Success! I went back to the telescope and tried to get the guider to plate solve. (See last entry). Unfortunately the peripheral stars in the field I think are too distorted and probably not enough of them to get a consistent solution. I did get it to work sporadically but this was not going to be sufficient to get the job done. Went to plan B which was to remove the spectrograph and put the regular focuser back on, then proceeded with the mount pointing model and polar alignment with the imager, the Atik 460EX. In other words, I did the mount calibration the usual way with the main camera like we do in the case for regular imaging. After we got a decent pointing model and more accurate polar alignment, I reattached the spectrograph. Moving around the sky confirmed satisfactory pointing accuracy. The star shows up in the guider field consistently. Although not centered, it was a simple matter to center the star using the cross hair in the software program. Interestingly, the spectrograph slit in the guider field is easy to see now due to the waxing gibbous moonlight! Thank you Moon! I found it was very easy to place the star onto the slit even if it was slightly off center! Next step was to test the guiding capability . I calibrated the guider on the target star because that is typically the brightest star in the field and the software will default to that if you select a dimmer star. Great! I see the calibration is successful. I select a star in the field other than the target star and start guiding! So the target star starts on the slit and lets see if we can keep it there after 10 minutes. Looks like we can! Now the guiding accuracy is OK. Generally we are staying under a pixel error but because of the moonlight probably there is a lot of noise that perhaps fools the guider thinking those pixels are part of the star (see image below). Looks like we are ready to take our first test spectrum! I slew to a nearby medium bright star, 4th magnitude. I’m not sure about the spectral type but this is a test. The star is centered manually. Guide star is selected and we start the guiding. I start a 10 minute exposure on the imager and wait…………..Image download……….A stellar spectrum is seen on the screen!!! I have never been so excited to see a thin streak of light across a computer screen. It actually works!
So this is a star in the constellation Cygnus which has a Hipparcos catalog number HIP106481 (see below). Turns out it is a “G” type star and as you can see there are lots of absorption lines. For now we won’t worry about what it means but we celebrate a successful spectrum acquisition! Next we slewed to the star Alhena in Gemini which I know from my early spectroscopy experience with a simple grating called a “star analyser” that it is an A0 star with a strong H alpha absorption line. After a 5 minute exposure there I was able to recognize a distinct absorption line which must be what we’re looking for. Won’t know for sure until we process it but I would say we are moving along pretty well now! Looks like we are ready to do a full project including calibrating and processing. Onward and upward!
Thanks for reading!
Live spectroscopy set up! The Lhires is connected directly to the C14. No intervening focuser. We are using the knob on the primary to focus.
This is the guide field. You can see the target star is centered on the spectrograph slit (dark oblique line running from upper left to lower right through the center). Notice as you go out from the center the stars are distorted. I am thinking this is due to the lens in the guide port. Doesn’t matter though as we can find several stars near the center that are suitable for guiding!
Active guiding session! The guide star is pretty well centered. Guider exposure here is 4 sec. Guide errors probably average less than a pixel although in this image you can see it’s a little more than that (left side guide set up window) . Notice the noise in the guide star live image (right) which is likely affecting guide accuracy. The Moon is 2 days before full on this night!
Very first spectrum captured! G type star, HIP106481. You can see multiple absorption lines. The broadest line (arrow) is probably H alpha
Spectrum from a known A0 star. Very strong H alpha absorption line! (arrow)
Less than 90 days to the Spectro conference. We’re still not at the telescope yet, although collimation is done (see last entry). Looks like the spectrograph was NOT properly focused. I visited the observatory of my friend Joe D who is an expert amateur spectroscopist and has the same equipment. Sure enough the spacing on his optical train was totally different. The adaptor on his measured about 42mm. Mine was only 30 -35. Way off. I went back to the bench testing, looked at all of the adaptors I had stored in various drawers etc and could not come up with any combination of sleeves etc that were not either too long or too short. I assumed the adaptor that came with the Lhires was correct when in fact it was not. So I checked with Francois over at Shelyak Instruments and he confirmed that my set up was incorrect. The slit on the spectrograph for Lhires is 23µ, and the camera pixels are 4.54µ (Atik 460 EX). Then the image of the slit should be ideally 23/4.54 = 5.06 pixel for 1×1 binning or about 1/2 that for 2×2. I was struggling to get just under 4 for binned images. What to do? Looked like there was an adaptor specific for Atik cameras on the Shelyak site with more appropriate spacing so I got one of those. The results are promising. For a 5 sec exposure of the calibration frames I am routinely getting 2.8 or so FWHM. Hopefully we’re good now. I have never had to fuss with spacing like this before for routine astroimaging. I mean we’re talking mm here! But apparently it is significant especially if in the future we plan to contribute to astronomical research!
Technical drawing for Lhires showing the optimal distance to the imaging ccd is 54.85mm. Subtract backfocus of Atik camera which is 13.5 and that leaves 41.35mm distance for whatever adaptor is being used
The idea is to focus the internal lens of the spectrograph using the calibration lamp. Make the line as narrow as possible. We have the camera open in the Sky X, 5 sec exposure and increased the resolution to 500%. We move the focusing ring on the lens by opening the side doors on the spectrograph and just reaching in there. Slight turns until the line does not get any narrower
In the processing program we can read the FWHM of a section of calibration line is under 3 pixels! In a perfect world we should be 2.5 but we’re 2.8. I think that is satisfactory but we will ask the experts!
So hopefully time to move on to the guider! Thanks for reading!
Presentations from last February’s conference are now available! Sorry it took this long.
And now the 2nd Annual Sacramento Mountains Spectroscopy Workshop has been announced! It will take place in Las Cruces NM February 22-24, 2019. They now have a dedicated website which is here: http://www.smswweb.com/
Due to the tremendous success of last years introductory meeting which was held up in Mayhill NM the meeting format has been expanded to accomodate the need. Any and all interested please take advantage! This is the only meeting of its kind in the US and its purpose is a practical introduction to spectroscopy to where after the meeting you will be able to understand spectroscopy, what it is, as well as basic acquisition and processing of spectral data. The faculty honestly is unparalleled and features Dr Stella Kafka, director of AAVSO, Francois Cochard, developer of amateur spectroscopes Lhires III and others, Christian Buil , optical engineer and French amateur who developed most of the relevant software for processing spectra, Dr David Whelan from Austin College and several others.
My job between now and then will be to have a functioning spectroscopy set up since the data acquisition part of the course is happening right here in my backyard! So over the next 4+ months hopefully you can follow the progress until the big event! Hard to imagine the world’s spectroscopy experts will be convening right here at the Talavera Space Hut on Friday Feb 22nd. Better get started!
Thanks for reading!