Sbig St-i User Manual

ST-i Spectrograph

With ST-i Spectro Software

SBIG Imaging Systems, A Division of Diffraction Limited©
59 Grenfell Crescent, Unit B, Ottawa, ON Canada, K2G0G3

Tel: [phone] | Fax: [phone]|

E-mail: [e-mail] | www.sbig.com©

Contents

Introduction ........................................................................................................... 2

Section 1: Setting up the ST-i Spectrograph ........................................................... 6

Section 2: Using the ST-i Spectrograph .................................................................. 8

Section 3: Installing and Using ST-i Spectro Software ............................................. 8

Section 4: Some Suggested Observations ............................................................ 10

Appendix A: HyperSpectral Imaging ..................................................................... 11

Appendix B: Calibration Data ............................................................................... 15

1

Introduction

Entrance Slit
Location

CCD Sensor
SF11 Prism

SBIG’s new ST-i spectrograph was designed specifically for our ST-i camera. This
unit is intended to enable an amateur to characterize his/her skies, flat field
sources, filter passbands, and other light sources. Its main purpose is to allow an
amateur to measure his sky spectrum, and optimize his flat field sources to better
match his conditions. The reason why this is important can be found in an article
by Alan Holmes titled Flat Fields – The Ugly Truth found at the SBIG web site:

http://www.sbig.com/about-us/blog/flat-fields-the-ugly-truth/

This unit also provides a good way for an amateur to compare his light pollution
situation to users at other sites. The cold reality of the world is that good paying
jobs are associated with cities and light pollution, and light pollution is fact of life
for most of us!

The optical design of the spectrograph is shown in Figure One. A prism is used as
the dispersing element instead of a grating. The reasons for this are two-fold.
Most important, the optical efficiency is high across the spectrum, roughly twice
that of a grating on average. This is valuable since the night sky is not that bright.
Secondly, the spectral range without confusion from multi-order light is from 430
to 1000 nm (4300 to 10000 Angstroms), encompassing the full sensitivity range of
SBIG CCD products. The skyglow in the near infrared (700 to 1000 nm) is
significant to the CCD camera, even though it is invisible to your eye. This near
infrared response is important in detecting faint stars and distant galaxies, but
also is not well baffled by many commercial telescope optical systems, so it needs
to be understood.

Figure One: ST-I Spectrograph Optical Design

Location

2

The design is simple: light enters the spectrograph through a 25 micron entrance

555.7 nm

slit and is collimated by an achromatic lens. It then passes through the Schott
SF11 glass prism, where blue light is bent through a greater angle than red
wavelengths. A second achromat focuses the light onto the CCD, with an
additional plano-convex lens to shorten the focal length and increase the
photographic speed of the system to F/3.66. The speed is important when trying
to capture the sky background. The plane of the CCD is actually tilted a little bit
relative to the angle of incidence of the light, as shown, to reduce the
contribution of chromatic aberration to the optical blur. The spectrum of a neon
gas discharge tube captured with this system is shown in Figure Two. Note that
the spectral lines are straight, top to bottom, even though the slit is seen to be
curved when inspected visually. The slit curvature was added to correct the
natural tendency of the off axis rays from the slit being bent by a slightly lesser
amount by the prism than on-axis rays. Curving the slit straightens out the lines,
enabling greater vertical binning of the CCD for faint targets. In these figures,
blue wavelengths are on the left, and red on the right.

Figure Two: Spectrum of Neon Discharge Tube

Figure Three illustrates the sky background from the author’s back yard, which

th

has about 5

magnitude skies. To capture this, the spectrograph is simply
pointed straight up. As a result, background starlight is also included in this
spectrum and is visible as the semi-uniform continuum baseline. The resolution
around the natural airglow line at 557.7 nm (marked) is adequate to separate it
from the pervasive mercury line at 540.6 nm, and the sodium line at 568.8 nm
(both from streetlights).

Figure Three: Airglow from my Backyard (Magnitude 5 Skies)

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A program for a PC is included with the spectrograph that allows the user to
control the ST-i and acquire spectra, to create a wavelength calibration for the
spectrograph, output text files of the data for processing with Excel or another
program, and re-bin the data into a format with uniform sized wavelength bins for
simpler comparison with grating data or from other sources. The intent is to
make this functionality easy to use, and to provide a window into the interesting
spectral properties of light sources around us. Figure Four illustrates a screen
shot from that program. A graph of the spectrum is shown, as well as a graph of
the spectrum binned into spectral intervals of equal width to correct for the non­linearity of the prism design.

Appendix A describes how to use this spectrograph for hyperspectral imaging.
Our software support for this interesting new capability for the amateur is a bit
simplistic at present, but we think it will intrigue many users and enable the
development of some new techniques. An example is posted in Figure 5 to
illustrate the power of this technique.

Appendix B graphs the nominal calibration data for the spectrograph, as well as
the dispersion in terms of Angstroms per pixel across the CCD. As you can see,
the dispersion is much lower in the near infrared.

We hope this simple-to-use spectrograph will become an essential part of
the amateur astronomer’s toolkit. It enables the amateur to visualize the
spectrum of the light around him, and to easily measure the transmission of
filters, the reflectivity of diffuse surfaces, and even atmospheric transmission with
a bit more time and setup. This technique will be described in a future paper.

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