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Thoughts on Celestron EdgeHD CG-5 8" w/ Full Frame Canon

astrophotography
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#1 johnfr

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Posted 04 August 2020 - 11:57 AM

I am currently using a Sigma 800mm telephoto lens intended for wildlife photography with my Canon Full Frame 5DSR. It has a lot of comma and chromatic distortion for DSO photography.   I am considering moving to a true telescope such as the Celestron EdgeHD CG-5 8" with the optional autofocuser . It is my understanding that this scope is designed to provide edge to edge comma free and low chromatic distortion images with a full frame camera.  I have a Celestron AVX mount and an Orion StarShoot guide camera w/ 50mm guide scope. I am mostly interested in DSO. I am curious what the forum members opinions are of this scope. Is Celestron's ad hype accurate?



#2 DRK73

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Posted 04 August 2020 - 12:52 PM

Ignoring the equipment part of this question (because you're going to get a lot of comments saying that that set will not work because of the mount, which is justified), what *kind* of DSO's are you interested in? 

 

Here's why that's important - a lot of emission nebula actually take up quite a lot of sky so you really don't need much focal length to capture them; they're just exceptionally dim. The Veil nebula, Eagle nebula, Swan, heck - just about all the popular ones can be imaged pretty nicely at ~500mm. The Andromeda galaxy takes up slightly more sky than the full moon - but again, it's dim. The 2000mm of focal length of the 8" SCT is just way too much and you'd be better off at the 300-500mm range. 

On the other hand, planetary nebula can be "tiny" - so if you're after the Crab Nebula you do need more focal length. Same thing if you're going to go after a lot of other more distant galaxies - but even then I tend to think of an SCT as too much focal length for those kind of objects, especially since (unless you invest in the reducer) you're trying to image a dim object using an f/10 lens. 

 

Yes, the EdgeHD has great optics (I've got one myself), but it takes a serious mount to image at 2000mm focal length and f/10, and neither the AVX (which I've also got) nor the CG5 are anything like what you need. 

 

If you wanted to image planets or the moon, though - then you've got a fine setup that will work. 



#3 Stelios

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Posted 04 August 2020 - 05:28 PM

First things first.

 

You will likely be imaging with the 0.7X reducer more than at the native F/10 (the primary reason being that you save 50% of the imaging time, the secondary reason being image scale at F/10 rarely yields benefits over F/7, the tertiary reason being guiding is harder at F/10).

 

The reducer covers APS-C, not full-frame. Not sure where you saw the "full-frame" guarantee for the Edge 8". All I found is a reference to "large CCD chips" (doesn't mean you didn't find it, means *I* didn't find it :))

 

The Edge 8" (I have one) is a wonderful scope. But you will be hardpressed to (you will very likely fail) to guide it well on an AVX or CG-5 mount--unless you're talking planetary photography. For DSO photography the smallest mount I would entrust it to would be an EQ6R-Pro, though a good HEQ5 *might* handle it with a light imaging load. I would *not* use a CGEM or CGX, although luck would be variable. 

 

Also, the Orion Starshoot autoguider is entirely the wrong guiding system to use with an Edge 800, even with reducer. Results rate to be from poor to unusable. An OAG is highly recommended. If you insist on a guidescope (bad idea), get one with at least a 400mm focal length and mount it securely on a top dovetail with at least 2 rings. 



#4 johnfr

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Posted 04 August 2020 - 10:12 PM

DRK73 and Stelios,
Thanks for the advice. Obviously a 2000mm+, f/10 scope isn't a good choice for my goals. Sounds like I need to do more research. Want to find something that will work better than my Sigma 800mm f/5.6 EX/DG Telephoto lens. It works but I would like to get rid of the comma and chromatic distortion.I have attached an example of the results I get with the Sigma 800mm.  I would welcome suggestions for a practical scope that would give me at least 800-1600mm that my AVX mount can work with.

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#5 spereira

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Posted 05 August 2020 - 07:04 AM

Moving to DSLR ...

 

smp



#6 DRK73

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Posted 05 August 2020 - 09:17 AM

DRK73 and Stelios,
Thanks for the advice. Obviously a 2000mm+, f/10 scope isn't a good choice for my goals. Sounds like I need to do more research. Want to find something that will work better than my Sigma 800mm f/5.6 EX/DG Telephoto lens. It works but I would like to get rid of the comma and chromatic distortion.I have attached an example of the results I get with the Sigma 800mm.  I would welcome suggestions for a practical scope that would give me at least 800-1600mm that my AVX mount can work with.

 

That's tough, and I don't think anyone should be telling you "oh just get a _________, it'll definitely work!" 

 

Imaging at 800-1600mm is not at all easy and for most folks starting out the recommended scope is an 80mm f/6 type (or smaller), used with a 0.8x focal reducer/field flattener getting you in the 400mm range. There are a lot of experienced imagers as well who stay in that range because going up in focal length increases the complexity to the point where it becomes expensive, and the frustrations can easily outweigh the rewards.

There are three major problems with larger scopes that have to be dealt with that I can think of (and I'm sure others can add to this list). 

 

The first is weight - longer focal lengths mean more glass and metal (or if you want to pay more, you can get a carbon fiber optical tube - but then, if you can pay more for that, you'd be smarter to pay more for a better mount), which means more weight. I think the AV-X is a decent mount, in the way that a Toyota Corolla is a decent car. It'll do the job reliably if what are you trying to is within reason: it can only carry so much weight before problems are incurred. Here's how the problems happen when you are trying to autoguide an AV-X carrying more than a *very lightweight* set-up: Normally the DEC axis should not have to be engaged, but if there is any error in your polar alignment your autoguiding program will issue a *move* command to the DEC axis, but there is mechanical static friction (stiction) in the gearing that must be overcome (this is similar to pushing a piece of furniture - it is harder to get that piece of furniture moving from rest than it is to keep it moving - gears in a telescope mount work the same way). For whatever reason, the DEC axis in AV-X mounts tends to be VERY sticky, meaning the impulse needed to get it moving from rest tends to be HIGH as well, so it is VERY difficult for this mount to make fine corrections needed to keep stars round. As focal length increases, the severity of the problem becomes increasingly magnified. As you increase the WEIGHT of your system, you increase the impulse needed to move the sticky DEC axis, which can easily lead to the mount/guiding system being unable to guide properly. 

 

A "dialogue" between the guiding system and the mount might look this:

 

guider to DEC axis: "move"

 

DEC axis receives command, attempts move, cannot overcome stiction, fails to move

 

guider to DEC axis: reissues "move"

 

DEC axis receives command, attempts move, cannot overcome stiction, fails to move

 

guider to DEC axis: reissues "move" command with added urgency

 

DEC axis receives command, attempts move, cannot overcome stiction, fails to move

 

guider to DEC axis: "I SAID MOVE!!!"

 

DEC axis receives command, overcompensates, and is now out of position on the opposite side.  

 

Repeat. repeat. repeat...

 

* * * 

The second is size - larger telescopes (other than SCTs) are longer and can be trickier to balance properly and can act like a sail if there is even a light breeze. Cords dangling from your camera(s) can also get blown by breeze and the gentle tugging on the mount from the telescope acting like a sail can also wreck your image. So now your guiding system is fighting the wind AND the weight of your system. 

 

* * * 

 

The third is going to be cost. Unlike your Sigma lens (and @Stelios pointed this out), most telescopes will simply not illuminate a full frame lens. They're simply not designed to, so you're throwing away sensor space. Heck, most less expensive telescopes may not even properly illuminate an APS-C sensor. A lot of the dedicated astronomy camera sensor sizes are actually kind of small - but that's totally fine, because, first of all, I don't think many amateurs are even thinking about making prints of their work, second - having that much sensor area actually works against you if you are imaging at shorter focal lengths. 

 

Here is a very good (and short read) on Pixel scale. 

 

Basically, what it comes to is that the easiest solution is this: if you don't want to or can't afford a better mount than the AV-X but still want to image, then if you use a shorter focal length optical system and a small astronomy camera with a sensor size/pixel pitch appropriate for that scope and your intended targets, you can still image the things you want, make pleasing images, and not be over-taxing your mount by making unreasonable demands. 

 

In short, it's not the focal length (and again, 800-1600mm on an AVX is asking for trouble) you should be thinking about, it's the image scale. 


Edited by DRK73, 05 August 2020 - 09:18 AM.


#7 bobzeq25

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Posted 05 August 2020 - 11:45 AM

You're likely headed here.  This is a common experience.

 

"I regret spending the first 6 months trying to learn imaging with an 8" Edge on an AVX mount. I lucked out and got one of the good AVX's, but with that scope/mount combo it was a losing effort. Fortunately got a nice little refractor, and not only have the quality of my images improved but I'm actually enjoying the process of learning how to do it!"

 

To avoid it, I'd get that small refractor up front.

 

This is an economical choice to learn on (and also make some fine images).

 

https://www.astronom...fpl-53-f-6.html

 

This is a great scope to learn on, that you'll keep for a long time as your big target scope.

 

https://www.skywatch...t-apo-refractor

 

There are a lot of good choices in between.

 

Whatever your choice, this will be the best $40 you'll ever spend in DSO AP.  <smile>

 

https://www.amazon.c.../dp/0999470906/



#8 johnfr

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Posted 05 August 2020 - 06:33 PM

DRK73
I understand the issues of Stick-slip friction in the mount. I spent 35 years doing motor controllers for the disc drive industry where we were moving objects in the sub-microInch range. As you can imagine stick-slip was always and issue. I have tuned my AVX mount to some degree already adjusting backlash out of the gear train. I haven't yet replaced the bearings with automotive roller bearing yet. That is my next tuning operation. In fact I just ordered some KRYTOX General Purpose Lubricant, -76° to 310°F, that I will use to repack the roller bearings with. The AVX uses same motors as a cgem2 (most people don't know that). I have a friend that uses a modified AVX carrying some 55 lbs. of gear and is able to guide down to about .35 or .4 on good nights... bad nights... .8 or so. I am more inclined to do this type of modes to my current AVX rather than spend 3x or 4x the cost I have invested in the AVX.

It sounds to me like the fact that I am attempting to image using a Full Frame DSLR is my down fall. I read the article on Pixel Scale your recommended and I realize that most folks doing DSO are using Astro Cameras with far less resolution and sensor size. That kind of puts me in a dilemma being a landscape photographer used to doing Gigapixel Wall Mural images. Even the highest resolution Cooled Astro cameras are at best 50% of the resolution of the Canon 5DSR. Granted the 5DSR is not the best low light model but it is what I have.

 

DRK73 and bobzeq25 -- You both make a good case for going with shorter focal length scopes combined with dedicated Astro cameras. Having gotten used to shooting at 800mm so going less feels like a step backwards. I have actually done a couple sessions using a 2x converter on the 800mm yielding 1600mm and find myself still want more reach. Being retired and living on Social Security, I don't have the budget to go out and spend 5 figures on additional equipment.

 

bobzeq25 -- thanks for the tip on the "The Deep-sky Imaging Primer" book. I am going to order it this evening.

 

In the mean time, it looks like for the foreseeable future, I will be using my Canon 5D and the Sigma 800mm f/5.6 EX/DG Telephoto lens/

 

Thanks for the advice.




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