good morning baltimore! (or, grism spectroscopy from the MSGC-observatory)

If there’s one thing I know about myself, its that if there’s a telescope in my vicinity I am going to try to drive it.

I recently started grad school at Johns Hopkins University, whose Physics and Astronomy Department building is host to the Maryland Space Grant Observatory (pictured below). I’ve made some awesome friends since moving here this past summer, a number of which have been the Observatory fellow, in charge of running the observatory open houses and coordinating reservations. I was recently able to get trained on the use of the 20in telescope (see below) and the CMOS camera JHU students have access to, and we’ve had two brief spells of good weather since. I’ll write a blog post about the most recent, for which I’m still processing data, but I wanted to write about the latter and post a goofy slideshow I made to present what I had been up to to other grads in the department this past week.

The observatory on the roof of the Physics and Astronomy department.

One of the first things I started playing with was imaging nebula, but this proved more difficult in the Baltimore than in my previous haunts. The combination of the city skyglow, nearby stadium lights (thanks bluejays), and super humid atmosphere made the transparency (how faint of a thing you can see) pretty poor, and the seeing (how sharp points in your image are) rather large. Even with a larger telescope, like the 20in (0.5m), I was unable to get great images of low surface brightness nebula the first night I was observing. Doesn’t mean I won’t keep trying (stay tuned).

Grism Spectroscopy

The next thing I tried was using an optic called a ‘grism’. A grism is a grating which breaks up light into its component wavelengths, and a prism which bends said light. In astronomy, grisms are usually designed to split light evenly and spread it linearly depending on it’s wavelength (like the diagram on the left). A grism works to split up light from an image into a spectrum (the brightness of the star at each color of light, see this post). So I placed a small grism between the telescope and the camera, and suddenly instead of images I was taking spectra!

The grism splits the light from a star in to the image of a star and the spectrum, both spread over the camera’s pixels. I can then measure the number of photons the camera recorded on each pixel along the spectrum to determine how bright the star is at each color of light

I used the SAO/DS9 tool to extract the brightness of each pixel along the spectrum in my images and save it to a text file. I then needed a transformation between the pixel value along the spectrum to the wavelength of light that pixel corresponded to. My next steps were to identify the spectra features in each spectrum that I could calibrate based on.

I first observed an A2 type star, 29 Cyg. I chose it because it was nearby Neptune at the time and because it has strong Hydrogen absorption. A type stars have the strongest hydrogen absorption features, because they are hot enough that they’ve excited lots of hydrogen in their atmospheres, but not so hot that they’ve ionized all of that hydrogen. The hydrogen in their atmospheres wiggles between excited and unexcited by gobbling up light of a series of specific frequencies that correspond to the energy of the electrons in those hydrogen atoms. This means that the spectrum of an A star will have dips in brightness at these hydrogen lines. The cool thing is, we know very precisely what wavelengths those dips should occur at, and therefore by measuring the pixel position of the dips in the spectrum of 29 Cyg, I was able to determine which pixels in my images corresponded to which colors of light. I was then able to apply this transformation to my other targets, namely Neptune and WR 137.

29 Cyg

I then used the matplotlib python package to plot these spectrum versus wavelength and color the lines with the color of light that wavelength corresponds to. I posted these to twitter and was retweeted by the official Matplotlib twitter page.

The spectrum of 29 Cyg, a A2V type star. I’ve annotated the Hydrogen beta and gamma absorption features I used to wavelength calibrate my observations.

Neptune

The spectrum of Neptune in reflected light. This spectrum is actually the sun’s spectrum that has bounced off Neptune’s cloud deck. Neptune’s atmosphere has a lot of methane, and so the broad dips you see are the light from the sun that is absorbed by the methane and not reflected back.

WR 137

The spectrum of a monster star, WR-137. This star is about (cosmically speaking) to go super nova. Instead of hydrogen absorption, it has fused all of its hydrogen and is now a super hot core of heavier elements. It shows strong carbon and helium emission which are the result of millions of years of nuclear fusion.

Barlow imaging

I then moved to more conventional amateur astronomy, trying to take pretty pictures. I wanted to take images of Jupiter and Saturn close up, so I attached a Barlow, or a focal reducer, to the telescope in between the camera and the mirrors. This effectively increased the magnification of my images. The observatory doesn’t have a focusing tool, so the images are slightly out of focus because I focused by hand, but what can you do. I send these to my mom for her birthday.

Jupiter and Saturn as seen with the Barlow focal reducer

That was my first few nights of observation, about two weeks ago now. I hope you found this stuff interesting! I have a lot more in the works, we had some great weather this past weekend (which is gone now). The next “what I’ve been doing on the roof in my free time” post should be very, very exciting if you’re a fan of exoplanets like I am. Until then, clear skies 🙂

Posted

September 20, 2021

prepare to astronomize, research log

William

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