Free planning tools · Check equipment compatibility

Guide-scope image-scale calculator

Compare guide-camera and imaging-camera scales without turning one ratio into a universal compatibility rule.

No account needed. Results update here as you change the inputs. Measurements stay in this tab unless you choose to share a link; shared links contain the inputs, including any location coordinates.

Your inputs

The starting values are examples. Replace them with your own measurements, prices or documented requirements.

What to have ready
  • The effective imaging and guiding focal lengths.
  • Native, unbinned pixel sizes for both cameras.
  • A measured RMS value in guide pixels if you want the error translation.
Measurements for Guide-scope image-scale calculator

Calculated from your inputs

Your plan

The example will calculate here when JavaScript is ready.

What to do next

    Calculation details

    Inputs used in this result

    Compare a change before committing

    Keep this plan, change the inputs and see the difference. The comparison stays in this tab.

    What does an entered guiding error mean at the imaging camera?

    A guide-scope focal length alone does not describe the relationship between guider and main camera. This image-scale calculator includes the pixel size of each camera and translates a supplied guide-pixel RMS measurement into the imaging scale.

    Use the result to read a guiding report or explain a proposed guider change. The sample RMS value is an editable example, not a prediction of how a mount will track. The scale ratio is descriptive and cannot approve a complete guiding arrangement.

    How to use this planner

    1. Enter both optical and camera combinations.
    2. Read the two image scales and the width of one guide pixel in imaging pixels.
    3. Substitute a real guide-pixel RMS measurement and investigate the physical system alongside the numbers.

    The calculation

    Each scale = 206.265 × pixel size in µm ÷ focal length in mm. Ratio = guide arcsec/pixel ÷ imaging arcsec/pixel.

    A worked example

    600 mm / 3.76 µm imaging and 200 mm / 3.75 µm guiding give 1.29 and 3.87 arcsec/pixel, a guide-to-imaging ratio of about 2.99.

    Example inputs are demonstrations. Replace them with your own measurements, prices or documented requirements.

    Assumptions and limits

    Unbinned, square pixels are assumed. Guiding algorithms can estimate subpixel star positions; this ratio alone is not a pass/fail test. Signal, focus, mechanical flexure, mount behaviour and guide exposure also matter.

    AAVSO CCD photometry guide

    For equipment-profile checks, see the PHD2 setup documentation.

    Questions about this calculation

    Can I enter an RMS value reported in arcseconds in the pixel field?

    No. This field expects guide pixels. Divide the reported arcseconds by the calculated guide arcseconds per pixel first, or use the scale figures to interpret the report directly.

    Does a particular scale ratio guarantee successful guiding?

    No. Guide-star signal, focus, centroid measurement, mount response and mechanical flexure all matter. Verify the equipment profile in the guiding software and assess actual images.

    Put the result in context