Installing a CDK24 Telescope at Penn State

09 March 2015


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.


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.


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.


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.

A Day in Pittsburgh

06 December 2014

Its 5:30am when I wake up. Its dark outside, and raining, and cold. I decide to not take an umbrella - I didn't really know where it is. I eat a big breakfast: cereal, banana, skyr, some Earl Gray, and some Lysi - special Icelandic fish-oil ("It's good for the brain, and the darkness", my grandma used to say.) I left the house, took my bike, and biked into the rainy dark, there was a trip coming up. Scheduled departure at 6:45am sharp.

I won't sacrifice safety for a schedule, I tell ya. I'm sorry, but I won't, said the busdriver of our bus - the only completely blue and white Grayhound bus covered with the Nittany Lion logo. There was no doubt where we were from. We were the Penn State Fulbrighters, and we were heading West. State College is situated in the approximate dead center of Pennsylvania, and the bigger cities all seem to perfectly lie on the perimiter of a 3-hour-car-ride-away radius circle around State College. We were heading to one of those cities, Pittsburgh. It was raining, but we made good headway nontheless.

I set next to a friend from India, the Eastern part, Kolkata. "We really love our fish, people even dry it and eat it like that - but dried fish smells alot!" - didn't sounds so far from Icelandic practices! In front of us was Myanmar, to our back was Turkey, to our side was Mexico, and Russia, behind them sat Pakistan, and then Colombia. There were more too: Latvia, Uruguay, Chile, Argentina, Thailand - and on and on and on. We were the internationals going to the seat of the Allegheny-Ohio-Monongahela river crossing, the city of the 400+ bridges. Unlike New York, the definitive World-City, we were heading for the quintessential American city, the City of Steel, the largest city of Appalachia, and the accesspoint to the West.

First stop: The Cathedral of Learning, it was built by immigrants in this typical American city, to show that every nationality was welcome to study there at the highest level of learning. The Cathedral has a number of classrooms, each bearing a name of a specific nationality: The Swedish Room, the Greek, the Chineese, the Swiss, the Indian, the Japaneese, the Turkish, and on and on. All of them were furnished with things that characterize the respective country somehow: Japanese scriptures about shining princesses, the red-like-the-flag seat cushions in the Swiss room, and on it went. Moreover, if celebrated, the rooms were decorated according to the season with typical national ornaments for the Holidays. These are classrooms that one can go and study in, study for finals, study for that essay that you need to work on for next Wednesday (or just sleep like some did!). We fit right in.

We went to the Andy Warhol museum. No pictures allowed, so just a few descriptions. The entrance is covered in shiny metallic color - the color of silver and aluminum, the color of the future according to Mr. Andy. Andy wasn't a man that went with the flow. He defined his own: Pop Art. You must have seen some of his art, it is popular - pop stands for popular! Then why not make portraits of movie stars and other artists, people who are themselves popular? Marilyn Monroes. Elvis Prestley's. Purple cows. And Maos. Why is the same portrait of Mao everywhere, in every house? Why not change it up a bit? Mao with red eyes. Mao with a green face. Mao on blue background. Pink Mao. Mao and the colors of the rainbow. 50 shades of Mao.

Mr. Andy was running a business, a corporation, an art generating machine. There really was no line between fine art and commercial art. But he wanted art to be affordable to all, not only for the elite. He found out crazy ways to make art quicker, and faster, but still somehow unique. He made wallpaper-art to make bigger large-scale art affordable for the masses. Another way was to make masks from photographs and use them as stencils to make a multiple portraits at once, all somehow slightly different, with different background colors, with different colored faces. Another way was to make ink stencils and then use them to trace an image with thick ink and blot another sheet of paper with it, to transfer the original image to another page but it was a bit different. The ink didn't transfer always the same way. Repeat, again, and again, and again. He needed help sometimes, and brought his friends with him on Watercolor nights, for fine wineing and dineing, and artmaking. It was fast, it was unique. It worked; people loved it. This was Andy's factory of Art.

We ended the day by going to the Duquesne Incline, where you can really see the crossing of the three rivers. Pittsburgh is really something else, it has got all the things of a big city without being one. You don't have the crazy traffic, but you have you have the longest continuous street of bars in the continental and non-continental United States, you have a cool incline, is not so far from State College, and it's the access-point to "the great state of Ohio". I'll be visiting again.

HET trip - Results

12 November 2014

We've finally come back from the trip from the Hobby-Eberly Telescope in Texas, and we are starting to see some results finally!

It was a productive trip, in a few ways:

1) HPF Thermal Enclosure Installed

We installed the HPF Thermal enclosure (see pictures before and after). Moreover, we drilled holes on the ceiling for the electrical conduits so it will be easy to get to later on when we install the clean-room where HPF will eventually sit in.

There are many things left to do on-site on the mountain before we commission HPF, but the next immediate thing will be to fill in the big gap seen in the latter picture below with a 6-foot sliding door that should be arriving to the mountain any time now!

You can see a short time-lapse video of the setup of the enclosure right here:

2) Preliminary temperature monitoring system installed

We installed a temperature monitoring system with 6 temperature sensors located in pairs in 3 different locations:

A) 2 High/Low in the HPF Calibration Enclosure;

B) 2 High/Low outside the HPF enclosure; and

C) Low/High inside the HPF enclosure

With just a few days of temperature data (see interactive plot below) we are starting to see some really important results:

1) The Calibration-box which is completely closed shows that it very effectively buffers out high-frequency temperature changes (see red curves below).

2) The Cal-box however drifts with the longer-term temperature changes, as expected, but these same long-term temperature variations seem a bit higher than expected. This issue will need to be addressed to provide HPF with the long-term temperature stability that it requires.

3) The high frequency changes between November 20th to November 21st are (most likely) due to movements of VIRUS spectrograph units into the basement which meant a lot of going in and out of the basement with open garage doors to the cold Texas mountain weather!

HET trip - day 1

11 November 2014

A quick update on the trip to McDonald Observatory in Texas (more to come later):

Purpose of the trip

To install the HPF thermal enclosure and start subsequent temperature monitoring (using Paul's awesome LakeShore - Raspberry Pi setup). The HPF - which needs to be extremely stable temperature-wise - will sit inside this enclosure which will buffer the temperature variations in the basement of the Hobby-Eberly telescope dome. We are hoping to achieve a factor of 10ish buffer; we can deal with a facor of 5ish.

Skies and weather

Excuisite, but cold, (~ -5 Celsius at night). Pictures below:

Black Moshannon State Park Observing

01 October 2014

There was a new Moon the other day, so a few friends went out, and looked at the stars. Here are a few pictures from the trip:

HPF MLI blanket fabrication

29 September 2014

A video of the Multi-Layer Insulation (MLI) blanket layout and clean-room fabrication that we have been working on for the past few weeks, for the Habitable Zone Planet Finder (HPF) spectrograph:

You can read more about what MLI blankets are in a previous post: here!

MLI Blankets

19 September 2014

In recent weeks much progress has been made in making Multi-Layer Insulation (MLI) blankets for the HPF spectrograph. But what are they, how do they work, and why are they needed for the HPF?

What are MLI blankets?

MLI blankets are blankets made out of multiple alternating thin sheets of a highly reflective material - in our case we use aluminized Mylar (looks a lot like aluminum foil but more durable) - and a netted spacer material - Tulle in our case which is yes, commonly used to make bridal veils. MLI primarily reduces heat loss by thermal radiation, but is ineffective at reducing thermal losses by heat conduction and convection. They are therefore widely used as thermal control elements for vacuum applications where radiation losses dominate. Satellites are a great example: MLI gives them the characteristic appearance of being covered with aluminum, or sometimes gold foil.

How do they work?

The main idea behind MLI blankets is the principle of radiation balance and the Stephan-Boltzmann law. Ideally, the perfectly insulating blanket would be a blanket that reflects 100% of the incident radiation. It is however very hard to fabricate a single-sheet blanket that accomplishes this, but by stacking many highly reflective layers on top of each other we can achieve higher and higher reflectivity, and reduce radiation losses further. The individual reflective layers can't touch, as then they would transfer heat between them resulting in no added insulation benefit (short-circuits the heat transfer); we need a spacer material to space them apart. A netted plastic material such as tulle (bridal veil) is great for this purpose. It is very thin and light allowing for very easy handling of the overall multi-layered blanket.

Why do we need them for HPF?

HPF is an infrared spectrograph which will operate at cryogenic temperatures of 180K cooled with liquid nitrogen. These low temperatures are needed so we don't saturate the infrared detector with background radiation. Moreover, the spectrograph operates under vacuum so radiation thermal losses will dominate, which makes MLI a great choice for thermal insulation.

Traditionally, MLI blankets are sewn together; the multi-layered blanket being held together by stitches. However, any kind of hole that punches through the layers tends to degrade the overall thermal performance of the blanket. Another method, of using tag-pins - the small nylon "I" looking pins that are used to hook price tags to clothes in stores - to fix the layers in place, has been mentioned in the literature, see paper by R. Hatakenaka, here). That way you don't need to punch as many holes as when you are sewing, and tagging - a few inches between tags - is faster and less error-prone than sewing around the whole perimeter of the blanket. Moreover, the tag-pins allow you to fasten the layers together without compressing them, which reduces stress around the holes. Lastly, the blankets tend to contract in the direction of sewing which might lead them to be to small if not oversized properly.

The blankets need to be properly sized, aligned, and held together to cover the whole radiation shield, liquid nitrogen tank, and copper thermal straps. Strategically placed Velcro-pads are used to align and hold the blankets in place on the instrument. We sew the Velcro on the blankets to strongly fix them to the blankets. This results in more holes punched on the area of the Velcro than the tag-only method, but sewing makes fixes them better to the blankets.

HPF subsystem assembly

11 August 2014

Update on research: Recently we finally assembled some of the subsystems we have been working on building for the HPF instrument this summer. Below is a quick timelapse video of the process:

Astrofest 2014

21 July 2014

I wrote a few words about Astrofest 2014 - an annual 4 night outreach event held by the department of Astronomy and Astrophysics at Penn State, published on the HPF blog.

Also, here is a short video that I shot of some of the activities we offered (mostly of the telescopes on the roof - as that is the coolest part, right?):

HPF - Keeping it cool

13 June 2014

The Habitable Zone Planet Finder (HPF) - being an infrared spectrograph - must be kept from being saturated by infrared radiation emitted from the surroundings. This can be done by keeping the instrument extremely cold, or at 180K, and must be kept at that fixed temperature to milli-Kelvin precision as any variations will increase measurement errors. How do we achieve this? Take a look at the figure below.

We need to consider four things:

  1. Cooling agent - We use liquid nitrogen (LN2) to cool the instrument. The LN2 tank is at a fixed temperature of 77K at atmospheric pressure.

  2. Conductive paths to the vacuum chamber. - To cool down the radiation shield we connect it with the LN2 tank with highly thermally conductive copper thermal straps. The straps need to be sized properly to not draw too much heat (resulting in a too cold chamber) nor too little heat (chamber too warm) from the vacuum chamber over long periods of time. The copper straps are sized to cool the vacuum chamber down to temperatures slightly cooler (around 160K - 170K) than the 180K end temperature goal.

  3. Heater Panels - These panels heat the overcooled radiation shield to the 180K temperature goal, and at the same time give us the milli-Kelvin precision required.

  4. Thermal Insulation - Lastly, all of the above components are kept under a high vacuum - the radiation shield, the heater panels, the thermal straps, and the LN2 tank - and are all covered with Multi-Layer Insulation blankets (MLI - commonly used for space probes!), to provide effective thermal insulation from the outside world.

Radiative equilibrium

The radiation shield is on one hand being heated up by the radiation from the surroundings (at room temperature) and the heater panels, and being cooled down from the copper thermal straps on the other. At equilibrium we can write: \begin{align} H_{\mathrm{rad}} + H_{\mathrm{Heaters}} = H_{\mathrm{Cu}}, \end{align} where \( H_{\mathrm{rad}}, H_{\mathrm{Heaters}}, H_{\mathrm{Cu}} \), denote the heat current from the net incoming radiation, the heaters, and the copper straps, respectively. Each of these factors are discussed below.

I. Incoming radiation

First off, lets consider the incoming radiation. All objects, regardless of their temperature, emit energy in the form of electromagnetic radiation: the warmth of the Sun and glowing coals in a fireplace is just infrared radiation emitted from these objects.

Then to the math. The net heat current absorbed by an object with surface area \( A \) and emissivity \( e_{\mathrm{eff}} \) (a dimensionless number between 0 and 1 - larger for darker surfaces) sitting in a room at absolute temperature \( T_{\mathrm{room}} \), can be expressed by the Stefan-Boltzmann law: \begin{align} H_{\mathrm{rad}} = A e_{\mathrm{eff}} \sigma (T_{\mathrm{room}}^4 - T_{\mathrm{HPF}}^{4}) \end{align} where \(T_{\mathrm{HPF}}\) is the temperature our object: the HPF instrument.

The emissivity of MLI blankets is very low (for good blankets \( 0.005 \lesssim e \lesssim 0.1 \); see here) and therefore offer very good radiative thermal insulation. By covering HPF in MLI blankets, the emissivity of the blankets governs \( e_{\mathrm{eff}} \) - the effective emissivity of the instrument. However, the actual value of \( e_{\mathrm{eff}} \) is highly dependent on the overall quality of the MLI blankets and the surface finish of the radiation shield, etc. and is very difficult to calculate the value exactly. Our best bet is then to empirically derive the effective emissivity from the APOGEE instrument - built for the Sloan Digital Sky Survey. This gives us a value of \( e_{\mathrm{eff}} \sim 0.0087 \). We will defer further MLI-blanket discussion - how we prepare and size the blankets for HPF - material for a whole blog post in itself!

II. Cooling with Copper

When a quantity of heat \( dQ \) is transferred through a conductive material in time \( dt \), the rate of heat flow is given by \( H = \frac{dQ}{dt} \). More specifically, we can relate the heat current to other properties of our copper thermal straps with the following equation: \begin{align} H_{\mathrm{Cu}} = \frac{dQ}{dt} = k A \frac{T_{H} - T_{C}}{L} \end{align} where \( k \) is the thermal conductivity of the material - copper in our case - and \( T_C = 77 K \) is the LN2 temperature, and \( T_H = 180 K \) is the cryostat temperature, and \( L \) and \( A \) are the length and cross-sectional area of our thermal strap, respectively.

There is one issue however: the thermal conductivity of copper varies with temperature, so \( k \) is not a constant in our operating temperature range (see figure below). By integrating over the temperature range: \begin{align} H_{\mathrm{Cu}} = \frac{dQ}{dt} = \frac{A}{L} \int_{T_C}^{T_H} k(T) dT, \end{align} we can account for this secondary effect - ignoring it, we would underestimate \( H_{\mathrm{Cu}} \).

The images below show a few photos from the copper straps preparation; we will need 16 straps in total.

III. Heater Panels

Like mentioned above, the heater panels heat up the overcooled radiation shield to the 180K temperature goal. Each panel has 4 thermal resistors (\( 150 \Omega \) each), which heat up in proportion to the current going through them. As we now know the heat current from (I) the net incoming radiation, and (II) the copper straps, we can calculate the heat current needed from the heater panels to keep the system at equilibrium: \begin{align} H_{\mathrm{Heaters}} = H_{\mathrm{Cu}} - H_{\mathrm{rad}}, \end{align} which can be used to calculate the current needed per panel and per thermal resistor. The exact current running through per resistor is controlled by a thermal feedback control system which monitors any external temperature fluctuations at the observatory - we can't control the weather!. The system then compensates for these changes by controlling the amount of current going through the resistors, warming them up as needed, keeping the instrument stable at 180K with the milli-Kelvin precision needed.

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