Using an Argo Navis with a Discmount DM-6

Introduction

This article is intended to be read after the general introduction to go-to mounts. It uses a specific mount, the DiscMounts DM-6, with an Argo Navis DSC computer, as an example to give a more detailed account of the procedure of setting up and using an alt-az mount with Digital Setting Circles.

We’ll be using a Stellarvue SV-105 refractor and small unit-power finder on the mount for this example.

Mount Summary

Various features of digital setting circles are mentioned in the general introduction. On this mount, the specifics are:

Mount geometry
Alt-Az.
DSC System
Argo Navis
Reference Position
Level tube horizontally for first reference alignment.
Alignment
2-star. User selects the alignment stars. Can also align on other objects (planets etc.)
Refinement
One of the 2 alignment stars can be replaced with a recently found object. As a more complex option, a multi-star pointing model feature is also available but not described here.
Backlash
None (not motorized).

Setting Up

Let’s go outside. It’s late evening in July, and we have couple of hours to observe. We are planning to look at some globular clusters, and maybe explore the region around Sagittarius, where we have heard the sky is densely filled with interesting objects. The beauty of our Discmounts DM-6 mount is that setting it up requires practically no time at all. We have assembled and balanced it, and aligned the multi-reticle unit-power finder, so mechanically, everything is ready to go.

Connecting the Argo Navis DSC System

On the DM-6 mount the encoders are built in to the mount head.
The result is that only a very short length of cable needs to extend from the mount and plug into the Argo Navis DSC box. This cable allows the Argo to monitor movement of the mount in both directions.

Initializing and Aligning the DSC System

Having set up the mount, it’s time to start the Argo Navis DSC system and give it its first reference — a level OTA.

We set the date and time, and told the Argo Navis that it was being used on an Alt-Az mount, in a previous session, and it remembers that information. Now, we power the unit on and after a few seconds it says

FIX ALT REF

This is the unit asking us to establish its first reference position, the “altitude reference”, which can be a perfectly level or a perfectly vertical optical tube. We press ENTER and it asks which of these options we prefer with a display that reads

ALT REF=0°

The “0°” is the setting for a horizontal tube reference. If we wanted to use a vertical tube, we could rotate the control dial one click and it would read “REF=90°”. We are, however, happy with the horizontal setting requested by the “REF=0°” reading.

A small level, which stores nicely inside the head of the mount, balances on the optical tube and makes it easy to test for precise level.

We just tilt the telescope forward on the mount until the level is centred, indicating the tube is level. It doesn’t matter which “azimuth” direction (North, East, etc.) the tube is pointing — it only needs to be level.

With the tube level, we press ENTER on the Argo Navis and it responds by saying

WARP=ALT FIX OK

This is the mount acknowledging the first reference position, the “altitude fix”.

Two-Star Alignment

Now it’s time to do the 2-star alignment. To pick good alignment stars, let’s have a look at the sky.

At our fairly dark location, looking up, we can see the spectacular spread of stars in the night sky. It’s a little confusing at first.

But after picking out a few bright stars, and then some familiar constellations, we feel oriented. There’s the Big and Little dippers, Cassiopeia, Cygnus, and others. Down in the South is the familiar Teapot of Sagittarius.
We pick 2 stars that are easy to see and find, that are not too close together, and that we’re certain we can identify. Let’s use brilliant Arcturus in the West (and pointed to by the arc of the Big Dipper’s handle), and bright Deneb in Cygnus, almost directly overhead.

First Alignment Star

We press EXIT on the Argo Navis to get out to the main menu level, then rotate the control dial until the display reads

MODE ALIGN STAR

Press ENTER to get to the list of suggested alignment stars, and rotate the dial until we see

ALIGN ARCTURUS

on the display.

The DM mount is simplicity itself. We hold the handle, and just push the telescope around to the correct part of the sky. Looking through the zero-power finder, we centre the reticle on Arcturus.
Now we switch to the telescope, with a medium-power eyepiece (a 17mm Nagler in this case). Arcturus is in the field of view, but not quite centred.

Again, just push gently on the handle of the mount until Arcturus is centred in the eyepiece view. With this system and this telescope, it isn’t necessary to have such precision that a cross-hair eyepiece is necessary, we just centre the star as accurately as we can by eye.

Press ENTER on the Argo Navis, and it replies

ALIGN ARCTURUS
WARP= -4.75 (1)

The “WARP” number is meaningless at this point — it is a measure of the distance from where the mount “thought” it was pointed, and it had no idea. It could be any number, and we just ignore it.

Second Alignment Star

Now we spin the Argo Navis control dial until it indicates it is ready for our second chosen alignment star:

ALIGN DENEB

We repeat the alignment steps above. First, use the zero-power finder to centre Deneb, just pushing the telescope into position.

In the eyepiece, Deneb isn’t centred, but it’s evident as the brightest star in the field.
We gently push the telescope until Deneb is centred in the eyepiece.

We press ENTER and the Argo Navis responds:

ALIGN DENEB
WARP= +0.50 (1)

The warp number is now fairly small, indicating that the star wasn’t too far from where the mount calculated it would be. The small error number is due to our centring the two stars only “by eye” and not with a precision reticle. This is good enough. The Argo Navis is now aligned and ready to use, and the whole procedure only took 3 or 4 minutes.

Using the DSC System

Setting up the DM mount and Argo Navis DSC is really simple, and using it is that simple too.

I’m anxious to show you the spectacular globular cluster M13. To get there quickly and easily, back the Argo Navis out to the main menu and rotate the control dial until it says

MODE CATALOG

Press ENTER to enter the catalogue, and rotate the dial to browse the list of available catalogues, stopping when it says

MESSIER

Press ENTER to enter the Messier catalogue, and use the control dial and ENTER key to change one digit at a time, until the display reads

M13

With M13 selected, we press ENTER and see

M13
GUIDE 38 → 40 ↑

(In case the characters don’t render properly on your browser, there are little right-pointing and up-pointing arrows in there.)

The numbers on the GUIDE line tell us which directions we need to move the scope and how far, measured in degrees.

We start gently pushing the scope to the right, watching the display count down:

GUIDE 30 → 40 ↑
GUIDE 22 → 40 ↑
GUIDE 15 → 40 ↑
GUIDE 8 → 40 ↑
GUIDE 4 → 40 ↑
GUIDE 0.6 → 40 ↑

Once the number is close to, or exactly, zero, we adjust the vertical position by pushing the scope upward:

GUIDE 0.6 → 40 ↑
GUIDE 0.6 → 30 ↑
GUIDE 0.6 → 21 ↑
GUIDE 0.6 → 12 ↑
GUIDE 0.6 → 6 ↑
GUIDE 0.6 → 2 ↑
GUIDE 0.6 → 0.5 ↑

Getting both GUIDE directions to less than one degree is good enough with this wide-field telescope and eyepiece.

Looking in the eyepiece, we find our target.

We can repeat this process all night — go to the catalogue, select an object, and push the telescope until the GUIDE figures are close to zero. Every hour or two it’s worth re-doing the 2-star alignment, or adding one additional alignment star, to compensate for the gradual accumulation of errors in our initial alignment.

Thanks for following along.

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