Installing a CDK24 Telescope at Penn State


09 March 2015 telescope pennstate

Introduction

During the past months I supervised the installation and alignment of Penn State's new CDK 24" telescope from PlaneWave Instruments, located in one of the 3 domes on the roof of Davey Lab. A lot of cold, Pennsylvanian, starry (and less starry) winter nights —and many a hot chocolate— later, the telescope is now up and running, and classes have already started to use it.

Below follows a somewhat technical description or summary of how it all went.

Before we start, I want to thank Emily, Eric, Sam, Ryan, Michael, Suvrath, Dave, Chris, Bob, and Christine for all their help and advice.

Let's get started.

Pre-installation

Dome OB1 for the new CDK24 used to house an older 12" telescope. It was moved to make room for its new, bigger brother: The CDK24.

The pier where the 12" used to stand was still in good shape, but the cords on the older telescope tended to get snagged on the corners of the aluminium base plate (see figure below) when slewing. A quick trip to the machine shop helped round the base plate corners. Moreover, a set of 4 mounting holes needed to be drilled and tapped.

Davey Lab vibrates. This has been shown by interferometric measurements in the building. We were concerned that these vibrations might degrade the telescope imaging performance. To help reduce this, we bought a polyurethane tubing (2" outer-diameter, and 1" inner-diameter) which we cut into 6 roughly 1/4" thick washers — our vibration dampers. Each bolt in the photo below got a pair. Both touched the base-plate, one directly above, the other directly below.

We checked if the base plate could indeed hold the telescope load without flexing. The telescope weighs ~240lbs, but the counterweights and mount do add substantial weight. An FEA displacement study (see the figure below) revealed that the maximum displacement for a total of 500lbs load would experience a 45micron flexing. No worries there.

Installation

The telescope is big. How did we get it into the dome?

The dome has two openings: a people door, and the closable slit in the roof — our opening to look at the night sky.

First choice: "Let's bring it through the people door." The telescope has a dual-truss design, and we were assured by our contacts at PlaneWave that the trusses can be safely disassembled. We proceeded. Everything went fine. Except: the door was half an inch short.

Not wanting to dismantle the door, we opted for bringing the telescope through the dome opening. With help from PSU OPP, a genie-lift, and a hoist, we brought the telescope carefully onto the mount where the telescope slides into a specially designed saddle plate.

Pictures are shown below.

Configuration

Various hardware and software needed to be installed to make the different components of the telescope system talk together.

First off, on the hardware side, the collimation of the telescope was checked with a Ronchi test. A Ronchi test places a Ronchi grating at the back-focus of the telescope, which is then used to check if the secondary mirror is tilted. You can also accurately place where the back-focus is on the telescope. The test went well, and little to no adjustments were needed for the secondary.

The camera, filter wheel, and focuser were then mounted on the back of the telescope, with a set of adapters and spacers to reach the back-focus. The IRF-90 rotating focuser has a focus-travel of 1.3". We therefore strived to stack the spacers in a way to use the full 0.65" of focus-travel in each direction. The figure below shows that we are around there: the camera is focused if we put the focuser at 0.3".

With the camera system properly installed, the telescope needed to be finely balanced in both axes, Right-Ascension, and Declination. The former is balanced by means of 7 counter-weights, which in our case needed to be placed as low as they could on the counterweight shaft. The latter is balanced by carefully adjusting the position of the telescope baffle in the mounting saddle plate. In practice, this meant a location precision of roughly \( \pm 1/8" \).

On the software side, there are three main programs that control the telescope and the accompanying subsystem:

The PlaneWave STI - For base telescope control and initialization.

MaximDL - For overall telescope, and camera control. Includes a comprehensive catalog of objects, and features for focuser control, and also image processing tools.

PWI 3 - For the IRF-90 focuser control (includes a rotator), and temperature monitoring, and telescope heater and fan control.

A more technical listing of the telescope system is summarized in the table below. More information about the telescope and the other accessories can be found on the CDK24 PlaneWave Homepage.

Telescope Optical Design: Corrected Dall-Kirckham
Diameter: 24"; 0.69m
Focal Length: 3.962m
Focal Ratio: f/6.5
Primary Mirror Material: Precision Annealed Borosilicate
Telescope (only) Weight: 240 lbs; 109 kg - (the total system is heavier)
Mount: Ascension 200HR German-Equatorial Mount
Camera: SBIG 8XME
Focuser: IRF-90 Rotating Focuser
Davey Lab Latitude: 40:47:53 N
Davey Lab Longitude: 77:51:46 W
Elevation: ~350m from sea level

Polar Alignment and Pointing Model

First, a couple of definitions:

  • Polar aligning is the process of aligning the Right-Ascension axis of the telescope to the polar axis of the Earth — the axis the Earth rotates around. In doing so, the telescope only needs to track in one axis. This is the main idea behind German-Equatorial Mounts.

  • Pointing model. Chances are you won't polar align the telescope perfectly. But, the telescope mount —being computer controlled and equipped with high resolution encoders— can correct for polar-misalignment. This is done by creating a sophisticated Pointing Model by comparing a series of mount positions against known sky locations with the help of a CCD camera. This is further explained in the video below.

Essentially the Polar Alignment and Pointing Model process goes like this:

  1. Align a finderscope to the CDK24. This is immensely helpful when starting out. You also might want to put an eye-piece on the primary focus. At first I put an eye-piece on the Ronchi-Spacer. Be careful: if you take the camera+focuser off, you might need to rebalance the telescope. This can give you slewing errors, and null a pointing model. Pointing models, are hardware-setup dependent, adding/removing weight changes how the telescope slews.

  2. You create a small pointing model by finding a few known stars in the sky, and you tell the telescope where it is pointing. The telescope control program can then calculate the polar misalignment for you. You adjust for it by adjusting a set of screws on the mount.

  3. You might have to repeat step 2 to get the polar alignment error within 10 arcminutes in each axis, which is recommended by PlaneWave.

  4. You create a much more sophisticated Pointing Model with the help of a CCD. A) You slew the telescope to a series of locations on the sky —with known coordinates— and you take an exposure of each location. B) For each location a program (called PlateSolve) uses the image to look up where exactly the telescope is pointing, and notes if there is a difference between the expected location, and the observed one. C) Doing this for a good number of locations distributed over the sky, builds up a comprehensive pointing model that dramatically increases the pointing precision.

We currently have 14 calibration locations in our pointing model. We can add more later on, but for now, it is already pointing very well.

Latest Results

With the current pointing model we are routinely getting stars with FWHM of 2" with exposure times around 300s. For these exposure times, we are thus limited more by the seeing in State College, rather than telescope pointing.

Classes at Penn State have now started to use it. Operation manuals are getting ready.

Current and future plans include:

  • Getting a guide-camera, and attach it to the MMOAG.

  • Get a new camera from APOGEE. We are still converging on a suitable model.

  • Use it for astro-major observing classes, and research projects for astronomy majors. Finding planetary transits would be cool.

  • Use it for outreach. This can be done by inviting people during Outreach events like Astrofest to see the telescope in action, and give them insight into how modern imaging-based astronomy is done. Also, having pretty pictures of galaxies and nebula taken by the telescope is a sure-way to spark an interest.

Below follows pictures of the telescope in action, and some of the photos taken by the telescope so far. Enjoy.


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