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iOptron 40cem - workable for EAA without North Sky views?

EAA equipment imaging mount dso CMOS
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#1 GaryShaw

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Posted 15 July 2019 - 01:39 PM

Hi All

I’ve been struggling with tracking problems in using my iOptron AZ Mount Pro. It seems fine for many of my EAA observing/Imaging sessions with short 3-8 second exposures but it is so far unable to handle exposures from 8-30 seconds long which I may want to do on occasion. If I ever wanted to take a try at full AP, it would be impossible unless sub exposures were less than 8 seconds. 

 

I can can probably trade the AZ Mount Pro in towards an iOptron EQ mount but my home observing sight does not allow me a view of the north sky and definitely not a view of the NCP. Best I can do for views northward are partial high-altitude views from about 270-340 degrees.

 

I’ve reviewed the iOptron CEM 40 manual and read about it variety of alignment procedures but I find it confusing and I’m still not clear on whether there’s a reasonable procedure to get a good NCP alignment without views to the north and within 15 degrees of the NCP. 

 

I’m hoping some of you have this mount ( or direct knowledge of it) and can provide me with some input on whether or not it will work for me and my limited sky views from south - northwest. Thanks in advance for any advice or suggestions you can offer - including alternate mount suggestions or ideas on improving performance of the mount I have. 

Cheers

Gary

 

PS:   If anyone is interested to view my video of the mount’s tracking error, the link below takes you to a short video showing Spica within the Sharpcap reticle view. I believe the movement of Spica that it shows takes place over 60 seconds. The camera is oriented correctly so I believe the error shown is indicating that the mount’s sidereal rate is tracking too slowly. Spica is not in an area affected by much field rotation.

 

https://www.dropbox....0 secs.m4v?dl=0

 

 



#2 Howie1

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Posted 15 July 2019 - 07:56 PM

It's in the cem40 manual on page 27 ... section 5.3.6 Polar Iterate Align .... it reads "This alignment method allows you to polar align the mount even if you cannot view the Celestial Pole" and it shows you bright stars near the meridian and horizon to do the iterations on. The iteration process for polar aligning is not well known but is in Craig Starks (PHD and Nebulosity creators) Nebulosity manual, and also I think I saw it on Uncle Rods astro blog once (?). Also, there's the DARV method of PA where you don't use objects near the pole. And then there's the Gold Standard for GEM's of drift aligning. 

 

Before splashing out / trading in ... if you really mean you wish to take up AP then you should definitely trade it in or return it, as an AltAz mount is not suitable for 'true' AP. If, however, as you are posting in an EAA forum you really wish to do short exposure EAA (not AP) then you can do EAA with Alt Az but as you rightly point out you will be limited to short exposures. Your statement about "high alt 270-340 gives you the longest exposures" is actually wrong according to this webpage (link down below). The table in that link says you should get sharp stars with up to a MAX of maybe 14 secs (and the author clearly says it is the MAX and depends on pixels size and focal length). But the table shows the areas where (for lat 40 close to your lat) the longer exposures SHOULD be possible. Try those areas where it's longer.

 

https://www.flickr.c...57674825468654/

 

Cheers


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#3 GaryShaw

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Posted 16 July 2019 - 08:00 AM

Thank you Howie, I appreciate your comments and suggestions. From reading the iOptron manual, it was not clear that the ‘iterate method’ allowed the use of stars in the southern portion of the sky. After reading some of the alignment procedures for Celestron EQ mount’s, I began to suspect, based on the clearer language in their manuals, that northern stars were not required.... but again, without hearing it from someone who actually aligns a GEM this way, I remain uncertain. It would be ideal to hear an iOptron CEM 40 user comment on the ‘iterate align’ process. 

 

I’ll definately check into the references you mentioned relative to alternate alignment methods. I suspect all of them are a bit more tedious than straight up polar alignment using a polar scope or iOptron’s iPolar system or even Sharpcap’s alignment procedure. Both of these last two, I believe, require being able to see within 15 degrees or so of the NCP- not an option for me. IOptron’s AZ Mount is pretty straightforward to level and align and I’ve been able to view and image some terrific DSOs using short exposures of 3-8 seconds. I have had trouble, though going above 8 seconds and I suspect this will limit my observing and imaging abilities - even for EAA - when lower gain and longer exposures (8-30 seconds say) may have benefit. Then again, perhaps that’s just a trade off I accept. Full AP is not my plan but having a more accurately-tracking mount appeals to my sense of quality even if it’s just a bit overkill for EAA. 

 

I’m clear on the field rotation issue you mentioned, my comment on high altitude, 270-340 degree sky sector was just a description of the portion of the northward sky that is visible at my home observing site. The rest is blocked. Thank you again for sharing your experience.

cheers

Gary



#4 Howie1

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Posted 16 July 2019 - 10:48 PM

Gary, I hope you get an answer from any CEM40 or other iOptron mount users who have used the iteration method. As I said in my post, not many folk know of it let alone use it. But you may get lucky and someone post up to help you out.

 

As you say, any SharpCap / Polemaster is going to want you to see the polar area ... and unfortunately you can't. PHD2 has methods which are in effect drift aligning, and are IMHO overkill for EAA.

 

Anyway, if you do end up going with a GEM, look at a Skywatcher GEM mount ... their PA routine built into the hand control (HC) does not require you to use any stars around the pole. I have gone 2min 30sec unguided and without PEC using that PA routine with a 1000mm 8" newtonian on my Skywatcher HEQ5 mount (IE way overloaded) ... so I know it works very well. The polar region right up to the equator can be totally obscured and the routine is still usable to do a polar alignment.

 

If you absolutely want a CEM40 mount, just re-read that section carefully ...  because like my HEQ5 PA rooutine it uses stars near the Meridian line which runs from NCP through Zenith and on towards the SCP. IE any bright star you can see along that very long line. And the horizon star it uses is any bright star you can see near'ish to the horizon in any direction. IE doesn't require you to have any polar view visible. More than half the sky can actually be covered for it to work.

 

Anyway, have fun and I hope you end up either getting that AltAz working well, or buying a GEM which will work for you.

 

Cheers


Edited by Howie1, 16 July 2019 - 10:49 PM.


#5 flyingcougar

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Posted 26 July 2019 - 08:33 PM

Gary, let's move this over to mounts for a larger exposure on the subject.


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#6 GaryShaw

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Posted 26 July 2019 - 09:21 PM

Gary, let's move this over to mounts for a larger exposure on the subject.

Perfect!

thank you Keith




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