ASI6200MM or Celestron EdgeHD 11?

Hi Brian,

well if you are honest to yourself, the question is only in which order you get the 6200 and the C11 … one random thought that crossed my mind a while ago: the Edge or non-Edge. at the end you most likely will operate them with a LF corrector, either the Celestron one for the Edge or the Starizona one for the non-Edge, so the corrected field/coma of the un-reduced C11 doesn't come into play. The price tag is 50% for the classical SC. Ok, there are goodies like mirror lock etc, … 

Matthias

Brian Boyle:
I am looking to make the next major upgrade to my observatory.  [SW Esprit 100, GSO RC8, ZWO 2600MC, 1600MM Pro, EQ6R-Pro mount]

While I like my Esprit and RC8 (the latter particularly for galaxies), I do crave more photons.   

A ZWO 6200MM Pro (complete with 2in filter wheel and Optolong NB/BB filters) will cost almost the same as a Celestron EdgeHD 11 with 0.7x reducer.    Neither will force me into getting a new mount. I think my EQ6R-Pro will handle an EdgeHD 11 with all the extras - at least based on some of the wonderful images I have seen taken with this setup on Astrobin.  

As I understand it the camera gives me a QE gain almost equivalent of going from my RC8 to an EdgeHD 11 (and also has no amp glow, or grav lens effects with filters like the 1600MM).   While I can't use full sensor area with my RC8, although I can wth my Esprit 100. 

The EdgeHD 11 with reducer operates at a focal length not well suited to my seeing (4-6 arcsec), but x 2 resample would give an ideal focal length, albeit with a small field of view on the 1600MM Pro.  [I could use the 2400MC on it of course, but I limit myself to OSC.]

Grateful for any advice.   I have been really impressed with images on Astrobin with taken with either (or both) this kit.

CS Brian

Brian,
I totally understand the desire for another, larger scope but with 4-6 arc-second seeing conditions, an Edge 11 won’t provide any improvement over your current scope.  Are you sure that your conditions are really that bad?  Seeing in the range of 4”-6” is VERY poor!  If they are that bad and you can’t use the scope in a better location, you will be very disappointed with the Edge.  With the ASI-1600MM Pro, on a reducer, you are likely to struggle with aberrations, you’ll be forced to bin, and you’ll struggle with moderately blurry images.  Things change completely if your conditions are actually closer to a 2” range.  If that’s true, then we’ll have to talk again.

John

I guess the mount. C11 is at the limit of an EQ6R pro (RASA11 probably overstretching it).

Hi all, 

Some great, wise and helpful comments above.  It is certainly true I will buy both, but they will be at least a year apart [and by that time something better may have appeared]. 

Really helpful comment about the mirrors on an SCT.  I hadn't realised how much work it is to get the sort of images the best photographers here on Astrobin get with an SCT (usually an Edge).

I had moved away from CDK or RCs simply because of the weight issue at 11-12 inch or greater aperture.  In my dreams, I would own a Planewave CDK 12.5 with reducer on a EQ8-R mount.  But its not in my budget.  

Interesting comment on the seeing.  During my professional career, I have been used to mountain top observatories where 2" seeing was considered fairly poor.  I had thought that seeing 4-6arcsec was pretty much average for other places.  

I am "stuck" in a broad flat valley with mountains rising up over 1200m within 10km of my observatory, so I wasn't expecting too much in the seeing stakes.  So when I get 4-6arcsec from my autofocus runs with 1sec images, I had figured that was the best I was going to get.   Nevertheless, I should investigate further if it is something I can readily improve with my existing rig.   The thought of transporting a larger telescope to a better seeing site fills me with dread.  Unfortunately for astronomy most of the accessible mountain tops around here have ski fields on them.  Which are floodlit at night… 

From these comments, I suspect my best route forward is the camera.  If I am to get a bigger scope, it may be best to go with a design other than an SCT - which means a new mount… And so it goes.

CS Brian

I’d say get the camera .. but I’d get the QHY version instead (QHY600) I have an EQ6-R Pro on a Tri-Pier, and there is no way I’d feel comfortable running a C-11 on it. I use to run a C-11 on my Schaefer mount (which you can see in my pics here) which is much heavier duty than my EQ6-R Pro. Having said that .. Skywatcher Johnny here on AB has some awesome images shot with a heavy 10” Skywatcher Newtonian sitting on a little Skywatcher HEQ5 mount .. so anything is possible I guess if all your ducks are in a row 😬

Brian Boyle:
I hadn't realised how much work it is to get the sort of images the best photographers here on Astrobin get with an SCT (usually an Edge).

Don't let mirror flop scare you off of the SC.  With the mirror locks on I've never had a flop. I usually leave them loose by about half a turn so that I can still focus.  At that setting they only flop during my meridian flips. 

The only time I've ever lost a frame to mirror flop was on M83 which had an elevation of only 12 to 16 degrees. Shooting that low gravity wasn't able to keep the mirror in one place, but the mirror locks on full did the job.

CS

Let me add my experience to the SCT mirror flop aspect:
Meade's major business was/is in SCTs (…and let's hope this continues if you're aware of the current news… and as far as I know Celestron's product line, it's not much of a difference. Hence, from what I can see, people are achieving excellent results. …or maybe my standards are too low…. 😅

My SCT doesn't have a mirror lock but there are ways to handle it, e.g. the famous (?) CCW focusing. Since I'm not doing "shoot-and-forget" but monitor the whole sessions, I observe the HFD curve and selectively the subs to make sure everything is in good condition. Usually it's about time to change the filter before the tube has turned that much before gravity is doing something it shouldn't do. And in case that I am uncertain if something happened (which could simply be due to changing seeing conditions), I go out and refocus. Takes just a minute.

David Redwine:
Let me be a bit more clear.  Once the scope is focused I set the mirror locks so that they are completely disengaged plus about about a half turn. During  a single night refocusing is seldom more than a few 10's of microns, so the stop isn't touching the mirror.

 The few flips I have experienced make it clear that the unrestrained mirror has two stable points that I think of as the forward and rearward positions.  Star trails of the flip  indicate that the mirror essentially loses its balance and flips forward or flops back in a second or less.  As long as the mirror lock is close enough it will block the flips most of the time, but a large force like a meridian flip, or having the scope nearly horizontal will still cause flops.   If I was pinching the mirror as you describe I wouldn't see the mirror flop when I do my meridian flip.

CS

Allow me to provide some background.  I worked as an engineering consultant for Celestron for about a year and half to develop a new zero backlash, zero mirror flop, precision focusing mechanism.  The design was successful and it is now used on their larger RASA systems and I use it in my highly modified remote C14 Edge out at DSW.  In the end, they made a business decision mostly for cost considerations not to offer the focuser with Edge scopes.  As a part of that work, I occasionally trained incoming techs on system assembly and alignment.  I also helped provide tolerance analysis in connection with some of their assembly and optical alignment procedures.  I've completely assembled and disassembled many C14 Edge systems from scratch and I designed and built many of the components in my own system.  As a result of that experience, I am very familiar with the Celestron systems and how they work.

The mirror locks on the Celestron scopes (mostly) work but they are far from kinematic and while they work pretty well to constrain tilt on the primary, they are less effective at preventing a lateral shift as the telescope tracks across the sky.  The entire focusing mechanism relies on machining tolerances that are not super tight so the mirror shift problem varies quite a bit between individual telescopes.  It is rare to see the mirror shift with the locks engaged, but it can certainly happen.

As long as you focus the telescope with the mirror locks completely disengaged, you are good.  Focusing the scope with the locks even lightly engaged will tilt the primary and that's not good.  If you do a full tolerance analysis of the system, even a small amount of tilt in the primary mirror introduces on-axis comatic wavefront errors and field problems.  This has nothing to do with "pinching" the optics.  It's all about pulling on the mirror slider from the side and the mechanism relies on a tight tolerance and low friction to operate properly.  It takes very little drag from those mirror locks to tilt the mirror using the focusing screw.  Frankly, I've never thought much of the way the mirror locks work but if they are used properly they go a long way to solving the mirror flop problem. (The old "Flop-Stoppers" were mechanically a much better solution.)

As far as focusing requirements during the night, a while ago I did a pretty careful thermal analysis of the C14 scopes.  You can find some of it here:  https://www.cloudynights.com/topic/617666-thermal-sensitivity-of-casses-scts-and-the-c14-edge-hd/?hl=+thermal%20+sensitivity
You didn't say what kind of Celestron you have but the thermal sensitivity of your scope and the temperature stability of your location will determine how often you should refocus.  That's important if you want to maximize your yield each night!  It's a waste of time to be imaging with an out of focus scope; not to mention really frustrating.


John

Ruediger:
Hello all,

there are very good points given above, so let me please add my experience coming from a Meade LX200 12" and other observations:

As John pointed our very precisely, the quality differs from brand to brand, model to model and even the batch.
Most mirror locks are definitely not suitable for serious imaging. I ended up in keeping the mirror locked permanently and attached a focuser (Baader Steel Drive). Even with that the shift was noticeable.

Three more things:
1. You have to use an OAG. Guiding with a guide scope is due to the mirror motions not successful.
2. The lock system is additional weight. So when talking about a light scope this might play a role.
3. With the planned camera combination the smallest tilt of the mirror will be visible due to it sensor size.

My experience within other local APs at my place: They all had more or less serious issues with their SCTs (all Celestron 9,11,14). Almost all have replaced their SCTs. So even if you get one of the well machined and well constructed ones, you introduce another potential source of trouble anger to your system. This something you have to consider when planning such a purchase.

And my final thought: Astronomical gear is like any serious tool: You buy it twice! Once cheap, and a second time expensive but working.
I guess almost everyone made this mistake in his AP career already. You always end up at the more expensive solution - sooner or later.

So I would always recommend to wait a bit longer, save some more money and go for the better option (here the mentioned PWI CDK12.5 which will definitely serve your needs and makes you happy from the very first day you play with it).

CS
Rüdiger

Rüdiger,
In some ways I agree; in other ways I disagree.  First off, I don't have much patience for low end instrumentation and that's one reason that I own a PW20 and an AP130GTX on a new Mach2.  However, I also have a lot of experience with Celestron C14 Edge scopes and if you configure them correctly, they can produce excellent results.  I've put 4 of them on a PhaseCam interferometer and three of them were outstanding optically and the other one was still "adequate."  In fact, the wavefront performance of my current C14 Edge system is actually better than my PW20 (the data for both systems is posted on my page).  That's not to say that the Planewave is poor--far from it.  The point is that Celestron has actually been delivering some very good optics.  A well aligned Celestron Edge on a good mount (i.e. NOT a Celestron mount) with an Optec SMFS and the primary locked down can deliver performance every bit as good as what I get out of my custom system out at DSW.  The folks at Celestron do a lot of things to keep the price low and some of that stuff drives me absolutely crazy, but at the end of the day, a Celestron Edge system can be an extremely cost effective way to get world class imaging performance that's equal to anything you can get from a Planewave.  The weak areas are in the corners and I will quickly admit that the Celestron systems are not nearly as effective at dealing with stray light nor are they as mechanically robust as any higher-end scope.  I had to screw around quite a bit to get my C14 Edge going, but I've had the exact same experience with the Planewave.  That whole story is up on my page as well (although it excludes a couple of behind the scenes episodes resolving issues with poorly designed components and other optical surprises.  I don't want to be harsh so I try to be somewhat judicious about what I report.)  As it is, I am fortunate that I can mostly work with high end gear, but for those with a more constrained budget, Celestron Edge systems can work surprisingly well.
John

Hi @John Hayes ,

maybe I did not express it well (always a problem, if you are not a native speaker).Therefore to make clear:

I have no doubt that the C14 is a great scope. Many excellent images were taken with that. It has proven its quality. I was also lucky to use one at my friends observatory and I have to say it is a great instrument. But I am still of the opinion, that you could avoid a lot of head ache, when chosen a scope with fixed mirrors. And same as you: Tinkering around with the equipment is nice, but not everyone has the expertise to put a C14 into parts, optimize it and put it together again. I prefer a "ready-to-use" construction. But I can totally agree to your argumentation and point of view. Nothing in there I would disagree.

CS
Rüdiger

Hi at all,

After reading the comments, I have to say that I totally disagree with Rüdigers comments. I don't know  on what an old  wreck Rüdiger made his expiriences. My edge eleven is my main instrument and I am very happy with it. Mirror is never moving while exposing, I do not even use the locking function anymore, because not neccessary. For imaging I use a guide scope an 90 mm Apo. The flipping while focussing you only can see under big magnification. Totally uninteresting in my eyes. Instead it is very sharp, stable and short. I use it with guiding scope on an EQ-6R. A perfect combination when there is no wind. Look at my gallery. 

But…..and this is the most important point. With your described seeing conditions, which seem to be extremely worse, you do not need an edge or an RC or an CDK. They all will only show the unsharpness of your night sky. Maximal focal lenght for your conditions should be 500 mm, I guess Brian.

CS, Christoph

Björn Arnold:
@John Hayes I always enjoy your technical insights (thank you!) but if may raise a (stupid) question here: which effects matter most?

I'm not referring to the skills of anyone specifically here but the question that this very detailed discussion raises is: what level of skill and experience are required to address this and that issue. What I mean by that? Example: take the best optics on an average mount and the people screwing the guiding control loop. I think at that point one doesn't need to care too much about wavefronts. Please, it's no critique of your arguments and explanation but I would wonder if we have left the point/question of the original post by @Brian Boyle.
As you've said in your initial reply: if the seeing is really that bad as Brian believes, I would assume the priority list of "most severe" effects looks different?

My very personal question that I am interested in is: where exactly is the benefit for Brian to switch to this and that optics? 
NB: count me as an AP novice but I've learned that asking questions may give a lot of insight as arguments are assessed again from time to time.

Clear skies! 

Björn

I agree that the topic has drifted a bit, but mostly to clarify some of the points made along the way.   Going back to my first post in this thread and your question, the tipping point is probably in the range of median seeing conditions around 2.5".  At that point (and below), you can achieve some pretty respectable results with almost any SCT.  Once seeing conditions drift above about 3"-4", you can tolerate pretty big errors in focusing, guiding, and optical alignment simply because everything is blurry to begin with.  Heck, at 6" seeing, you probably don't even need to focus anymore!  

The ultimate challenge is to know how to assess your local conditions.  When I first started imaging in central Oregon, I was convinced that the seeing was almost always pretty bad.  However, as I tuned up my equipment and reduced all of the systemic errors, I began to realize that the seeing conditions weren't always as bad as I first thought.  I even reached a point where I achieved 1" FWHM star images on some nights and I could take one look at my guider images and tell almost immediately how good the conditions were.  I also learned that the conditions can fluctuate tremendously during a night.  I've seen it go from below 1.5" to well over 4" in a short period of time.  That's why it's important to configure each imaging system so that it has the capability to take advantage of the best possible conditions--usually down to 1" (though that may depend a bit on the scope and camera.)

I recognize that most folks don't enjoy screwing with their equipment as much as I do and that's where a small(ish), high end refractor on a good quality mount comes in.  That configuration can produce spectacular results and do it with a minimum amount of messing around.  Once you get into multiple mirror, large aperture scopes, things become inherently more complicated.  I would never recommend a RC, SCT, Cassegrain, or CDK to anyone who isn't at least a little interested in fooling around with it to get it to work properly.  I've seen an amazing number of folks order up a 17" CDK, a camera, an auto-guider, PCs and other stuff, have it all crated and shipped to DSW or even Chile where the observatory staff puts it all together only to find that there are massive problems.  I know of one guy who has spent over a year screwing remotely with his "new" telescope in Chile and still can't take any decent images with it.  Those guys should have just started with a small refractor and they would have been a lot more productive, a LOT happier, and have more money in the bank.

John