This calculator tells you whether a guide camera can keep up with your imaging train, whether it sits behind a guide scope or an off-axis guider. It compares the two pixel scales, tells you whether to bin the guide camera for PHD2, and rates how easily you will find a guide star.
How to use it
- Start with the imaging telescope. Pick it from the catalogue or type its aperture and focal length in mm, add a Barlow or reducer if you use one, and choose its design: refractor, Newtonian or Schmidt–Cassegrain.
- Then describe the imaging camera. Pick it from the catalogue or type its pixel size in µm, and set its binning anywhere from 1 × 1 to 4 × 4.
- Next, choose the guider. For a guide scope, pick one of the five sizes, or choose Custom and type its aperture and focal length in mm. An off-axis guider needs nothing typed, because the guide camera borrows the telescope’s focal length.
- Choose the guide camera from five ZWO models or Custom, and enter its pixel size in µm, its resolution in pixels and its binning. Each model comes with its own read noise and QE. For Custom, the tool assumes 2 electrons and 60%.
- Set the seeing and the guide exposure, which start at 3″ and 2 s. Seeing here means the FWHM the air blurs a star to, so 2″ counts as good, 3″ as average and 4″ as poor.
- Now read the gauge. It shows the scale ratio and its verdict, both pixel scales, the target guiding RMS and the measurement floor. If you guide with a guide scope, it also tells you the guide focal length you would need to reach Suitable and Excellent.
- Finally, check the sampling advice and the guide stars. The tool tells you whether to bin, and gives a rating for three regions of sky.
How it works
A guider can only correct what it can measure, and the scale ratio is how the tool judges that. It is the guide camera’s pixel scale divided by the imaging camera’s, or in other words, how many imaging pixels one guide pixel spans. Thanks to sub-pixel centroiding, a guider can hold a few times its own scale. So a ratio up to 3× is Excellent, up to 5× Suitable, up to 7× Marginal, and anything beyond that is Not suitable. A ratio that lands exactly on a limit counts as within it. The target guiding RMS is half the imaging scale, and the measurement floor is a tenth of the guide scale. With an off-axis guider the focal length cancels out, so the ratio comes down to the two pixel sizes, each multiplied by its binning.
PHD2 locks on best when a star spans 2 to 5 guide pixels at half maximum. The tool finds that width by dividing the seeing by the guide pixel scale. If the star covers more than 5 pixels, it suggests the smallest binning, up to 4 × 4, that brings the star back into range, and gives you the new scale ratio. Should that ratio pass 5×, the tool calls it too coarse. Below 2 pixels the star is undersampled and centroiding loses accuracy, which is the usual limit of short guide scopes.
To count as a guide star, a star has to reach a signal to noise of 10 in a single guide exposure. The tool takes the aperture, the exposure, the QE, the read noise and a fixed suburban sky, and works out the faintest star that makes the cut. It then counts how many stars at least that bright fall in the guide camera’s field. Since star density depends on galactic latitude, it rates the Milky Way, typical sky and the galactic pole region separately. Two or more usable stars on average is Easy, and fewer than 0.5 is Very difficult.
Formula
scale ratio = pixel scale (guide) ÷ pixel scale (imaging), with pixel scale = 206.265 × p ÷ f
Pixel scale is measured in arcseconds per pixel (″/px). p is the pixel size in µm, multiplied by the binning, and f is the focal length in mm once any Barlow or reducer is included. The scale ratio itself has no unit.
Worked example
Say your imaging camera has 3.76 µm pixels on an 800 mm telescope, unbinned: 206.265 × 3.76 ÷ 800 = 0.97″/px. Your guide camera has 3.75 µm pixels behind a 200 mm guide scope: 206.265 × 3.75 ÷ 200 = 3.87″/px. The scale ratio is 3.87 ÷ 0.97 = 4.0×, which is Suitable. To reach Excellent, you would need a guide focal length of at least 266 mm.
Common questions
What is a good scale ratio for a guide camera?
Anything up to 3× is Excellent, and so is a guide camera that is finer than the imaging camera. Up to 5× is Suitable. Up to 7× is Marginal, which can work on good nights with bright guide stars, though you should expect elongated stars when the seeing drops.
Is my guide scope good enough for my telescope?
That comes down to the scale ratio. If the verdict is Not suitable, the tool will suggest a longer guide scope or an off-axis guider instead.
What binning should I use for PHD2?
The tool recommends binning when stars span more than 5 guide pixels. Take an off-axis guider on a 2000 mm telescope, with 3.75 µm guide pixels and 3″ seeing. Stars come out 7.8 pixels wide, so the tool advises 2 × 2, which brings them down to 3.9 pixels. With a 3.76 µm imaging camera, the scale ratio then becomes 2.0× instead of 1.0×.
Do I need an off-axis guider for a Schmidt–Cassegrain?
Yes, and the tool explains why. A Schmidt–Cassegrain’s primary mirror moves during a session and shifts the image, and a separate guide scope cannot see that shift. So use an off-axis guider whatever the scale ratio says. Its pick-off loses most of the light and coma swells the stars, and the guide star rating takes both into account.
How accurate is the guide star rating?
It is based on the average star density of each region, so the target you choose may be richer or poorer, and the sky brightness is held fixed. Treat the rating as a comparison rather than a promise.
Does it need an account, and is anything stored?
No. You don’t need an account, the calculations run in your browser, and nothing you type is sent anywhere or kept.
Related tools
In the store
Equipment this tool helps you choose, at David Astro: