Astronomical Odds

This seems a very simple subject but experience taught me how to get the scope as accurately level as possible in the minimum amount of time.

First thing to remember is that on soft ground always push down on the legs before mounting the scope and always place the level on the mounted OTA.

At first I was using a small spirit level from a DIY store to set up my tripod, but it proved difficult to place the level in order to check all the axes. As part of the ETX alignment process the OTA has to be level and it was hard to balance the DIY level on the telescope tube.  I tried a circular bubble level and although better than the DIY level it took quite a few separate adjustments to get the bubble centred. So I bought the two Engineering standard levels shown in the pictures below.

spirit_levels 2_axis_level levcelling_diagram

The two axis “T-level” makes levelling the telescope and tripod very easy and I can normally level with two, or maxiumum three, adjustments of the tripod legs. The level is aligned with axes A and B as shown in the diagram below. First adjust leg 1 to level axis A then adjust leg 2 to level axis B. Simplicity itself! Sometimes it is necessary to make one more adjustment but most times it has not been necessary.

The small square level is used for levelling the OTA during the ETX alignment. It’s small enough to sit comfortably on the curve of the OTA and is visible from all sides even using a red light torch.

There are two aspects to getting good performance from the ETX125 “GOTO” system. Training the drives and the equipment set up.

Training the ETX drives

Training the drives and calibrating the motors deals with the accuracy of the gears and the scope’s movement. Good training ensures that once an object is found and centered, it stays centered in the eyepiece and the scope tracks accurately.

The set up function in the Meade ETX125 telescope allows the drives to be trained. The manual says to use a terrestrial object in daylight, but common sense tells me to use the most distant fixed object visible……in other words Polaris. It also makes sense to me to train the drives in darkness in case external light sources affect the optical sensors used by the ETX drives to measure the motion of the OTA. I use a home made 26mm reticle eyepiece to accurately position the star in the field of view.

EXT125 Set up and Alignment in Alt-Az mode

Setting up the equipment properly ensures the alignment stars are close to the field of view and the scope can be aligned accurately. Getting both these elements right ensures that objects can be found in the eyepiece in the first place and stay in the field of views for minutes, if not hours. Given practice it should be possible to center objects in a 26mm eyepiece without too much trouble.

For this summary, I have also assumed the Autostar is programmed with the correct coordinates (longitude and latitude) and set to Alt-Az mode in the set up menus. High precision mode should be on and the slew controls at their slowest settings.

Secure the ETX securely to the tripod and then place it so that the panel with the power supply and Autostar connections faces due West. If the tripod is on soft ground push it down firmly.

Level the scope and tripod, using a bubble or a two axis spirit level by adjusting the tripod legs.

Rotate the OTA (tube) anti-clockwise until it hits the hard stop. Then rotate the OTA clockwise until it roughly points North then aim it at Polaris. I have done this often enough to simply to do this optically but it would simple enough to use the declination setting circle on the fork arm to get the approximate angle. I use a 26mm reticle eyepiece and it’s possible to see Polaris’ companion star very close by to make sure I have picked the right object. If there is any doubt I leave the scope for a few minutes and Polaris should hardly move in the field of view.

Once Polaris is correctly located, lock the horizontal or Right Ascension (RA) drive. This is a good point to zero in the spotting scope and by making sure Polaris is in the center of the cross hairs.

Drop the the OTA until it is horizontal using a small spirit level to check the angle and then lock the declination wheel by turning the circle until it just holds the OTA. It is important not to over-tighten the friction clutch.

The OTA is now in the “home position.”

Turn on the Autostar and enter the date and time using the 12 hour system. Answer Yes or No to daylight saving time according to whether the clocks are on Summer or Winter time.

Select Easy Align and start the Alt-Az alignment process to let Autostar find the first alignment star. It automatically searches for the brightest stars based on the location, time and date. You cannot assume that the scope will slew exactly to the right star but it should be the brightest object in the spotting scope. Center the star in the eyepiece using the direction controls on the Autostar. At this point it’s still best to use a reticle eyepiece. Sometimes aligning early in the evening is best because only the brightest stars are visible and there’s less chance of selecting the wrong star. There are documents that list all the Autostar alignment stars but it helps to be able to identify them just by looking up. If the Autostar selects a star that is not visible then it is possible to skip the selection and select another star using the keys.

It’s possible to take your time centering the star in the eyepiece, Autostar will absorb any delay.

After the two star alignment select the object you want to view from the menus. Press ENTER so that Autostar calculates the coordinates then press GOTO. The alignment can be checked by going back to Polaris or another bright star or object.

The GOTO calculation is more accurate on deep sky than planetary objects but I can usually see the object in the eyepiece. If the object moves quickly in the field of view, hold down ENTER for 2-3 seconds to activate synchronization. Move the target to the center of the eyepiece and hold ENTER down for a couple of seconds. This is sometimes necessary if a camera is put in place of the eyepiece.