IEXOS-100 Go to Issues #iEXOS-100 #astrophotography #alignment


Hello all, newbie here. I have been using my iexos-100 Mount for about a month now, along with the ES AR102 scope and still have issues with locking onto the actual target. I have to manually slew to my target to get it into my camera display. Also, I have star trails after 30 sec exposure. I perform a rough polar alignment, then do a 2 star alignment. Could my issues be caused by not getting the stars directly in my display during my alignment? It's hard for me to tell if they are exactly in my display since I live in light polluted area. I'm thinking I need to get a polar alignment scope and use sharp cap or pole master. Also, I know my site is too heavy for the Mount, I am hoping to trade it in for the EXOS2-GT(Hopefully Jerry sees this haha) I love the scope and Mount, just wish I had known more to pair the scope the the Mount. Any input would be much appreciated.!
Here is my first attempt at capturing Andromeda.

Jim Norwood

I’ve found with mine that it is important to:


Level the tripod.
Enter Lat/Lon in Explorstars

Put the mount on and locate Polaris through the alignment tunnel.

Balance the equipment on both axis.

Perform 3 Star Alignment. (Center object in FOV, press SYNC, press ENTER for each object. Not sure if pressing SYNC is necessary,  but I've been doing it)
Balance again after mounting the camera.


I can then seem to track reasonably well for hours. I haven’t really done much more than 30 second exposures yet though. I figured I’d just wait until I was ready to attempt guiding before doing that.




Hi Tommy, I have the same basic setup as you, but have added an ES ED80 CF to my collection.
Here are a few things I have learned that may be of help.

First, the AR 102 does work ok for visual observation on the iExos 100, but it can be a little “jiggly” if you have to touch it (change eyepieces, etc.) The mount can find and track a target ok for visual use. Yes the better the PA the better your results. I have a Rigel Quickfinder for a finder scope, and for visual targets, it works great. Another item I have found very helpful is the  1.25" Wireless Illuminated Reticle Eyepiece with Focusable Dual Crosshair - 20mm. I don’t know what eyepiece you use initially to center the target for your alignment, but something with a large wide FOV to start works great. Another option would be a 26mm 2 inch eyepiece. I have found that its not unusual to have to slew the mount a little to center the target with ExploreStars to complete the alignment. You don’t mention what brand of ExploreStars your using. I have both iPad and Fire HD8. I use the Fire most of the time.

Regarding astro photography,  (and I consider myself a beginner), I got the ED 80. Its much lighter, and the shorter focal length (480 vs 663) makes it easier to find targets. I do use a .8x focal reducer as well. You could use a focal reducer with the 102, but it would add a little weight and extend the length. I use the AR 102 for visual use, and the ED 80 for visual and astro photography. Regarding oblong stars I have found 45 secs the max and 30 secs works best. I don’t use a laptop, so mine are all unguided.  I do use my DSLR and a few lenses a lot with my iExos 100, and have had great success (I think). Taro Seki is a member of this forum and has posted outstanding photos on his website His are unguided and usually 30 secs. Hope this helps.
Vero Beach, FL
Scopes: ES  ED80CF, ES AR102,  Meade ETX 90 EC (Deforked)
Mounts: ES iExos 100-PMC Eight, ExploreStars IPAD & Fire HD8, Explore FirstLight EXOS Nano
Nikon D600, D5500

Wes Mcdonald


The basic issues with round stars in your setup is this:

1.  Explorestars 2-3 star alignments don't fix polar alignment error.  It builds a mount model in its little brain that it uses to convert actual celestial coordinates to mount coordinate system.  If the mount is polar aligned, and the cone error is zero, and the dec home offset is zero then the celestial coordinates are the same as the mount coordinates, but this is seldom the case.  So while a 2 star will HELP you find the target it will not provide good, rotation free, tracking of stars.

2.  The iExos has quite a lot of periodic error.  Frankly so does the Exos2.  ES designed these mounts with the idea that rather than spending big bucks for precision gear and mount assembly, a very usable mount could be built less expensively and with the addition of guiding it could be be made very good for a lot less overall cost.  And they did that.  The Exos2 can be routinely guided to less than 2 arc seconds, and often much lower.  On this forum we have similar reports for the iExos.  Even assuming a perfectly polar aligned scope, periodic error will move stars around the focal plane over time, thus causing them to be not round and smeared, more and more over time.  The Exos2 has a periodic error that cycles in 600 seconds.  This means half a cycle in 300 seconds, and quarter cycle in 150 seconds.  As with most sinusoidal"ish" waveforms, the rate of change of position is greatest as the thing crosses zero...If you happen to take an image at this time in the cycle you can get some pretty good motion in 30 seconds. 

These two sources of position error during an exposure pretty well puts a limit on how long you can expose, especially at longer focal lengths.  If you want to do unguided astrophotography get short focal length scopes that will provide a wide field of view in a camera, and get a camera with big-old pixels.  The game is to get the per pixel arc second scale big enough that 10-15 arc seconds of motion of the star is not visible.  A side benefit of the short focal length is the ability to get a fast scope, that is one that will permit shorter exposure times.  In my view this is evident as you consider the photons hitting a single pixel emanate from a larger sky area and thus are just more per second so exposure time for equal photons collected goes down.    

Sharpcap is a good way to get polar aligned as it uses your guide camera.  If your imaging camera is also a guide camera, I suppose you could use sharpcap to get polar aligned, then employ the same camera with your astroimaging software.  

I have a polemaster because I already had one for my Meade, and it works well on the Exos2.  On the iexos mounting it will be problematic, although it is not hard, one simply puts it on a vixen bar with a right angle 1/4-20 mount, align the mount, then replace it with the imaging scope.  It will cost you though.  Sharpcap, if it works with your camera, is a cheap date.

If you want to delve deeper into imaging it will require a commitment to go to guiding with all that entails.  Not hard, just a nice new pile of stuff to buy and get running.  

Regarding accuracy of GOTOs.  This is an interesting topic, one which I have been exploring.  To get accurate gotos, you have to do a few things, even when using explorestars 2-3 star alignment.  Here they are:
a.  Figure out how to repeat your set up with regard to level.  Fact is your level is sort of unimportant to the 2-3 star alignment, but being able to repeat it each time you set up is important to another thing you have to find, the DEC home. 
b.  Dec home error is the error between where the scope is pointing in DEC when you turn it on, and the actual DEC = 0 position (excluding mount polar alignment error).  The iexos I believe is lacking any alignment marks on its RA and DEC axis, and I recommend you find the polar home position and mark them.  While RA home is nice to have, the 2-3 star alignment removes RA home error it seems, so that one is not super important.  But this is not the case for the DEC home error.  It affects every pointing angle around the sky dome.  And while the 2-3 star alignment will provide great pointing at the stars employed, as you stray away from them in your viewing error will creep in.  I have a paper on finding DEC home in the Mounts section of the forum (or is it Main) in the user contributed files sections.  One of the subgroups has an empty folder, the other has a folder with the notes in it.  You might find it helpful.
c.  Telescope cone error also causes pointing errors across the sky dome.  Jerry published a link to the Astronomy Shed channel on youtube that tells you how to mechanically remove cone error.  I suggest you find that one and do it.  Once you have it corrected, it will not be an issue.  For now that is the best way to deal with it..

But for you the best method for finding objects is probably to use a red dot finder to go to a star located near a desired deep space object (DSO).  Do a goto to the star, and center it in the red dot (make sure the red dot is collimated with your telescope line of sight).  Then center the star in your guide scope (again colllimated).  Then SYNC.  Now do a goto to the DSO.  In the local area of the star you centered, the Gotos will be accurate enough.  Ideally you could use a flip mirrir to center the star through the main scope prior to SYNC.  The better you get the star centered, the better your goto to the DSO will be...But you can figure out the camera field of view as it compares to your guide scope field of view, and so long as you ability to center the star in guide scope is lots better than the camera field of view, the DSO should appear when you do a goto after a nearby SYNC.  Also, don't be afraid to do gotos to other "nearish" objects after a star SYNC.  Sometimes you get lucky in a pretty good usefully accurate region of the sky. 

When you have your telescope under computer control with a guide camera and imaging camera you can do a platesolve and have the mount move with high precision to a DSO, regardless of the inherent goto accuracy.  When you do it you will be experiencing one of the most magical things.   But that is another discussion.  Suffice it to say that the iexos, exos2 can both do all this stuff well when you want to go there.

In summary

1.  Stars are gonna smear for long focal lengths and Explorestars 2-3 star alignments because the mount is not polar aligned and there is periodic error.  Cure is to get polar aligned, and shoot short exposures with 400mm or less focal lengths.  Improve your polar alignment with sharpcap if you can,  or perhaps the iterative method as can be found on the web or in the forum posted by Jerry I believe.  

2.  GOTO accuracy is best if the mount is level and DEC home position is known (that is the DEC home position that was found with the scope "level" is repeated with the same "level" each time you set up).  Cone error must be eliminated mechanically one time, after which it should be good to go going forward.

That's my take on things.  Final thought is this -- all of this gets better with practice.



Wes, Southport NC
PMC-8, ES ED 127, 10" LX200GPS, Astro-Tech 8" Newt, ETX-90
Polemaster, Orion ST-80 and SAG
Electrical Engineer, Retired


Thank you for all the excellent info! I think I need to do a better job at PA. I have been doing a PA with my DSLR attached to the scope, trying to PA through the display. I will try with the eyepiece to see if I get better results. I also need to do a cone alignment. Lastly, I think I am going to sell my AR102 and go with an 80mm scope to reduce weight and the focal length.

Jeff Snell

Hi Wes,
Great explanation above.  WRT two or three star alignment on the EXOS2 GT PMC-Eight, can I simply slew to 2 or 3 known stars and sync on them to achieve the same accuracy?  The stars selected by Explore Stars are not the easiest to see from my location.

You've probably heard this already but a Polemaster is a great accessory.  To be clear, I am a novice at best but I just received my QHY Polemaster yesterday and had it up and running in minutes last night.  Under a Bortles 4/5 sky in Ogden Utah, and thin high cirrus clouds with barely any stars visible and bright 3/4 Waxing Gibbous, the camera performed magnificently puling in the data the naked eye couldn't.  The process was a breeze,  Can't say enough about spending the ~$270 it cost for the device.  You'll need an adapter for your specific mount and they run about ~$30.  Best move I've made thus far for a "must have" accessory purchase.


EXOS2 GT PMC Eight, Polemaster, Canon T5i (Modified)
ES ED 80 APO, Celestron 8" Edge HD

On Wed, Jan 8, 2020 at 7:21 AM Tommy <darko7411@...> wrote:
Thank you for all the excellent info! I think I need to do a better job at PA. I have been doing a PA with my DSLR attached to the scope, trying to PA through the display. I will try with the eyepiece to see if I get better results. I also need to do a cone alignment. Lastly, I think I am going to sell my AR102 and go with an 80mm scope to reduce weight and the focal length.

Wes Mcdonald


Short answer to your alignment question is no. Sync is not the same as an alignment.  The 2-3 alignment algorithm takes data based on your measurement (centering the calibration stars) and does a computation to solve for the mount misalignment.  With that knowledge, Exorestars then points the mount to selected objects by correcting the pointing angles for the mount alignment error.

Sync by comparison simply tells Exorestars that the computed position is correct at the new mount location to which you moved to center a star which you synched.  Basically Explorestars accepts the motor pulse counts and sets that position equal to the star celestial  position.  If you think about it you will see that after doing that, you kinda have a locally accurate place from which to move the mount to nearby positions....basically think of it as moving from the synced position incrementally by Delta Ra/Dec.  But note the mount is still not aligned and thus the Delta move can't be exactly correctly positioned on the celestial sphere....but the errors should be small for small Delta moves.  Now how small is small?  Well that depends on lots of stuff, but logically you will agree the better the polar alignment the further you could offset by Delta and have acceptable pointing accuracy (meaning you see the desired object in your telescope  field of view).  Happily sync operations do not Cascade.   Thus you can sync again elsewhere and get some pretty good pointing around there.   And that is what happens.

Interestingly, if the mount is polar aligned by, for example, use of a Polemaster and you have your cone angle close to zero and you started from a good Dec home position, there is really only one more error that needs to be removed to get great goto performance...Ra home error.  In that case you can slew to a star, center it in the field of view and Sync.  In this case the sync wiped out the Ra home error and now you can enjoy the full accuracy of pointing these mounts and PMC can offer.  One sync.  No 2-4 star alignment required.   

If the mount is polar aligned but there is Ra and Dec home error and cone error,  sync can't do the job alone.  This is because these errors cause the mount to point incorrectly in both Ra and Dec in a rotationally coupled way.  Thus the ra and Dec errors observed at various places in the celestial sphere are different.  A sync can correct the Ra and Dec error at an individual point in the sky.  When you move to another place, which should be correct since the mount was polar aligned, pointing will be wrong because the incorrect Ra and Dec offsets got baked into the pointing cake at the previous sync.  So you have to find the object and sync again.  I have to check this next point but I believe that even when polar aligned, if Dec offset exists (remember the sync did not find the globally correct Dec or Ra home error) then tracking unguided will slowly deviate from the object over time.  

Hope this TLDR helps.


Wes, Southport NC
PMC-8, ES ED 127, 10" LX200GPS, Astro-Tech 8" Newt, ETX-90
Polemaster, Orion ST-80 and SAG
Electrical Engineer, Retired


So, I decided to see what the Mount could do with just my 300mm Nikon lens, no scope and finally got decent results. I did a rough polar alignment and then a 2 star alignment. Then pointed towards Andromeda. It slewed almost directly to the target, with minor adjustments. Tracking was pretty good as well. Reducing my focal length and telescope weight really helped with the performance. Here's my final image of Andromeda. 
Mount: iEXOS-100
Camera: Nikon D5600
Lens: Nikkor 70/300
15x30sec lights
15 darks
15 Bias
Stacked in DSS, editing in Lightroom