Last updated: 9 April 2000

Autostar Alignment and GOTO Tips

From:	mark@carmine.deom.co.uk (Crossley, Mark)
I've see lots of messages about alignment problems, and GoTo
inconsistencies with the Autostar. I offer the comments/rules/hints
below as a result of my experiences and thoughts about the process.

Whilst some of it will be teaching you how to suck eggs, I'd be
interested to hear if those people who do have inconsistent results
would try these steps and see if it improves matters for them.

Using these 'rules' I find that virtually every object I 'GoTo' is in
the FOV of the 26mm EP + 2x barlow, and certainly within the FOV of the
26mm EP on its own.

Autostar firmware version 2.0h has changed things somewhat in that an

that is in the FOV at the beep, may drift off slightly during the
subsequent 30 secs. But if you're looking thru' the EP when the beep
sounds, at least you'll see which way it goes!

I've hesitated about posting this, but what the heck. Let the debate


1. OTA balance. You must set the OTA so that it has a consistent balance.
   Balance your OTA so that it  has either a nose or tail heavy 
   balance.  By default (no EPs, a single light EP, no cameras attached), the
   ETX90 is nose heavy, but add a heavy EP and barlow, and it becomes tail 
   heavy. Add weight to ensure that with all the equipment combinations you use 
   the OTA remains either nose or tail heavy.  I prefer to keep mine nose heavy
   by adding a 'small' weight to my dew shield.

    Reason: If the OTA changes from being nose to tail heavy (or vice versa), 
    the Autostar cannot compensate the change in backlash characteristics. It 
    'assumes' that the OTA will behave in the same manner as when you perform
    the drive training.

2. Accurately align your 'spotter' with the eyepiece view. You need a 'very'
   distant object to do this to eliminate parallax errors.

    Reason: If you don't do this, you may pick up the wrong star when 
    performing the alignment process.

3. Check the zero setting of the Alt scale (mine was 8 deg out).
   a. Put the ETX on a level table (check with a spirit level). 
   b. Level the OTA with a spirit level. 
   c. Lock the Alt clutch. 
   d. Check Alt scale reads zero. Adjust if required by loosening the smooth
      central dome.

4. Carefully paint the Alt pointer on the OTA fork white, otherwise it is very 
   difficult to see in the dark.


1. Don't perform your alt/az training during the day. Do it at night after your 
   scope has 'cooled down'.

    Reason: The backlash is dependent on metal & plastic components (gears, 
    pinions, axles, gearbox housings etc) that all expand and contract 
    thermally at different rates.  The backlash is likely to be different during 
    the day when the 'scope is at say 15-20 degrees C, that at night when it
    may be at 5 deg C.

2. As you're now 'training' at night, use Polaris as your 'stationary'

    Reason: Polaris is for the training purposes a stationary object. Any other 
    star will have a apparent motion that may affect the training. If Polaris 
    isn't visible from your normal viewing site, relocate temporarily to a 
    position where you can see it.

3. Use a reasonably powered EP. I use the standard 26mm EP, with a 2x barlow. 
   (This probably results in about a 10-12mm effective EP as the Celestron 
   Ultima short barlow I use will not fully insert into the EP holder)

    Reason: The higher the power used, the more accurately you can perform the 
    alignment. However, too high a power makes the process rather difficult, I 
    get very good results using the above set-up.

4. Use a slow slew speed for the training. I use speed 3 with the above EP 

    Reason: It is a one shot process, if you overshoot you can't reverse 
    direction and you'll have to repeat the process.

5. Repeat the training regularly, especially when the 'scope is new and 
   components are still 'bedding' in and the grease is dispersing.
   Once you've done it a few times it only takes a couple of minutes and you're 
   going to have to find Polaris anyway to put the 'scope in the home position.

   Reason: For the stated reasons, and also to compensate for changes in the
   ambient temperature as you move through the seasons.


1. For best GoTo results always use AltAz alignment, not Polar.

    <Unconfirmed theory now follows!>
    Reason: The backlash in RA (Az) will reverse direction as the OTA swings
    across the meridian (so long as you don't have a neutral OTA balance). I
    don't think the Autostar can cope with this.

    In AltAz mode, the Autostar maintains a consistent approach the Az 
    compensation as it is always 'pushing' the OTA in the same direction.

    In Polar mode, on one side of the meridian the drive will be 'pushing' to 
    OTA, the other side it will being 'pulled' by the OTA.
    This is the same as the Alt problem described in Prereqs #1 above.

2. Put the OTA in the 'home' position. I use the following method:
   a. Roughly align the base with North.
   b. Point the OTA at Polaris.
   c. Lock the Az clutch.
   d. Swing the OTA to zero on the Alt scale.
   e. Lock the Alt clutch.
   Note: make sure the clutches are tight enough to prevent any slipping.

3. Perform the "two star easy align".

4. Don't press ANY buttons whilst the 'scope is slewing to the alignment stars.
   Always wait for the beep.

5. Use a reasonable EP power to centre the alignment star. I use the same 
   26mm EP + 2x barlow as before. Don't rush the alignment, get it right.

6. Be absolutely sure your aligning on the correct star.

    Comments: The initial pointing of the Autostar may be some distance out.

    Make sure the star you're aligning on is the one you think it is.  Also 
    make sure that you know which star is which - it may say Alioth for 
    example, but actually be pointing closer to Megrez. I've used the wrong 
    star a couple of times, it doesn't help!

    Also the excellent quality, easily used finder supplied with the ETX 
    doesn't help when trying to identify the correct star.

GoTo GoTo

1. Try the GoTo feature. it should now be working and putting the desired 
   object with the FOV of the 26mm eyepiece.

2. DO NOT expect good results when using GoTo on moving objects (planets, Moon

    Reason: The Autostars algorithms for calculating the position of these 
    objects does not seem to be very accurate (when compared with the SkyMap

    computed position).  Also it is highly dependent on you correctly entering 
    the date/time, daylight saving, TZ offset, latitude and longitude.

Comments welcome!

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