Grating To Sensor Calculator

The video below is a "must see" video. Please watch it. It's our best explanation of how to best use the Star Analyser grating and has lots of hints and suggestions. The video also explains how to use the calculator further down on this page.

Are you using a DSLR? Please be sure to watch this video: link.

If you plan to use a standalone DSLR, please read this: link. (pdf)

In a hurry or want some help getting started? Submit your equipment details in our Equipment form and we'll send you a short report that shows you the best way to use a Star Analyser to capture spectra. Click on this link to get started.

How to mount your Star Analyser grating

Note: Using a Star Analyser grating requires that you already be somewhat proficient at capturing images of stars.  You need to be able to locate specific stars in the sky. And you will need to be able to capture images of them that are properly exposed, well-focused, and not smeared from their movement through the sky. We're happy to coach you on how to get started in astronomical imaging if you need to acquire these skills.

The first thing is: don't worry about getting your setup exactly right. The tolerances in spectroscopy with the Star Analyser are very forgiving. Use the calculator here to come up with approximate settings, and then go out and have fun capturing spectra!

For 80% of amateur installations, you'll be able to just screw the grating onto your camera nose piece or filter wheel and be good to go.

Are you using a standalone DSLR? You don't need to use the calculator on this page. Just read the this paragraph and skip everything else on this page! First, thread our AD-58 adapter: (link) onto your camera's lens cap threads. A variable (zoom) lens is strongly recommended. Then, for the 18 - 55 mm standard kit lens, use a Star Analyser 200. Or, for a 70 - 200 mm lens, get a Star Analyser 100. It can be difficult to capture spectra without tracking, so for best results, your camera should be on a tracking mount like these. (Note to non-Canon camera owners: if your lens cap threads aren't 58 mm, you'll need a thread adapter, as discussed here: link).

For a more technical discussion on the above, see the Objective Gratings section at the bottom of this page: link.

Are you using a DSLR with your telescope optics? Use our AD-T2 adapter: (link) on your camera's T-Ring. You'll most likely want to use our Star Analyser 100. The AD-T2 puts the grating 60 mm from the camera sensor. Plug your camera specs into the calculator below to see the entire configuration.

Are you using an astronomical video or FITS camera on your telescope? For almost all telescope and camera combinations (unless you're using a filter wheel) the calculator below will probably recommend our Star Analyser 100. Use the calculator to confirm the distance to the camera sensor with the Star Analyser 100. Or, if you'd prefer, we can send you a custom report based on your specific equipment: link.

To get started, please download and review this document: "Choosing a Star Analyser grating" from this link.

After reading the above pdf document, use the calculator below to confirm your configuration.

Have a question? Get immediate answers. Send us a question via our Contact form: link. Or, we're often available for a live chat: click on the green chat window in the bottom corner of your screen.

The Calculator's Input

The calculator starts out with sample values for the Star Analyser 100 (SA100) grating used with a typical 8" SCT and a small video camera. To use the calculator, replace the sample values with values that describe your own equipment. (We recommend the default value of 3 arcseconds in the Seeing field, unless you know that your local conditions specifically call for a different value.)

NOTE: Using the Star Analyser as an "objective grating" on the front of a DSLR camera lens is a special case. Click here to scroll to an objective grating discussion at the bottom of this page.

If your Input values are out of range for satisfactory performance, a red message offers advice on how to modify your setup. To eliminate a red message, change your setup to modify one or more of your input values. For instance, you might shorten or lengthen your camera's nosepiece (if any), or add our 10mm spacer, to change the Grating to Sensor value. Or you could use a focal reducer to change the Telescope Focal Ratio.

If there is no red message, you can capture good spectra with your setup. You might see other messages that make optional suggestions for getting even better results. The fine tuning described in the optional messages is not required, but it is worth thinking about. Often these messages point out a simple change which can significantly improve the quality of your spectra.

The Calculator's Output

If the Calculator shows three green flags, you can ignore the details in Output section. It simply shows some technical details that aren't critical. The key Output fields show you how the grating performs with your Input values:

When should I use the Star Analyser 200 instead of the 100?

In some applications it can be difficult to get three green flags when using the SA100. In these cases the Star Analyser 200 (SA200) is often helpful. The SA200 generates the same spectrum length at half the spacing.

Where optimum spacing is feasible with the SA100, it should be chosen for best performance. However, there are three cases where the SA200 is particularly useful:

Note that where the optimum spacing can be achieved with SA100, it should be chosen for best performance.

If you have any questions, chat or email us here to contact us. We love answering questions!

In a hurry? Rather than you filling in the calculator below, we can help you get started. Just tell us about what equipment you plan to use. We'll send you a free report that shows you the calculator results and how to mount the grating. Get started at this: link.

For additional help on these 3 fields, please see: link)

Want some help?. The calculator can little overwhelming when you first see it. We can help! Send Tom your equipment list and he'll plug it into the calculator, sending you a free report that recommends the best configuration for you to use. Click on this link to get started. Why not?!

pixels FWHM
This calculator Copyright 2014 - 2023. Field Tested Systems, LLC. Click this link to see how easily you can process the spectra of stars

How can I use the calculator to improve my setup?

If you see three green OK-flags, you have optimized your settings. If not, you can try out various changes in the calculator before actually making changes in your equipment. You might add one or more spacers. Or change the focal ratio. Once you achieve settings that satisfy you, you can adjust your equipment accordingly.

Consider the sample Input values. (If you have already changed those values, click Reset to see them again.) They result in Field 10's Dispersion value being 14.0 Angstrongs/pixel. No red messages appear, so the setup is satisfactory for capturing good spectra.

And yet the Dispersion field shows the message "Resolution could be improved..." This is not a critical problem which prevents you from using the grating. However, it explains changes you could make to get better results. For instance, you might add a 10mm spacer. To see the effects of doing that, change the Grating to Sensor Distance (field 5) from 40 to 50. Increasing this distance optimizes the resolution by spreading out the spectrum. You can see this in the Dispersion value (field 10), which changes from 14.0 to 11.2.

Now the "Resolution could be improved..." message is gone, replaced by the green OK. But the message next to the Spectrum Coverage (field 11) is no longer the green OK. The new message, "Difficult to fit..." is not red, so this setup too would be satisfactory for capturing good spectra. However, you could still improve on it. Instead of adding a spacer, you might try inserting an f/6.3 focal reducer to reduce the Telescope Focal Ratio (Field 2). To do this, click Reset (to undo the spacer) and then change field 2 from 10 to 6.3.

Now all three messages are a green OK. You have found an optimum setup for use with this equipment.

You could, of course, go even further, but that is not always a good idea. As we mentioned above, useful values for Dispersion (field 10) are typically in the range of 7-20 Angstroms/pixel. The smaller this value, the more spread out your spectra. To make this value smaller, you can increase the Grating to Sensor Distance (field 5) or use a Grating (field 4) with more lines/mm.

It's tempting to make this value smaller and smaller, in order to spread the spectra farther and farther. However, just as when you add more and more magnification to your telescope, there is a point beyond which the image degrades due to optical limitations and merely gets dimmer and dimmer. Even before that, it is important to remember that the more you spread out a spectrum, the fainter it gets, requiring more exposure time. The optimum value depends on your particular setup. The calculator warns you if you might be using too much dispersion.

Where should you start?

We recommend that you start by capturing the spectrum of a midrange "easy" star: a type A star such as Vega. The spectra of these stars have very clear Hydrogen Balmer lines, which makes calibration easy. Once you have calibrated a type A star, you can easily calibrate spectra of more difficult stars that don't have such clearly identifiable lines. To do this, use the zero order star image in a one-point calibration procedure. For a video example of this procedure, using RSpec software, click here.

What kind of resolution can you expect?

The best possible resolution is twice the dispersion, according to the Nyquist sampling theorem. So in theory, the best resolution you could get with a dispersion of 10 Angstroms/pixel is in the range of 20 Angstroms. However, there are other factors that affect the resolution before reaching this theoretical limit.

First of all, the resolution of any grating in a converging beam setup (or any other slitless system) is limited by star size. And once you apply enough dispersion to overcome the star-size limit, the resolution is limited by aberrations because the beam is not parallel through the grating. Increasing the dispersion further does not overcome these aberrations. It only magnifies the effect and makes the spectrum fainter.

In practice, the maximum resolution you can get when using a grating as described here, regardless of dispersion, is about R ~100-200. For instance, at 6000 Angstroms, this yields a resolution of 30-60 Angstroms. This maximum is independent of the number of lines/mm in the grating; that is, you cannot improve the resolution by switching from the SA100 to the SA200.

Doug West provides a more theoretical treatment of this topic, available here: here.

Objective Gratings

When you mount the grating on the lens cap threads of a DSLR camera, it's called an objective grating. In the calculator, set the Grating To Sensor Distance (field 5) to the focal length of the lens, and ignore any messages about the focal ratio (field 2). In this setup, the distance between the grating and the lens has no effect on the spectrum. However, it is usually best to mount the grating as close to the lens as possible, to minimize vignetting.

The advantage of an objective grating setup is that, because the light in the beam from a distant star is parallel, the spectra produced are significantly sharper than those achieved with the grating between a telescope and a camera. In this setup you can gain resolution by lengthening the spectrum. Zooming your DSLR lens can achieve values as low as 3 Angstroms/pixel. (Here you can ignore the high-dispersion warning in the calculator.)

The disadvantage of an objective grating setup is that, because a DSLR's aperture is considerably smaller than that of most telescopes, it is suitable only for use with bright stars.

Objective grating summary: To mount the grating on your DSLR lens cap threads, use our AD-58 adapter: link. Then, if your lens is 35 - 100mm, use the SA-200. If your lens is 70 - 200mm, use the SA-100.

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