Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Recommendations for home observatory mount

astrophotography equipment mount observatory
  • Please log in to reply
40 replies to this topic

#26 rgsalinger

rgsalinger

    Skylab

  • *****
  • Moderators
  • Posts: 4476
  • Joined: 19 Feb 2007
  • Loc: Carlsbad Ca

Posted 29 April 2019 - 05:14 PM

Now that my CEM120EC2 has been working for a year and has even been transported to a new home - last week - I think that it's proven itself to be a bargain. I have not had one single night (when seeing and windcooperated) when the RMS guiding was anything ever got above .40 arc seconds. Note that this is on a permanent pier. Note that the most weight I've put on it was my AP155 F7 which is 40" long and weights with rings/camera/wheel/etc about 40 pounds. I can't imaging paying more for a mount than the 7K that one of these costs unless I was convinced that I had some payload or use case that made something else more desirable.

 

The Mach 1 was a great mount for me because it was so light and transportable, I do not have the same good feeling about the 1100 and 1600's and I don't know why I'd want to give up 40lb of capacity and get a Mach 2 instead for a bit more money. 

 

 

Rgrds-Ross


  • aneeg likes this

#27 gotak

gotak

    Surveyor 1

  • -----
  • Posts: 1805
  • Joined: 18 Sep 2016
  • Loc: Toronto, CA

Posted 29 April 2019 - 10:26 PM

Thanks Tim, that's a question I had earlier (do I need DEC encoders).

Anyone else care to him in on the benefits of the EC2 vs EC?

CF


When modeling works you could in theory with some effort go unguided.

And maybe it also ensure no dec backlash? Not sure about that one.

#28 rgsalinger

rgsalinger

    Skylab

  • *****
  • Moderators
  • Posts: 4476
  • Joined: 19 Feb 2007
  • Loc: Carlsbad Ca

Posted 01 May 2019 - 12:27 AM

First thing that I would do is to ask iOptron about whether or not mount modelling will be effective (assuming that they ever release it) with just one encoder. Based on what I've been able to glean they are trying to put out something but they are finding it "harder" than they thought it would be. At the same time, I can easily go 4 minutes unguided with my refractors and 2 encoders. I can't wait to get a big scope on my mount to see how far I can go. With my CMOS cameras, exposures longer than 3 minutes just have everything burned out in the middle anyway.

Rgrds-Ross



#29 spokeshave

spokeshave

    Surveyor 1

  • *****
  • Posts: 1876
  • Joined: 08 Apr 2015

Posted 01 May 2019 - 04:00 AM

First thing that I would do is to ask iOptron about whether or not mount modelling will be effective (assuming that they ever release it) with just one encoder. Based on what I've been able to glean they are trying to put out something but they are finding it "harder" than they thought it would be. At the same time, I can easily go 4 minutes unguided with my refractors and 2 encoders. I can't wait to get a big scope on my mount to see how far I can go. With my CMOS cameras, exposures longer than 3 minutes just have everything burned out in the middle anyway.

Rgrds-Ross

I want to make sure that I was no misunderstood. If the intent is to do unguided imaging using multipoint modeling, I would not recommend a single encoder mount.

 

I'll add that unguided imaging is hard, and IMO, not worth the effort. While it is true that my mount will easily do exposures of 10 minutes or more unguided, it won't do that for every sub in every part of the sky. Even the best modeling software only produces a model and all models are flawed. Actual tracking is still open-loop and at any time, the mount is only guessing where it is pointed. Closed-loop guiding has become so easy, inexpensive and effective, that to me at least, it makes no sense to try to image unguided.

 

Tim



#30 ClownFish

ClownFish

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 6532
  • Joined: 26 Apr 2005
  • Loc: Kampala, Uganda

Posted 01 May 2019 - 04:09 AM

I actually do not know the advantage of unguided astrophotography, when an OAG is so inexpensive.   

Please debate the pros and cons ($$$) of accurate unguided imaging.   I have been doing well with guided imaging.

 

CF


  • roofkid likes this

#31 RaulTheRat

RaulTheRat

    Mariner 2

  • -----
  • Posts: 230
  • Joined: 09 Oct 2018

Posted 01 May 2019 - 06:21 AM

This is an interesting thread to me as although I'm probably a bit longer away from buying than the OP, my budget and requirements are similar.

Particularly, I'm interested in hearing more pros and cons between the Mesu 200 and the CEM120EC2.

#32 WadeH237

WadeH237

    Skylab

  • *****
  • Posts: 4168
  • Joined: 24 Feb 2007
  • Loc: Snohomish, WA

Posted 01 May 2019 - 08:02 AM

Regarding the encoder on just the RA, vs both dec and RA, I would choose the two encoder solution.

 

I am not familiar with the iOptron implementation, but I am making an assumption that it uses the encoders to ensure zero backlash.  If that's the case, the dec encoder will make a difference in guiding - more so than the RA encoder, since a properly set up mount will not see backlash in RA.

 

I've never actually understood the utility of having an encoder on the RA axis only.  The only practical benefit is to eliminate periodic error.  This makes the spec sheet look good, but if you are going to be guiding anyway, you shouldn't generally care about periodic error.


  • ezwheels and Der_Pit like this

#33 alphatripleplus

alphatripleplus

    ISS

  • *****
  • Moderators
  • Posts: 56940
  • Joined: 09 Mar 2012
  • Loc: New Jersey

Posted 01 May 2019 - 08:34 AM

I actually do not know the advantage of unguided astrophotography, when an OAG is so inexpensive.   

Please debate the pros and cons ($$$) of accurate unguided imaging.   I have been doing well with guided imaging.

 

CF

Please start a new thread if you want to discuss the pros and cons of accurate unguided imaging. Thanks.



#34 gotak

gotak

    Surveyor 1

  • -----
  • Posts: 1805
  • Joined: 18 Sep 2016
  • Loc: Toronto, CA

Posted 01 May 2019 - 08:45 AM

Regarding the encoder on just the RA, vs both dec and RA, I would choose the two encoder solution.

I am not familiar with the iOptron implementation, but I am making an assumption that it uses the encoders to ensure zero backlash. If that's the case, the dec encoder will make a difference in guiding - more so than the RA encoder, since a properly set up mount will not see backlash in RA.

I've never actually understood the utility of having an encoder on the RA axis only. The only practical benefit is to eliminate periodic error. This makes the spec sheet look good, but if you are going to be guiding anyway, you shouldn't generally care about periodic error.


I'd say that ioptron is using the encoder as a means to tame PE. They have a PE spec but if you seen the copies for non-EC versions it can be very smooth or in some cases has a few spikes. If it's in the +/- specs it's still sufficient for ioptron, as least I have never heard of them saying otherwise.

So the encoders are essentially used to give a very flat PE without having to achieve that natively with the mechanical bits. Some would argue that's a bad thing, but then again very expensive fighter jets are designed to be only flyable with the assistance of electronics. And our world is becoming more and more controlled by electronic bits anyhow, so I for one don't really care about this use of encoders.

The other aspects is the the encoders removes the need to do PEC. Which some people feel improves guiding.

My own experience is that the encoders do provide a measurable improvement. So I averaged 0.4-6 on RA and now with encoder min the cem60ec I am down to 0.4-0.5 total with 0.3X being normal for RA. And overall just a better behaved mount.

So in the end it's not that you can't achieve similar with guiding, but I think you'd find it harder and less reliable.

In a nut shell the EC versions are really for people who want a bit better and see still going to accept guiding as a normal thing to do. And it's not too far fetched as eliminating all sources of movement outside of the mount is a serious task and often frustrating.

#35 spokeshave

spokeshave

    Surveyor 1

  • *****
  • Posts: 1876
  • Joined: 08 Apr 2015

Posted 01 May 2019 - 11:25 AM

Regarding the encoder on just the RA, vs both dec and RA, I would choose the two encoder solution.

 

I am not familiar with the iOptron implementation, but I am making an assumption that it uses the encoders to ensure zero backlash.  If that's the case, the dec encoder will make a difference in guiding - more so than the RA encoder, since a properly set up mount will not see backlash in RA.

 

I've never actually understood the utility of having an encoder on the RA axis only.  The only practical benefit is to eliminate periodic error.  This makes the spec sheet look good, but if you are going to be guiding anyway, you shouldn't generally care about periodic error.

The belt drive system in the CEM120 is virtually backlash-free. My backlash in DEC is too low to be measured by PHD2.

 

The advantage of having an RA-only encoder mount for me is simple. It makes RA guiding error equal to DEC guiding error. My old mount - a CGE Pro - was excellent. It would always guide at less than 0.5" RMS error in each axis. However, even though the guiding was very good, guiding in DEC was always a little bit better than RA. So, even when both axes are guiding at less than 0.5", if the RA error is 0.45" and the DEC error is 0.30", the stars will be eccentric in the RA direction. That's because the RA axis must be corrected for PE and the DEC axis is just along for the ride. Correcting for PE in guiding is by definition reactive, so some error must occur in order for PHD2 to correct it. This inherent error is eliminated by the encoder on the RA axis. The difference, particularly at long focal lengths, is dramatic.

 

For a permanently-mounted mount in an observatory, I see no need for the DEC encoder, unless a multipoint modeling approach for ungiuded imaging will be used. In normal closed-loop guiding, the only corrections that the DEC axis should make are to correct polar alignment error and much more infrequently to correct for atmospheric dispersion. When permanently mounted, perfect polar alignment can easily be dialed in and maintained and guiding corrections in DEC would be very rare. Set up correctly, the DEC axis has to literally do nothing the vast majority of the time.

 

Tim


  • roofkid likes this

#36 WadeH237

WadeH237

    Skylab

  • *****
  • Posts: 4168
  • Joined: 24 Feb 2007
  • Loc: Snohomish, WA

Posted 01 May 2019 - 11:48 AM

Thanks for the comments on the RA encoder value.

 

My experience with earlier mounts (mostly a pair of CGEs), was that declination guiding was much harder to tame than RA guiding, even without using PEC.



#37 John Fitzgerald

John Fitzgerald

    In Focus

  • *****
  • Posts: 5447
  • Joined: 04 Jan 2004
  • Loc: ROR Obs. near Pettigrew, AR

Posted 01 May 2019 - 02:13 PM

Sometimes the ground can shift a little over periods of months or years, so polar alignment may have to be tweaked. Depends on mass, width and depth of pier, subsoil conditions, and seismic activity.

#38 spokeshave

spokeshave

    Surveyor 1

  • *****
  • Posts: 1876
  • Joined: 08 Apr 2015

Posted 01 May 2019 - 04:42 PM

Thanks for the comments on the RA encoder value.

 

My experience with earlier mounts (mostly a pair of CGEs), was that declination guiding was much harder to tame than RA guiding, even without using PEC.

DEC can certainly be the problem child, particularly in lower-quality mounts and when DEC has to deal with PAE. I was describing my use case in which the mount is permanently installed in an observatory where PAE is virtually non-existent. My CGE Pro was very well-behaved in DEC when installed in the observatory. RA-only encoders would obviously not be of the same value in a mount when DEC error is the limiting term. However, my comment was specifically in the context of the CEM120XX. From what I know about my mount and from those who also have the mount, it is extremely well-behaved in DEC, having virtually no backlash. 

 

More broadly speaking, if a mount does not have a lot of backlash and stiction that cause DEC to behave wildly and that has decent polar alignment, there can be many minutes between a legitimate need for a DEC correction. However, even on mounts with relatively low native PE, a legitimate RA correction will be needed every few seconds. So, in otherwise well-behaved mounts, there should always be more error in RA than in DEC. 

 

Tim



#39 photoracer18

photoracer18

    Vanguard

  • *****
  • Posts: 2436
  • Joined: 02 Sep 2013
  • Loc: Martinsburg, WV

Posted 06 May 2019 - 07:19 PM

If by "close", you mean the distance between St. Lois, Missouri  and Boston, Massachusetts.  Yes, then I guess he is close.  wink.gif

I don't think a lot of people really know how big many of the western states actually are.



#40 ClownFish

ClownFish

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 6532
  • Joined: 26 Apr 2005
  • Loc: Kampala, Uganda

Posted 07 May 2019 - 02:37 AM

I don't think a lot of people really know how big many of the western states actually are.

More that Mexico is huge, 15% larger than Alaska, and this town is in the middle of that country.



#41 PFitzhorn

PFitzhorn

    Vostok 1

  • *****
  • Posts: 155
  • Joined: 20 Sep 2017
  • Loc: Fort Collins, CO

Posted 07 May 2019 - 01:00 PM

This may be obvious, but I chose my mount based on two major criteria.  First was weight of the payload and the carrying capacity of the mount including accuracy and precision for guiding - yada, yada, yada - like you're doing.  Second was the software I would be using to drive the system to ensure compatibility between the two.  Since I use TheSkyX, compatibility with mounts wasn't much of an issue, but I notice my current TheSkyX Pro build (12164) has a driver for the CEM120EC, but not the CEM120EC2.  I've no ideas what the differences are and this may or may not be an important issue.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: astrophotography, equipment, mount, observatory



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics