Telescope Image Scale Calculator
Effective focal length = telescope focal length × optical factor.
Effective pixel size = camera pixel size × binning.
Image scale = 206.265 × effective pixel size ÷ effective focal length.
Field width = sensor width pixels × image scale ÷ 60.
Field height = sensor height pixels × image scale ÷ 60.
Seeing sampling = seeing FWHM ÷ image scale.
Diffraction radius = 251.64 × wavelength in microns ÷ aperture in millimeters.
Mosaic panels estimate panel steps after reducing each field by the chosen overlap.
How to use this calculator
- Enter the telescope focal length in millimeters.
- Enter camera pixel size and sensor dimensions from the camera specification.
- Use 1 for native optics. Use 0.8 for a reducer. Use 2 for a two times extender.
- Enter your normal seeing value. Use a local average if you do not know it.
- Enter target size and mosaic overlap when planning a large target.
- Press Calculate. The result appears below the header and above the form.
- Download the result as a CSV file or a PDF file.
Example data table
| Setup |
Focal length |
Pixel size |
Binning |
Optical factor |
Image scale |
| Wide refractor |
400 mm |
3.76 microns |
1 |
1.00 |
1.939 arcsec/pixel |
| Reduced imaging scope |
800 mm |
3.76 microns |
1 |
0.80 |
1.212 arcsec/pixel |
| Long focal setup |
2000 mm |
2.90 microns |
2 |
0.63 |
0.950 arcsec/pixel |
| Planetary extender |
1000 mm |
4.63 microns |
1 |
2.00 |
0.477 arcsec/pixel |
Image Scale Planning Guide
Image scale connects your camera to your telescope. It tells how many arcseconds of sky fall on one pixel. A smaller value records finer detail. A larger value covers more sky. Neither is always best. The right value depends on seeing, tracking, and target size.
Sampling and Seeing
Good sampling starts with the atmosphere. If your typical seeing is three arcseconds, a scale near one to one and a half arcseconds per pixel is often practical. This places two or more pixels across a star image. That helps stars look round. It avoids wasting signal on too many pixels.
Focal Binning and Field
Focal length changes scale quickly. A reducer shortens focal length. It makes the scale larger and expands the field. A Barlow or extender does the opposite. It increases focal length. It makes the scale smaller and narrows the field. Binning also matters. Two by two binning doubles the effective pixel size. That doubles image scale and improves signal per binned pixel.
Field of view is important. Wide nebulae need a larger field. Small galaxies and planets need more focal length. The calculator compares target size with sensor coverage. It also estimates mosaic panels with overlap. This is useful before a night session. It can prevent framing mistakes.
Practical Planning
Diffraction and seeing give useful limits. A large aperture can resolve finer details. Real skies often set the stronger limit. When the result says undersampled, stars may look blocky. When it says oversampled, images may be soft and noisy unless seeing and tracking are excellent.
Use the numbers as planning guidance. They do not replace test exposures. Focus, guiding, tilt, collimation, filters, and processing affect detail. Still, image scale is one of the fastest checks before choosing a camera, reducer, or telescope setup. It helps you match equipment to the sky and to the target.
Advanced planning supports comparison. Save one result for a native setup. Then test reducer or Barlow values. Compare pixel scale, field size, and panel count. The best option is balanced. It captures the target fully. It keeps stars sampled well. It also leaves enough signal for clean stacking and calibration. That makes planning easier before the sky gets dark.
FAQs
What is telescope image scale?
It is the sky angle covered by one camera pixel. The unit is arcseconds per pixel. It depends on focal length, pixel size, and binning.
What image scale is best?
There is no single best value. Many deep sky setups work well near one to two arcseconds per pixel. The best value depends on seeing, target size, and tracking.
Does a reducer change image scale?
Yes. A reducer shortens effective focal length. That makes image scale larger and field of view wider. It is helpful for large targets.
Does a Barlow change image scale?
Yes. A Barlow increases effective focal length. That makes image scale smaller and field of view narrower. It is common for planets and small targets.
Why is seeing included?
Seeing describes atmospheric blur. The calculator compares seeing with image scale. This helps identify undersampling or oversampling before imaging.
What does binning do?
Binning combines neighboring pixels. It increases effective pixel size. This increases image scale and can improve signal per binned pixel.
How is field of view estimated?
The calculator multiplies sensor pixels by image scale. It then converts arcseconds to arcminutes and degrees. This gives width, height, and diagonal field.
Are mosaic panels exact?
No. They are planning estimates. Real panel count may change with camera rotation, framing choice, target shape, overlap, and usable edge quality.