Modelling your Telescope – Measurements to take

Introduction

The most fundamental thing you need to know when building an observatory is the dimensions of your equipment. Here we present our simple guide to measuring your telescope and mount, and getting the pier height just right. If we’re designing your observatory for you, this will provide some useful background, but you can skip ahead to the measurements (described in the grey boxes).

Why We Need This

We often don’t! Actually we maintain a library of models we’ve made of popular telescopes and mounts over the years, so there’s a good chance we can model your telescope in the observatory without needing any new measurements. However, if your rig is something we haven’t measured before, getting these measurements means we can model it and make sure that it will fit under the closing roof, as well as maximising your comfort (and minimizing your frustration) when using the telescope.

Getting Comfortable

The first step in the process is to know whether you use your telescope for visual astronomy (at the eyepiece) or imaging (with a camera) or both. If you’re a visual astronomer (even occasionally) you need to prioritise making the eyepiece comfortable to access. If you’re only ever going to use a camera with your telescope, you can skip this step.

First, find a convenient, flat location and set up your telescope tripod, mount and optical tube. Set the tripod at a height that allows the best comfort in looking through the eyepiece in a range of positions: check it with the telescope pointed at Polaris (the ‘home’ position on equatorial mounts), at the Zenith (straight up), horizontally East or West, and due South. Ideally, you’ll be able to comfortably reach the eyepiece in all positions without bending or stooping too much, but those with long refractors in particular will have to accept a bit of compromise, particularly with the telescope at the zenith.

Measurement 1: Pier Height

Your first measurement is of the height of the tripod itself when it's set at the most comfortable observing height - measured from the ground to the top of the tripod, where it meets the mount.  This represents the ideal height of the pier above the observatory floor.

For Astrophotographers

As an astrophotographer, you generally don’t care about where the eyepiece might end up, because you won’t be using one. So it’s tempting to simply say you’d want to your optical tube to be as high as possible in the observatory, but there’s a little more to consider:

• Observatory Type
  If you have a dome, there’s an optimal place for the telescope, which is usually just above the bottom of the observing window, when the telescope is pointed due South. This will allow it the best range of movement within the dome in all directions.
  If you have a clamshell, the telescope position needs some calculation specific to the clamshell itself. It’s unlikely, for example, that the telescope would be mounted centrally, as a clamshell is usually sized to contain the equipment and no larger. So your supplier (hopefully us) will make sure the clamshell, mounting and pier are all sized correctly to ensure all is well.
  If you have a roll-off roof observatory, the main imperative is that when the telescope is parked, you can close the roof. But it’s a bit more complicated than that…. read on!
• Getting the Position Right
  Remember there’s no need to leave your telescope parked in the ‘home’ (sometimes called ‘switch’) position, pointed at Polaris, if you have a roll-off roof. In fact, you can mount your telescope higher up if you park it with the counterweight bar pointing East-West and the optical tube horizontal, pointed South.
• Remember Wind and Neighbours
  One of the downsides of mounting your telescope high up can be wind… the further your telescope pokes up over the walls of your observatory, the more likelihood there is of wind moving/vibrating the telescope while in use. Equally, if your neighbours have an annoying habit of leaving their outside lights on, they will be more likely to interfere with your imaging runs if the telescope is poking out into the sky. Mounting it a little lower can reduce both of these issues. Neither of these is an issue (generally) with domes.
• How Important are Low Horizons Really?
  It’s easy to assume that you need views right down the horizontal in all directions, but consider carefully whether that’s really the case. It’s rare to image successfully below about 25 degrees altitude, simply because atmospheric water vapour, pollution and general mush means it’s incredibly rare to get good seeing. So again, it’s worth considering having your telescope a little lower, out of the wind, because you’re not sacrificing much in the way of quality skies.
• Future-Proofing
  A serious consideration because in our experience, astrophotographers do tend to upgrade their equipment, or add more, fairly regularly. Consider whether having a few extra inches below the roof will allow you more flexibility when upgrading your optical tubes or mount in the future.

Measuring the Telescope

We ask for three more key measurements when we’re working things out:

Measurement 2: Height of the point where the mount axes cross.

Equatorial mounts have two axes - the Right Ascension (RA) and the Declination (Dec) axes - about which they rotate.  These axes cross in the front part of the mount, and this is a crucial measurement when working out the position of the equipment in different configurations.  Measure from the top of the tripod to the cross:

Measurement 3: Overall Height

Next we need to know the overall height of the telescope.  For the sake of our modelling, we ask you to measure it in three positions (shown left to right in the illustration below): at Polaris, at Zenith and pointing Due South horizontally:

Measurement 4: Eyepiece Height

Using the same three positions as for Measurement 3, measure from the bottom of the mount to the top of the eyepiece in all three positions.  By 'top' we mean the heighest point of the eyecup.  This measurement is not required if you only do astrophotography.

A Handy Table

Here’s a handy table you can copy and paste for your measurements: