining the more complex setup of German EQ mounts - but anyone can get the idea. The GEM gives terrific flexibility in OTA choice and for many of us (including myself) that's worth the additional complexity. For quick field visual setups, though, such as public outreach, the forkmount is a winner. On a wedge, the forkmount can do a fine job (I used them in my observatory for years) but then you have the complexity of an EQ mount with the inflexibility of a forkmount.

#94 WesC

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Posted 23 September 2013 - 12:00 PM

I can pull out my disassembled CGEM and have it setup, polar aligned, 2+4 star aligned in less than 30 minutes. Anywhere I want, with no back pain, straining or struggling. I usually have the OTA on the ground with the cooling fans running while I polar align too. And don't let polar alignment freak you out, once you learn how to do it, its a breeze. I do it in just a couple of minutes. but DO get a polar scope as it helps immensely.

No argument, for pure visual convenience nothing beats alt/az mounts. But I got around that on my GEM by figuring out the perfect height for my tripod (easily) and using a Baader click-lock visual back to rotate my diagonal so that I can view comfortably no matter what orientation the GEM is pointing using a standard observing chair... no step ladders or laying on the ground. ;)

In my own personal opinion the flexibility and benefits of the GEM far outstrip the minor visual convenience of the alt/az fork for SCTs... for how I use my scope.

In the future I would like to have a smaller alt/az setup for a refractor. Sort of a semi-grab and go. Something like a DM6 or a Nova Hitch--that does not have a permanently mounted OTA. But after a few experiences helping setup and use a fork-mounted SCT. No way would I ever own one. The weight and awkwardness and the permanently combined fork and OTA are a deal-breaker for me.


You just need to decide what is more important to YOU. That's why there are options, everyone's needs are different.

#95 BWAZ

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Posted 23 September 2013 - 12:19 PM

You have only told half the story, however. Everything you said in your long post applies to extended objects only.

The ability of a telescope to see fainter and fainter stars is entirely to do with aperture and has nothing whatsoever to do with f/ratio.


Dave


Due to the diffraction and the size of the pixel, the statement is not correctly which is generally true for visual observation. In fact, the star image does have a actual size (airy disc plus a few diffraction rings) that is too small for the human being to resolve but the CCD can under certain conditions. As Rich mentioned if the star image is within a pixel, reducing focal length while maintaining the aperture has no effect to shortening the exposure time. But if the star image hits more than one pixels at longer focal lengthen and falls into one pixel at certain shorter focal lengthen, that can reduce the exposure time for sure.

#96 WardyNew

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Posted 23 September 2013 - 03:08 PM

Only just saw your pictures with the LX850. Amazing!

#97 Alph

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Posted 23 September 2013 - 03:28 PM

The ability of a telescope to see fainter and fainter stars is entirely to do with aperture and has nothing whatsoever to do with f/ratio.


That's right. Check out this web page The algorithm was provided by Bradley E. Schaefer. The focal ratio is not part of the equation. If in doubt, check his scientific credentials.

#98 Alph

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Posted 23 September 2013 - 04:49 PM

I have developed a non-dimensional brightness coefficient to allow different telescopes to be directly compared with each other, and to make it possible to predict limiting magnitudes when migrating from one system to another.

Brightness= 1000/(f/#)^2


This will not predict telescope's limiting magnitude. Google Bradley E. Schaefer limiting magnitude. No need to reinvent the wheel.

#99 gavinm

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Posted 23 September 2013 - 05:26 PM

Thanks for the link Alph. Nice

#100 Starhawk

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Posted 23 September 2013 - 05:28 PM

That's about visual limiting magnitude. It's not the topic at hand.

-Rich


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