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CEM40 Unguided Accuracy

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#1 Drwonga

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Posted 22 September 2020 - 11:39 PM

Hello All!

 

i just got a CEM40 (non EC) and noticed with my Orion ST80 and Nikon D500, I can only get about a 10-13” exposure before I see stars trailing. I’ve aligned via the iPolar software. Am I missing a step? I would have expected at least 30” of exposure unguided. Is the solution auto guiding?

 

Thanks!

 

Larry



#2 Tapio

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Posted 22 September 2020 - 11:47 PM

Auto guiding is the solution.
But I would have expected better unguided than 13".

#3 Linwood

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Posted 22 September 2020 - 11:50 PM

It certainly can do better than that, I imaged at 400mm and could fairly consistently get 30" exposures and frequently 60" with decently round stars (of course, unless you want to do statistics on the images, one person's round is another person's oval). 

 

Guiding works great.  If you guide, it's also easy to record PEC so you can do un-guided better (PEC playback helps correct for recurring errors on the RA axis; you basically record manual corrections, but manual corrections are painful so most people use a guide scope and camera to record it).

 

Polar alignment is critical of course. 

You might try hooking up PHD2 and using the D500 on the scope as a guide scope (clearly you can't image at the same time).  If you then run the guide assistant for a while, it will double check polar accuracy (there are a couple other things there related you can do).  You can also see how it is tracking while not guiding, graphically and numerically. 

 

A guide scope and camera is not all that expensive, the camera probably the most expensive part, plus you need to figure out how to attach the guide scope.  It will certainly give you a lot more flexibility.  But I suspect there's another problem there somewhere; unless you are really picky about roundness, you should do better than that. 



#4 terrypaula

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Posted 23 September 2020 - 12:33 AM

That's very surprising for an IOptron mount. "My strange example, somewhat... I have an AVX that was having the same issue, even though it is not capable of performing with the IEQ30 I could get a better measure proportionately with the AVX unguided so I decided to check the axis' against level and found the Declination was off level by a little more then 3mm.  I put reference mark on the declination and reset to the new Home position and the star trails I had were significantly better even without an auto guider in place."  I did go back to auto guiding for almost all AP after that.  EAA is a little different but strangely that's where I saw the most significant improvement.  Thankfully auto guiding isn't necessary for EAA.


Edited by terrypaula, 23 September 2020 - 12:34 AM.


#5 bobzeq25

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Posted 23 September 2020 - 12:42 AM

Hello All!

 

i just got a CEM40 (non EC) and noticed with my Orion ST80 and Nikon D500, I can only get about a 10-13” exposure before I see stars trailing. I’ve aligned via the iPolar software. Am I missing a step? I would have expected at least 30” of exposure unguided. Is the solution auto guiding?

 

Thanks!

 

Larry

Autoguiding is necessary for all but _very_ expensive mounts.  It's also a relatively easy (there is some learning and care needed) and cheap way to improve your images significantly. 

 

It's just not intuitive how important very precise tracking is to good images.  You can get a clue from two numbers.  Autoguiding is mediocre until you get below 1 arc sec.  The periodic error on a CEM40 is specced at +/- 7 arc sec.

 

So, 99+% of us autoguide.  The CEM40 is a good mount, but it still needs autoguiding.  Whether or not your mount should do better unguided is really not important, and you've no doubt got better things to work on.  If you can't get it to autoguide well, that's a different story.

 

Minor point.  The predictive algorithm in PhD2 is a very easy and effective form of Periodic Error Correction.


Edited by bobzeq25, 23 September 2020 - 12:46 AM.


#6 sg6

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Posted 23 September 2020 - 01:59 AM

The mount is a pretty good mount, which leaves the polar aligment aspect. I would suggest that you check both what you and iPolar managed and then check the accuracy by an alternative - look through the polar scope.

 

There is less chance of "What is wrong with my mount?" then "What is wrong with my app?".

 

Your presumption is that it is accurate and right. What if it isn't?

Basically use the app, if it is easier or quicker or more accurate. However I would say don't expect to get all 3 out of it, but do check the result, don't blindly use the app, know what it is doing, then check that it has done it. If you read there are a number of polar alignment "fails" posted around CN. Sometimes you need to know what to do, and I suggest that you, well everyone, needs to be able to check that the result is actually right.

 

With good, and relatively simple polar alignment with your eye and a polar scope you should be getting greater then 12-13 seconds. 30 easily and if you spent the time and care with a polar scope I would almost suspect 60 seconds is achievable.



#7 Tapio

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Posted 23 September 2020 - 02:02 AM

Alternative method to polar alignment is drift alignment (I personally trust this method).



#8 barnold84

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Posted 23 September 2020 - 04:51 AM

Hi,

 

I also ordered the CEM40 mount a few days ago and expect delivery within a few days from now.

 

I'm expecting that with this mount doing unguided photography at f=400mm should be possible up to 90 to 120 seconds. I succeeded doing 120 seconds exposures at 2000mm (yes, three zeros) with my LX85 and I consider this a quite cheaper mount compared to the CEM40 (yet to be proven by experiment). This was only possible with an extremely careful polar alignment which I achieved through drift alignment (my personal favorite) since the LX85 is per default not shipped with a polar scope.

 

As bobzeq25 said: get into auto guiding. What I said above was for the sole purpose of getting to know my mount. Here's a list why you won't be able to shoot for longer exposures without auto guiding:

1. Lack of perfect polar alignment. If you want to get the accuracy from above, drift alignment might take an hour if done with only a reticle eyepiece. If you're using software, your just one step away from auto guiding (so this doesn't count).

2. Periodic worm error. Every! worm drive has periodic error. Depending on the money you spend, this is more or less. Nevertheless, it's there and especially the longer focal length you are using the more you will notice it. Typically a full worm revolution is about 7 to 10 minutes. So the gear has sufficient time to ruin your image. Periodic error correction will help a bit but won't eliminate everything. It mainly "helps" the auto guider as it doesn't need to correct that much.

 

I don't know if I'll give it a shot to see how far I can drive the CEM40 without auto guiding. Maybe only for testing the mounts overall quality but not for daily use. 

 

The more interesting questions with the new mount would be: how accurate will the iPolar be? How well is the drive responding to guiding commands? Is the dec backlash within a good range or excessive (my current problem with the LX85)? How about max payload? Does it handle 50%, 80%, 100% max payload well?

 

Björn



#9 ShortLobster

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Posted 23 September 2020 - 08:05 AM

You should have better than that with that mount. Most of the likely reasons have been covered above: 

  • Check polar alignment with a second method
  • Check your balance in both axes
  • Ensure tracking is enabled in the mount, sidereal if you are imaging DSOs
  • Ensure everything on your rig is tight (saddle, alt & az knobs, focuser tube, camera, etc)
  • If you aren't using plate solving, make sure you have done a good alignment (2 or 3 star), and that you have cleared alignment data first. 

As others have said, guiding is the best answer. With my CEM25P and IEQ45 Pro I can get sub .8" RMS all night long. 



#10 Drwonga

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Posted 23 September 2020 - 08:37 AM

Thanks so much for all the feedback! It has confirmed where I ultimately want to go with this setup, which is auto guided with a wide field refractor. I think right now I just need to bite the bullet and move forward with assembling the other components of the rig. Ie: imaging telescope and guide scope with camera. In the interim however, I will work on polar alignment, balancing, and a 2-3 star alignment. I was also using King tracking, but will switch to sidereal.

 

Thanks so much! waytogo.gif




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