CEM40 - To EC or not to EC

[Buzzard75]

WARNING!!! Wall of text explaining my situation before getting to the questions that have probably been asked a million times. 

I've been planning on getting an EQ mount for quite some time to take the next step in my astrophotography journey. I was pretty well set on the EQ6-R Pro after seeing one in person and the results it can produce. It appears to be a rock solid mount, but I was somewhat put off by the size and weight of the thing. I know that weight can be a good thing in terms of stability, but it has to be portable and as easy to haul around as possible as I go from location to location and tear down every night. Then iOptron came along with the CEM40, which appeals to me much more in terms of size, weight and portability. The payload is comparable to the EQ6-R Pro, but it weighs less than half as much. In addition, I already have a familiarity with iOptron mounts as I have a SkyGuider Pro and have recently been using our club's SmartEQ Pro. I have been able to get 120-second subs at 250-300mm reliably on the SkyGuider Pro. The SmartEQ tracking on the other hand is horrible and I'm limited to about 20-seconds at the same focal length before I start seeing star trails even with good polar alignment. The benefit of the SmartEQ Pro is the GoTo though. I'm currently using a William Optics RedCat 51 and camera lenses for all my AP. I have plans to get a triplet in the 102 to 127mm range, but not any time soon. For the time being, it will be short focal lengths and unguided. I can keep using the SkyGuider and continue to limit my exposure lengths, but that is obviously hindering me on the extremely faint targets and limiting my capabilities. Not to mention the fact that it's extremely difficult to locate those objects manually just using a Telrad. I can do it, it just takes a considerable amount of time that I could be imaging. The SkyGuider is great for Milky Way and widefield work though, where your targeting and framing doesn't have to be so precise. However, after using the SmartEQ Pro, I realized how much of a time saver the GoTo can be. I can get the target in frame every time rather than having to take dozens of test exposures and trying to figure out what I'm looking at or if I'm even close.

So, question time. I'm sure the standard CEM40 is an improvement over both of the mounts currently available to me. Given my current situation, future plans and that I will be shooting unguided for the foreseeable future though, is the EC model worth the extra money over the standard CEM40? I understand that once you start guiding, it all comes out in the wash and I've heard the argument to save the money and put it into a guiding setup. I don't plan to guide until I get the 102-127mm scope so that would require the purchase of the scope and another camera, possibly two, so more than the difference in cost between the standard and the EC model. It would also mean more equipment to haul around and complexity in the setup. I'm not ready to take that step yet.

I guess the real question is, all things being equal (PA, seeing, etc.) and shooting unguided at 250-300mm, will there be any marked improvement in the length of my subs with the CEM40 or would I need to step up to the CEM40EC? What sort of difference in length of exposures could one expect between the CEM40 and the CEM40EC with the improved tracking accuracy? Would the difference be more noticeable at a longer focal length of 750-900mm?

[vlaiv]

Let me have a go at this, although I don't have experience with such mount.

I think that encoders are good idea in your case and they are worth it. Are they worth the money difference? Can't be the judge of that, and it depends on your budget and if you can afford them. If guiding was an option, then I would say - get plain version  and do the guiding, but since you don't want to get into that yet, then encoders are very good thing.

They won't help you with PA, but PA errors are quite often much less than people believe. Main thing that is limiting your unguided exposure time with such mounts is periodic error, and that one gets handled by encoders.

[Buzzard75]

16 minutes ago, vlaiv said:

Let me have a go at this, although I don't have experience with such mount.

I think that encoders are good idea in your case and they are worth it. Are they worth the money difference? Can't be the judge of that, and it depends on your budget and if you can afford them. If guiding was an option, then I would say - get plain version  and do the guiding, but since you don't want to get into that yet, then encoders are very good thing.

They won't help you with PA, but PA errors are quite often much less than people believe. Main thing that is limiting your unguided exposure time with such mounts is periodic error, and that one gets handled by encoders.

Thank you! This was my understanding on the purpose of the encoders as well and that it was probably the better choice for me. I don't want to say money isn't a concern, but I can afford it. If it's the only mount I will buy for a long time, I'm also willing to make the investment. The only thing I wasn't particularly clear on was how much of an improvement there would be on the length of the subs due to the improved tracking accuracy. I doubt that's something anyone would be able to give a detailed answer on without doing some testing. I know that at longer focal lengths the error of even a few arcseconds can make a huge difference in elongation of stars.

[Freddie]

Before spending your money, I would have a good read through of the numerous threads about the Ioptron EC mounts (40 and 60) on the cloudy nights forum.

[vlaiv]

4 minutes ago, Buzzard75 said:

Thank you! This was my understanding on the purpose of the encoders as well and that it was probably the better choice for me. I don't want to say money isn't a concern, but I can afford it. If it's the only mount I will buy for a long time, I'm also willing to make the investment. The only thing I wasn't particularly clear on was how much of an improvement there would be on the length of the subs due to the improved tracking accuracy. I doubt that's something anyone would be able to give a detailed answer on without doing some testing. I know that at longer focal lengths the error of even a few arcseconds can make a huge difference in elongation of stars.

You could try to estimate what sort of improvement you are going to get if you have enough info on how mount behaves.

Not sure if anyone published related info on CEM40EC, but there are at least a few posts on how CEM60EC behaves (and CEM120EC for that matter). I think it's safe to say that CEM40EC will perform similarly.

Start by calculating sampling rate of your setup (arc seconds / px) and also doing some basic measurements on FWHM. For elongation to start showing you need something like 10-20% of difference in two axis. I'll do a simple calculation as a guide on pretty much made up (but still assumed based on real numbers) data.

You are shooting at 300mm FL, and using apertures below 70mm, so FWHM should be around 3.5" or so. PE of about 0.7" for duration of exposure is going to be upper limit (20% of FWHM).

CEM40 is listed as having up to 7" P2P PE over 400s. Roughly speaking if we assume that perfect sine wave, 7" will be covered in half that time, so we can say that for 20s or so exposures you will have perfect stars always. If you relax what you perceive as round star, and do about 50% (so its obviously elongated but still not "streaking") you can do one minute exposures with non EC version.

In contrast to this, EC version has error listed to 0.25" RMS on 400s - so you can safely go with much longer (half a dozen minutes or more) exposures and you won't see any elongation due to PE. With this mount, PA error and atmospheric refraction will be main source of error

[vlaiv]

This is very interesting:

There is section on both guided and unguided exposures and star shapes, there is also guide assistant in PHD2 showing that at this particular setup (PA error) drift rates were 0.7"/min in RA and 0.6"/min in DEC.

Elongation started showing in 5 minute subs without guiding.

[Buzzard75]

 

18 minutes ago, Freddie said:

Before spending your money, I would have a good read through of the numerous threads about the Ioptron EC mounts (40 and 60) on the cloudy nights forum.

Thanks! I've read a lot of them and they honestly make my head hurt. 🤣 A lot of them come down to the argument of guided vs. unguided and that you should just be guiding and it won't make a difference. Got it, understand it, not where I'm at with my skills and abilities or equipment right now. Some of the arguments appear to have absolutely nothing to do with the mounts and that just causes me frustration, so I'm probably missing some nuggets of information in there. *sigh*