What is the best mount for my set up?

[Icesheet]

Based on a previous post here: Help- CEM40, CEM60 or EQ6-R Pro, I decided to dig a little deeper as to how I could choose a suitable mount for my current set up (and potential future set up). If I had the money I would just buy a Mesu or CEM120EC and stop worrying that the mount may be my limiting factor. Unfortunately my pockets are only so deep and significantly emptier due to this hobby! A caveat to this post is that it is theoretical and not based on any real life personal experience. Also, all of the information has been sourced elsewhere so please correct any inconsistencies or errors I have made. 

 

Right now the most demanding equipment I have of a mount is an Esprit 100ED (550mm FL) coupled with an ASI1600mm (3.8μm)This set up weighs in at ~9kg and gives me an image scale of 1.43"/px. Therefore, regardless of anything else the resolution of my images will not exceed this threshold. Based on that my first instinct was as long as I have mount that can handle 9kg and guide with <1.43" Total RMS then my image quality would not be degraded by mount performance. As is always the case in this game it is not as simple as that. 

 

I stumbled upon a thread on CN about arcsec/pixel and guiding RMS in which a user explained that an approximation of your final output resolution can be determined by:

output resolution = sqrt(seeing^2 + sampling^2 + guidngRMS^2)

This is a simplification in itself and another user pointed out that there other inputs here, known and probably unknown. Have a read for more info. From my perspective this is a good enough approximation. Based on that equation my final output resolution will never be better than the largest input.

 

So if my seeing is 2", my sampling is 1.43" and my guiding is 1" my output resolution would be:

sqrt(2^2 + 1.43^2 + 1^2)

= 2.7"

That is the stars in my final image would measure 2.7"FWHM when ignoring any other factors.

 

Taking a look at this keeping the seeing and image scale the same but with examples of bad guiding (2"), good guiding (0.8") and god like guiding (0.3") you get the following outputs.

Bad = 3.2" FWHM

Good= 2.6" FWHM

God like= 2.5" FWHM

 

So by improving the guiding by the same factor sees diminishing returns in terms of final resolution output. Again, ignoring other factors.

 

If I know consider the image scale I would have based on the scope I think I would buy next, an Edge HD 8" it would look like this:

Edge with x0.7 reducer (1422mm FL) and ASI1600mm (3.8μm) gives an image scale of 0.55"/px (maybe I would want a new camera too!)

Using the same guide RMS for bad, good and god like (assuming I could achieve that at this FL) it would give the following:

Bad= 2.9" FWHM

Good= 2.2" FWHM

God Like= 2.1" FWHM

 

So while the unrounded numbers show a bit more disparity between Good and God like, the same diminishing return is apparent. Even at longer FL and higher resolution where I would have expected the difference in the improved guiding to really count. Only if I change the seeing to exceptional do you start to see a noticeable difference between good and god like guiding. Unless I'm not appreciating the difference 0.1" FWHM of stars has in resulting image quality? Perhaps the difference will be related to the amount of subs you have to throw out?

 

Now the two mounts I have been considering are the EQ6-R (£1179) and CEM60 (£1699) which if I include the price of tripod for the CEM60 likely takes it closer to £2000. It seems to me that the common perception is that the CEM60 is a significant step up to the EQ6-R. Based on the specs I can find online the peak PE of the mounts are, CEM60 <+\-5" (manufacturer spec) and EQ-R between +/-6-7" (no manufacturer spec but found some who had measured it online). Furthermore, based on the guiding experience I can find online there wouldn't be much that in it, certainly not the difference between good and god like I outlined here anyway. Perhaps iOptron are really conservative in their peak PE error and perhaps there will be a lot more variance in an EQ6-R so that I can be pretty sure of better consistency from the CEM60 but am I really going to see the benefit of that in my final images? I mean the difference between the mounts just now is a good chunk of the price of a brand new Edge HD 8"! 

 

I know much of this is a generalisation and perhaps I'm missing key aspects here? I know the CEM60 has higher resolution stepper motors, is lighter and has a higher rated payload (although, I note FLO recommend to consider 2/3 of quoted payload iOptron's products which would actually bring it line with the EQ6-R), but does it justify the extra cost? It seems I would have to move up to the CEM60EC to perhaps see the difference? 

 

I appreciate anyone's input here especially if you have experience with both mounts as real life experience is a better gauge than the potential the numbers might show. 

Edited July 1, 2019 by Icesheet

[cotak]

This is a game for people who likes rabbit holes.

What is your current mount? Do you really need to change your mount for a c8? The c8 isn't that heavy in the grand scheme of things and it's very compact so unlikely to really exceed the limits of all but the lightest class of mounts. Even try cem25 has been known to swing a c8 now and again.

Chasing star FHWM it might be worth your while to get an AO unit instead. It would make it much more likely to get godly guiding.

As to the 1/2 and 2/3 rules. Those are really poor way to describe a problem with many facets. I have seen someone throw a 12" Meade on a cem60 and have reasonable results on his first try at imaging. I have also seen someone overload a heq5 with great success. What is likely true is that the eq6 type mounts with fixed worms are stiffer in relation to external disturbance. While the spring loaded ones likely is best used when calm or inside wind protected observatories. Even the way you balance them are different. The fixed worm types often requires biased balancing, while I have found my iOptron mounts has subtile but significant performance improvements when 3D balancing is achieved. I say subtile because unless you are using an EC version you might not even realize there is soemthing there due to the noise of regular guiding. I didn't even realize that I had terrible guide frame download lag and variances until I tried to better understand why short and long guide exposures resulted in such different behaviours. As it turns out the lag contributed to inducing oscillations in I believe both the EC and the ieq45 pro, but just that with the ieq45 there was a lower expectation on my part to what it should look like so I never noticed a problem. So from the above my conclusion is that there could be a lot of people who tried ioptron only to discount them due to those final few factors which might seem small but could have a big impact.

As to your choice specifically, both mounts will work well with a c8. Your real question should be if the other factors like future OTA upgrade or features like the cable management setup on the cem60 matters. 

[Icesheet]

Thanks @cotak, and my reference to ‘god like’ guiding was based on your graph

I sold my HEQ5 so I need to find another mount now and may as well make it worthwhile that’s why I started looking at this. From reading around forums you commonly hear the CEM60 touted as a step up from the EQ6-R but that the EC version is not worth it. Based on the spec (and your experience) it seems to be the other way round. The CEM60EC is the real step up and there’s not much between the EQ6-R and the standard CEM60. 

I’m not chasing star FWHM, although like anyone else I would like them as tight as they can be. So instead of taking it for granted that the CEM60 would be the mount that gave me that, I wanted to try and confirm it. It now seems to me that I would not be giving up much by going for the EQ6-R over the CEM60. Aside of weight and cable management benefits which do not justify the extra cost for me. Again, all this being said without real world experience.

I guess my mind is made up about it really and I just wanted to hear if anyone could justify the extra cost of the CEM60 over EQ6-R, preferably through experience. Actually, I really thought about the CEM25EC given it may have been able to handle the payload but it only has latitude adjustment to 60deg and I’m slightly north of that. 

As to future upgrades I’ve learned to never say never with this hobby but I can’t imagine needing to load >20kg and I need a mount now. If I could stretch to the CEM60EC I would but I can’t and with the money saved the EQ6-R seems the better bet just now. No doubt I’ll curse the day I thought the weight and cable management were not worth it!!

 

Edited July 1, 2019 by Icesheet

[cfrommen]

I really think that the EQ6-R Pro is the way to go in your case. Honestly, if weight wasn't my major concern compared to the CEM60, I would also choose this mount. But heart surgery forces me to really cut the load when carrying equipment. That's why I will probably opt for the CEM60 (non EC) as the next new mount for me. Trevor Jones over at AstroBackyard.com has gained a lot of experience with this mount (and also the CEM60) over the past months and he highly recommends the EQ6-R Pro.

Don't worry about cable management so much. The money you save from buying the EQ6 vs the CEM60 can be invested in a Primaluce Eagle 3 (mixed reviews) or similar that helps you to cut down on cables and connections dangling around  😉

 

P.S. If weight is not an issue for you...I have a trusted Takahashi NJP mount  (Temma PC) which should take care of your needs for many more years to come. The 25kg is too much for me to lift around that's why I was looking into the CEM60/EQ6-R Pro mounts. We just have to get it from Oslo to Bergen

 

[Icesheet]

It sounds like the CEM60 is the one for you @cfrommen.  It's a shame you can't benefit from your NJP now. I couldn't find too much info on it but I'll send you a PM to find out more (in English, I'm a Brit in Norway!)

If I do go eventually go for the EQ6-R I will most certainly put the saved money to good use, thanks 

 

 

[cotak]

The EQ6 is really a good choice. I am sure you'll enjoy using it. 

Weight isn't a big deal for me but even with the CEM60 I wouldn't call it something you don't notice when you have it in your hand, quite the opposite. Actually as weight is one of it's selling point I was quite surprised at how heavy the hard case is with the mount inside.

 

Edited July 2, 2019 by cotak

[souls33k3r]

Just to add to this, the weight of a EdgeHD 8 and Esprit 100 is the same

[ollypenrice]

I think too much theory can take you into a land of dreams! I have a simple rule of thumb, which is that I want my guide RMS in arcseconds to be no more than half my image scale in arcseconds per pixel. And I also found from experience that, at my site, there was no point in imaging at 0.63"PP because I was only getting 'empty resolution,' meaning a larger image containing no more information than a smaller one would have contained even when my guiding requirement was met. I feel that my site will support 0.9"PP often enough for it to be worth having a rig which offers that, but it is far from uncommon for us to shoot only colour on less stable nights and wait for good ones for the luminance. Personally I won't be trying 0.55"PP but that's me and at my site.

Given that CMOS cameras are coming in strongly and have tiny pixels (which don't bin to great advantage) I wonder about the future of scopes with much more than a metre of FL in amateur AP. I now use a 140/1015mm refractor to reach my 0.9"PP.

The new EQ sixes have certainly improved if they run a PE of +/- 6 arcsecs. My older ones are at about four times that but they guide at between 0.5 and 0.8 arcsecs.

I run a couple of second hand Mesus here very happily but my choice in your shoes would be the iOptron, I think, for the accuracy. 

Olly

[Icesheet]

Your rule of half seems to be consistent with what I’ve read and what the theory above suggests actually. I’ll be honest I hadn’t really thought about all this until I started considering this mount (and possible scope) upgrade. I’m glad I did though as I now feel I’m in a better position to chose a suitable set up based on my likely sky conditions.

 

Im curious to know why binning on CMOS sensors with small pixels would not be beneficial?

[pete_l]

On 01/07/2019 at 03:38, Icesheet said:

So if my seeing is 2", my sampling is 1.43" and my guiding is 1" my output resolution would be:

sqrt(2^2 + 1.43^2 + 1^2)
= 2.7"

That is the stars in my final image would measure 2.7"FWHM when ignoring any other factors.

So in practical terms. your final resolution would be a little worse than your seeing. Although using the RMS figure for guiding does, itself, hide a multitude of other problems and assumptions. Ones that make any real accuracy in modelling performance rather suspect.

While the CEM60 does look better on paper, can anybody actually point a finger at the images from that mount and say they are consistently any better than the same images (scope, camera, target) taken from an EQ6? Or that the imaging process s significantly easier?
Those are the factors I would investigate if comparing the mounts: the results, not the specs.

[ollypenrice]

7 hours ago, Icesheet said:

 

 

Im curious to know why binning on CMOS sensors with small pixels would not be beneficial?

The discussion in this thread turns to the subject of binning CMOS. 

Olly

[Icesheet]

5 hours ago, pete_l said:

While the CEM60 does look better on paper, can anybody actually point a finger at the images from that mount and say they are consistently any better than the same images (scope, camera, target) taken from an EQ6? Or that the imaging process s significantly easier?
Those are the factors I would investigate if comparing the mounts: the results, not the specs.

This is exactly what I’m getting at but as hard as it is to properly compare on paper due to the variables it will be just as hard to compare the results in final images due to the same factors. Unless someone does numerous side by side tests with exactly the same equipment. Perhaps beyond it all is what @ollypenrice and others have said previously, we’re probably going to be seeing limited anyway. FWIW, in my opinion, at shorter focal lengths (<1000mm) and a typical image scale of >1.5”/px I don’t think you’d notice the difference between the two mounts. Longer focal lengths and smaller image scale is where you might see the benefit from the CEM60 (on nights where seeing permits). 

 

3 hours ago, ollypenrice said:

The discussion in this thread turns to the subject of binning CMOS. 

Olly

Thanks! Interesting read.