About two months ago I acquired a used Optec TCF-s focuser for the telescope in the North Dome.  The focuser is designed to compensate for the changing focus position of the telescope as a function of temperature (TCF = Temperature Compensating Focuser).  It uses a temperature probe attached to the telescope tube to determine the temperature, then moves focus based on a coefficient that expresses the number of encoder steps (each resulting in 0.0022 mm movement of the focuser) per degree Celsius decline in temperature.  According to Optec’s documentation, the coefficient is around 80 steps per degree Celsius for a typical 8-inch f/10 Schmidt-Cass telescope (like a Celestron or Meade 8-inch).  Basically, as the temperature falls the telescope shrinks, reducing the distance between primary and secondary mirror.  That results in the focus point of the telescope being pushed further out (increasing “backfocus”).  Of course, I’m not using an 8-inch f/10 telescope – the ‘scope in the North Dome is a 14-inch f/11, so I needed to determine the coefficient specific to my rig.

Over the course of about a month I kept the main mirror stationary and only focused using the TCF.  I would typically re-focus every couple of hours, making note of the temperature and the position of best focus.  The chart of the results is shown below.  Given these data I arrive at a value of 161 steps per degree Celsius.  One thing that is somewhat vexing is the scatter in the plot.  One possible source of the scatter would be inaccurate temperature readings, and really is my main suspect.  For one thing, the telescope uses a dew heater strip near the front corrector plate.  That likely results in the aluminum telescope tube not being the same temperature throughout.  In the next few nights I’m going to run the dew heater at a slightly lower level to see if that reduces scatter while still keeping the corrector plate clear of dew and frost.

 

 

The observatory was very good this year so Santa brought it a very nice present – a new telescope for the South Dome!  Actually it isn’t all that new.  It’s a Meade Instruments 16-inch Schmidt-Cass likely from around 1994 or so.  It has some mileage on it, and the previous owner had gutted the electronics to upgrade them with Sitech controllers and encoders which seem to be the best drive and control system for these telescopes.  Actually none of that will be of much concern as I plan to “de-fork” the telescope to place it on the Astro-Physics 1200 mount in the South Dome.  Another nice addition is a three inch Optec TCF-S3 focuser, which should be more than sufficient to support the spectrograph and camera.

It’s sort of a shame to waste the mounting as I understand it tracks really well.  The telescope’s previous owner was able to use it to track satellites across the sky keeping the object centered in the field the whole time.  But the AP1200 is actually a much better mount and is much more compact – definitely an issue.  Furthermore, because of the short fork, the camera and spectrograph would limit how far north the telescope could point as they would not pass between the forks without hitting the base.  Anything northward of about +55 or +60 degrees would be out of reach.  That’s not a whole lot of sky but there are objects in my observing programs that would no longer be available.

Once the dovetail rail (used to attach the telescope to the mount) arrives I’ll begin testing the optics.  Meade Instruments were known to deliver some clunkers, optically speaking.  I’m also already aware of other common problems with this model, including mirror flop.  That’s why I acquired the Optec focuser.  I plan to lock the mirror down to prevent it from “floating” around – which can affect pointing and image quality.  Not surprisingly it has been raining since the telescope arrived home.

 

Last summer and fall I participated in the “BRITE Stars in Cygnus” campaign monitoring the radial pulsations of Deneb’s atmosphere.  This involved measuring radial velocities by determining the wavelength shifts of the Si II λλ 6347 & 6371 lines, where expected velocity differences might be 10 km/sec or less.  After measuring a number of nights’ data it appears I’ve achieved less than “stellar” results.  On most nights the standard deviation in my data was around 2 to 3 km/sec where my goal was for a precision of 1 km/sec or better.  So I began to wonder about the wavelength stability of the LHiRes III and read a couple of posts from other observers who shared the same concern.  So I decided to conduct a few tests.  This article presents the results of a couple of those tests.

Test #1:

The first test was very simple. I took a series of calibration lamp exposures over the course of about an hour. The spectrograph was on the telescope and the telescope pointed at the pole with the drive turned off. That is, the telescope was not moving. I turned on the lamp and took an image about every two minutes. I then measured the mean shift of the three Neon lines visible in the wavelength range (Ne 6532, 6598, & 6678) recorded using an ST8 camera and the 2400 lines/mm grating, resulting in a measured dispersion of about 0.11A per pixel. Using the IRAF task “reidentify” I measured the shift from the first image and created the plot shown in Figure #1.

Figure 1: The measured shifts of calibration lines from frames taken while the telescope was “parked” – standing still, and not touching the spectrograph.

Note that, except for the very beginning, the shift in positions for the Neon lines was fairly constant wrt time. I show two linear least-squares fits – one for the entire data set and one “cherry picking” the final 3000 seconds or so. Basically the only thing this should be showing is the effects of changing temperature since nothing else was moved, other than to switch on the lamp at the beginning of the test. The telescope had been parked for over a day at the time of the test. Since the test was taken in the late afternoon the temperatures were falling – around 2 degrees C over the course of the test. The cause of the rapid drop in the roughly six-minute span near the beginning of the sequence is a mystery.

 

Test #2

Test #1 should give an idea of “best case scenario”. The telescope was standing still and the spectrograph was not touched throughout the duration of the test. When I’m gathering actual “science” data I always take calibration images before and after each spectrum, or around every 30 minutes in cases where I have a bright target for which I take several images within that time. Using two calibration images the wavelength solutions for surrounding calibrations are interpolated over time to match the time of mid exposure of the science image. Of course that means I need to switch the lamp on, rotate the source onto the slit, take the calibration image, rotate the source back away from the slit and turn the lamp off for each calibration image, all of which involves manipulating items which are parts of the spectrograph. Furthermore, the telescope is moving from east to west causing the direction of pull due to gravity to change throughout the sequence. So a second, more “real world” test was conducted monitoring a star with a well-determined radial velocity (HD 82885 = 11 LMi) over the course of four hours comprising a total of 11 images.

 

The second plot shows heliocentric-corrected measurements of radial velocity using the H-alpha line in the spectrum of HD82885. In this case the standard deviation of the measurements was 1.13 km/second. Which, to be honest, I found quite surprising. I took an additional step and used the telluric correction task in ISIS to determine the precision of the wavelength solutions and found a standard deviation of 0.04 Angstroms with a mean of essentially zero. Obviously my skill at trying to “eyeball” match the telluric lines was not all that good on an individual basis, but on average they confirmed the wavelength solutions as being generally good.

 

So, again, I find this result very encouraging. During the sequence of spectra the star passed across the meridian. The sequence began at hour angle -2.6 hours and finished at hour angle +1.5 hours, so a little over four hours. That means the stresses due to gravity on the spectrograph and camera changed a lot throughout the sequence. Note how the slope of the measurement curve changes at meridian crossing, hinting at slightly non-linear changes wrt hour angle likely due to gravity.

 

The one thing that is quite puzzling, however, was that my mean radial velocity determined from the experiment was 17.5 +/- 1.1 km/second – the published radial velocity for HD82885 is 14.4km/second. So my measurement is about 3.1 km/sec (or nearly three sigma) from the star’s known radial velocity. On the other hand, using Telluric lines in ISIS seems to agree with my wavelength solutions since, on average (despite a larger scatter in the measurements) the mean difference was zero (actually about 6e-10 Angstroms – WAY down in the noise). I have no idea what the source of the 3.1 km/sec difference is. I used 6562.801 Angstroms for the in-air wavelength of H-alpha. And I’m almost certain I measured the correct star; 11 LMi is the only 5th magnitudes star in the near vicinity and the spectra I measured match those of the star’s published G8+V spectral type.

Figure 2: Radial velocity measurements based on a four-hour sequence of 11 spectra of HD82885, a star whose radial velocity is well-known to be 14.4 km/sec. My result of 17.5 km/sec, while consistent, is consistently off by an average of 3.1 km/sec.

 

As a final check I used the cross-correlation task in ISIS to measure the differences of each reduced spectrum against the first spectrum. In each case ISIS reported shifts wrt the first spectrum which agreed with those I’d determined by simply measuring the position (in IRAF) of the H-alpha line.

 

So, in summation – it appears the LHiRes, at least for the duration of these two experiments, behaved rather well. Really surprisingly well given my previous attempts at radial velocity measurements. However, despite the standard deviation of the 11 measurements I made being just slightly above 1 km/sec, the mean ws off by 3.1 km/sec

The Sequel:

Harboring a bit of doubt about the result, above, I re-ran the experiment on the night of 2015 January 15/16. This time I only acquired 7 spectra of HD82885, but they were all a bit better S/N as I took 1800 second exposures each, as opposed to 1200 seconds for the December data. The results were even, to me, simultaneously more astonishing and perplexing. I basically found the same radial velocity, this time Vr=17.18 +/- 0.39 km/sec. So while the mean radial velocity was not significantly different the standard deviation of the measurements was even better. Note that, in both cases, I had to actually hold the knob that rotates the Neon lamp onto the slit each time. It was fairly cold outside (around -7C) and the wires inside the spectrograph that attach to the Neon source were stiff and would, if I did not hold it, cause the Neon lamp to spring back out of the light path. So I actually had to hold on to the knob during the short (2-second) calibration exposures. Figure 3 illustrates the result.

Figure 3: Result from re-running the radial velocity test illustrated in Figure 2. Within the error margins the mean radial velocity found here is the same as for the previous test, but the standard deviation is even better.

 

 

Conclusions:

Based on these two experiments (hardly an exhaustive sample):

1. I seem to have a systematic offset of around 0.065 Angstroms, resulting in a roughly +3 km/sec error (or a bit more than 1/2 a pixel for my setup) in the region of 6600 Angstroms.  I’m not sure if that arises from instrumental issues or from my method of measuring the H-alpha absorption line in my spectra.  All measurements were done using the IRAF “splot” task.  For H-alpha in air I used 6562.801 Angstroms, and c=299700 km/sec
2. The LHiRes III can be used to measure radial velocities within +/- 1 km/sec, perhaps better, at ~6500 Angstroms.
3. While there is some movement within the instrument those movements seem largely linear over time. Note this says nothing about the source of such movements just that they can be calibrated out by using surrounding calibration exposures and interpolating the wavelength solutions to the central time of exposure of the “science” image.
4. The only explanation I have for the much higher standard errors I found in my Deneb measurements is that I was adjusting the grating twice each night in order to capture both H-alpha and Si II λλ 6347 & 6371 lines. My current working theory is that after moving the grating micrometer it takes some time for things to settle.

 

Bye Bye CGEs!

The era of the Celestron CGE mounts here at Beverly Hills Observatory are numbered.  Yesterday I purchased another previously-owned Astro-Physics 1200GTO mount, to be used for the photometric telescope in the North Dome.  As soon as the pier adapter plate arrives I’ll be retiring the CGE and will put it, and the one I removed from the South Dome, up for sale on AstroMart.  Both mounts have served very very well for eight years.  Each mount has all the usual foibles so familiar to other CGE users.  Yet every single bit of data and every image and spectrum, save for spectra gathered in the past year, has been acquired using a CGE.  For the money you just can’t beat ’em!

Figure 1 is an image I took one evening of the globular cluster M3, combined from 12 2-minute subexposures.   Given the image scale of about 0.72 arcseconds per pixel the star images are nice and round, indicating solid guiding.  If you ask many imaging experts they will tell you the CGE is not up to handling a 14-inch Celestron at full imaging scale and I would largely agree with them, particularly for objects near the celestial equator.  But that’s from an imaging perspective where one is trying to get the perfect “pretty picture”, where nice round and tiny star images is paramount (pun intended all you Bisque fans).  But for photometry it just isn’t as important if a few images out of a multi-hundred image sequence is trailed.  It does negatively impact the signal-to-noise for measurements in trailed images, but I was willing to sacrifice a few images until I could afford something better – and that time has come!

Hello AP1200s!

Last year I acquired a 1200GTO mount for the South Dome and, after a whole bunch of issues with the motors and encoders (essentially both motors and encoder assemblies had to be replaced) the mount has performed brilliantly.  Furthermore, the support I received in determining the source of the problem and making the necessary repairs was surely beyond anything I’d experienced.  The first AP1200 I purchased was, as far as I could determine, 12 years old, thus well out of warranty.  Furthermore, Astro-Physics doesn’t even manufacture the mount any longer (so it’s a “legacy” product).  They surely were under no obligation to go to the lengths they went to in order to get the mount working.  After their expert repairs had been made the AP1200 in the South Dome has performed flawlessly.  Both the build-quality and product support convinced me that I never had to look elsewhere for a telescope mount; I’m an A-P customer for life!

 

In the past year Astro-Physics has released two new mounts to their lineup and one, the AP1100, looked to be the perfect match for the C14 telescope.  It weighs a bit less than the 1200 but has about the same carrying capacity as the older AP1200.  Both were well above what I need for a C14.  But a week ago I spotted another AP1200 for a nice price on AstroMart.  This mount came with lots of the accessories that I would have had to purchase individually, as well as a “portable” pier that would allow me to take one of the telescopes out for dark-sky viewing (whatever that is).  Yesterday I drove to Ashland, VA to meet the seller and after a quick listen to the motors paid the man his asking price and brought the mount home.  Over the next week or so I’ll take it apart to check it out, maybe clean and re-grease the gears, but by all appearances this mount is in perfect working order.  It will be installed as soon as the pier adapter plate (which attaches the mount to the concrete pillar) arrives from Dan’s Peir Top Plates (another very much recommended company, by the way!)

 

In November of 2012 I purchased a new spectrograph for the observatory from a fellow amateur and, as it turns out, long-time ProTools victim, Ron DiIulio in Texas.  It’s a LHiRes III (for Littrow High Resolution), manufactured in France by Shelyak Instruments.  Ron had purchased the instrument early in 2011 but just did not have the time to use it.  I found his ad on Astromart where he’d listed it for sale back in January.  It was quite a nice package deal as it included every grating and every slit width that Shelyak provides and so is quite versatile, offering resolution as high as 0.11 Angstroms.  Including considerations of slit width, camera pixel size, and sampling theory it should allow me to achieve a systematic maximum resolution of around 0.4 Angstroms.

Over the past month or so I’ve had the opportunity to learn a bit about the instrument and to do some calibrations and finally to baptize the new instrument with first light, that having occurred while many of my friends were either in or just off the coast of Australia for the total solar eclipse that happened on November 13th.

Calibrating the Grating Micrometer

The first thing I calibrated on the spectrograph was the micrometer that adjusts the grating angle, thus bringing different parts of the spectrum into the field of view of the acquisition camera.  Each grating will need to be similarly calibrated, but for the time being I only measured the 2400 line/mm grating – the grating which provides the highest resolution for the instrument.  I took a series of spectra of both the internal Neon calibration lamp as well as a Mercury gas discharge tube and measured the micrometer reading required to place various emission lines at the center of the ccd chip.  The figure, below, shows a plot of the results.

The wavelength range for the 2400 line/mm grating and ST8xme camera is about 175 Angstroms, at a scale of about 0.11 Angstroms per pixel.  In practice, however, I’ll likely be using a slit width that is about four times the detector’s native pixel size.  So I will usually be binning the data 2×2, resulting in a system resolution in the area of 0.4 Angstroms. Also, just looking at the plot and noting the locations of the Neon and Mercury lines it’s pretty clear that I’ll need to build or acquire an improved calibration lamp setup as wavelength calibration will be difficult or impossible anywhere blueward of about 5600 angstroms. I see that some people are using Thorium-Argon lamps, which makes sense given the wealth of bright lines across the entire spectrum, but at well over $2000 it is something that will have to wait!

Focusing the LHiRes

The next item up was to focus the instrument.  In the Littrow design the same lens is used both to collimate the light coming from the spectrograph’s slit onto the dispersing element (in this case a reflective grating) and to focus the light returning from the grating onto the acquisition camera’s CCD.  In fact, the distance from slit to collimator must be the exact same as the distance from the collimator to the imaging device.  The process of focusing the spectrograph is achieved in two steps; first the collimator is adjusted until the light coming from the slit is collimated (the rays made parallel) onto the grating, then the camera is adjusted to best focus by moving it either closer or farther from the collimator.

Focusing the Collimator

When the collimator is at the correct distance from the slit the light rays heading towards the grating are made parallel.  This is the same condition for light rays originating from an object which is at infinite distance,  So the strategy for setting the collimator position is to remove the spectrograph’s grating and in its place insert a telephoto camera lens and camera.  It is then easy to simply take exposures of light coming from the slit and adjust the collimator position until the slit is focused in the camera.  I used my Nikon D5100 DSLR along with a 300mm telephoto lens which I’d previously adjusted to focus at infinity.  The collimator lens in the LHiRes is a simple two-element achromat housed in a helical focusing device with a 1mm thread pitch; that is, a full turn of the lens in it’s focusing holder will move lens 1mm either towards or away from the slit.  The images, below, show the focusing setup and an image of the focused slit.

Focusing the Camera

Once the collimator lens is locked into position it is time to assure that the acquisition camera is at the proper distance from the collimator.  Shelyak Instruments makes a variety of camera adapters for this purpose and the spectrograph includes, as standard equipment, an adapter for the Santa Barbara Instrument Group ST-series cameras.  That’s plenty convenient since that’s precisely the model of camera I use.

In a perfect world that would be the end of it; simply attach the camera and away you go.  There can, however, be small manufacturing inconsistencies in both the focal length of the collimator optics and in the positioning of various components within the spectrograph itself, typically on the order of small parts of a millimeter.  The ST cameras, also, have undergone several revisions and changes.  Taken together these considerations mean that once the camera is attached it may be necessary to tweak the collimator to get the best focus onto the camera’s CCD chip.   Tests conducted by several people, most notably by French astronomer Christian Buil (one of the true pioneers in the field – his website is a treasure trove of information), have shown that measurable image degradation occurs if the collimator has to be adjusted more than about 250 microns (one quarter millimeter) from it’s optimal location relative to the slit.

Once I finally got the correct adapter attached I was relieved to find that the collimator required only very little adjustment, about 60 microns, to bring images of Neon emission lines from the internal wavelength calibration lamp into focus.  Shown below is one of the first spectra taken with the LHiRes from its new home.  It shows the absorption lines of H-alpha (Hydrogen, on the left side) and Helium (to the right).

Just after completing the first dome, in 2006, I began to purchase backup parts or “spares” for the telescope and it’s mounting. In 2007 I purchased a new Santa Barbara Instrument Group (SBIG) ST8xme camera to replace the ST9xe camera I’d been using. I decided to spring for the “class 1” chip on the ST8, and began using it in September of 2007. Not long thereafter I realized that I had an entire second setup other than another Celestron 14-inch telescope assembly. When I found one for a reasonable price on Astromart I decided to buy it and thus had two complete and nearly identical setups. Originally I figured I’d use the second rig for trips to dark-sky sites. That way I would not have to take the main telescope apart every time I wanted to observe somewhere other than my badly light polluted back yard. As it turned out, the second rig made exactly two trips to dark skies, and both times I ended up bringing both of the rigs anyway. I also began setting the second rig up on the driveway next to the existing dome. I had a “Scope Roller” contraption that allowed me to roll the entire rig out of one of the bays of my garage onto the driveway where I had painted spots indicating where the leveling bolts were to sit. It only took a few minutes to set the second rig up so I ended up using it in that fashion for about a year. In 2009 a local amateur astronomer, Norm Lewis, who was also a local TV weather man, offered his 10-foot Technical Innovators “ProDome” for sale on Astromart. The price was very attractive so I contacted him and immediately bought it. Norm helped me move the dome back to Baltimore from near Mt Airy, Maryland where he had used it for some years. Similar to the first dome I first had to break up and remove a big chunk of my concrete drive way (which had been poured twice and is over 8 inches thick). After opening the necessary space I built a 10-foot octagonal deck and then assembled the dome on that.

A freak early snowstorm slowed construction in early November, but by late November the observatory was complete. First light for the new observatory was in January of 2010. Like the first dome this dome currently houses a Celestron 14-inch Schmidt-Cassegrain telescope on a Celestron CGE go-to mount. Each of the telescopes is controlled by a laptop in their respective dome, then linked via a home wi-fi net to my main computer in the study in my house.

The main instrument for Rig #2 is the ST9xe camera originally purchased for Rig #1. It has a 5-position automated filter wheel attached which contains a set of Custom Scientific B,V,R, and I band filters and a “clear” filter. Because the ST9 has a chip with relatively few pixels (512×512 pixel array with 20-micron pixels) it can be read out a lot more quickly than the ST8 in the adjacent dome. So I tend to use this rig for objects which are brighter and will benefit from the faster imaging cadence.

 

 

 

Building the First Dome

 

The first building was designed and built during the summer of 2006. It is an octagonal building roughly eight feet across made largely of plywood with a dome whose shape was inspired by the WYNN observatory on Kitt Peak in Arizona. The key advantage to the dome’s design is that it is all made from flat surfaces. Essentially there are five squares and four equilateral triangles in the design, mounted on an octagonal upper section that has wheels mounted to it’s bottom side allowing the dome to rotate. The top and side piece were replaced by hinged (top) and removable (side) door panels that allow an almost 40-inch wide “slot” for the telescope to look through.

The telescope in dome #1 (which I refer to as “Rig #1”) is the better of the two otherwise identical appearing telescopes in every way. The differences are not large, but discernible. An SBIG ST8xme camera with a grade-1 chip (1530×1020 9-micron pixels) has been used with this ‘scope since September of 2007. The vast majority of data acquired have been images, mostly through a “clear” filter. The camera is equipped with an automated five-position filter wheel containing standard B,V,R, and I filters. However, since the bulk of what I observe are fairly faint cataclysmic variable stars I make the trade-off of being able to go a magnitude or so fainter at the expense of not being able to reduce the magnitudes to a standard system. For the most part these data are used for timing of periodic phenomenon in these systems, a goal whose requirements can often be served without precise color information. On the best nights here I can often achieve 1% photometry of objects as faint as 15.5 magnitude using 60-second exposures.

This camera can also be attached to an SBIG Self Guiding Spectrograph (SGS). Using the stock 600 line/mm grating the instrument is capable of a spectral resolution of around 2 Angstroms, and can acquire decent signal-to-noise over about 800 Angstroms in a single image. The little bit of work done using the SGS has been to observe a few long-period pulsating variables (Mira-type) in the 0.5 to 0.7 micron range. For the most part I am still assessing the capabilities of the instrument and, frankly, searching for projects which might benefit from those capabilities. By combining sets of three or more 30-minute exposures I’ve achieved 20-to1 signal-to-noise for red stars as faint as 12th magnitude. I tend to concentrate on the red part of the spectrum due to the camera’s higher quantum efficiency in the red and because my primary wavelength calibration source, a Neon gas tube, has most of it’s lines in the red. I also have a Mercury tube for the occasional foray into the bluer regions, at the expense of less precisely calibrated wavelengths and much reduced sensitivity.