The Astrophotography Framing Assistant is an astrophotography field of view calculator that draws its answer on the sky. Choose a Telescope and a Camera, add a Barlow or a reducer if you use one, and it shows how much sky they frame, and at what pixel scale, as a rectangle over real survey images.
How to use it
- Pick the Telescope with Pick the telescope, or type the Aperture and Focal length in mm.
- Set Barlow / reducer, from 0.33× to 5×. The factor multiplies the focal length.
- Pick the Camera, or type the Resolution in px, the Pixel size in µm and the Binning, 1 × 1 or 2 × 2.
- Press Add to view. The combination’s Outline appears in the middle of the view and a row joins the Equipment history table. Add several to compare them. Clear equipment empties both.
- In the Sky View, type an object such as M31 in Go to an object, press Enter, and drag the imagery to put your target inside the Outline. The Survey menu offers DSS2, 2MASS and allWISE.
- Turn Rotation, from 0° to 360°, to match your Camera’s angle on the sky.
- Read the Equipment history table, one row per combination: effective focal length, focal ratio, field of view, diagonal, pixel scale and the Dawes and Rayleigh limits.
- Switch to the Solar System View to set an Outline over the Moon, the Sun or a planet. Save image downloads the view as a JPEG. Never look at the Sun without a proper solar filter.
How it works
The tool works from the Telescope’s aperture and focal length and the Camera’s pixel size and pixel count. A Barlow or reducer multiplies the focal length, and the sensor’s size is the pixel size times the pixel count. The focal ratio is the effective focal length over the aperture.
The field of view uses the small-angle form. The exact form, 2 · arctan(sensor size ÷ (2 × focal length)), is less than 0.1 % smaller for a field up to 5° across, far below the accuracy of the numbers you type. Binning 2 × 2 doubles the effective pixel size and halves the pixel count, so the field is unchanged and the pixel scale doubles. The Dawes and Rayleigh limits are 116 and 138 over the aperture in mm, in ″, and depend on the aperture alone.
The Outline is the ideal rectangle of the sensor, placed by angle over survey imagery. It guides composition; it does not preview your picture, and vignetting, distortion and the image circle of your Telescope are not modelled, so the corners of a real frame may be weaker. The Solar System View draws the Moon at 30′, the Sun at 32′ and each planet at its widest.
Formula
Field of view = (180 ÷ π) · s ÷ F ; pixel scale = 206.265 · p ÷ F
The field of view is in degrees and the pixel scale in ″/px. s is the sensor’s width or height in mm (pixel size × pixel count ÷ 1000). F is the effective focal length in mm (focal length × Barlow or reducer factor). p is the effective pixel size in µm (pixel size × binning). 180 ÷ π = 57.2958.
Worked example
A 200 mm aperture Telescope of 1200 mm focal length with a 0.8× reducer, on a Camera with 3.76 µm pixels and 6248 × 4176 px. F = 1200 × 0.8 = 960 mm, so the focal ratio is 960 ÷ 200 = f/4.8. The sensor is 3.76 × 6248 ÷ 1000 = 23.49 mm by 3.76 × 4176 ÷ 1000 = 15.70 mm. The field is 57.2958 × 23.49 ÷ 960 = 1.402° = 84.1′ wide and 57.2958 × 15.70 ÷ 960 = 0.937° = 56.2′ high. The pixel scale is 206.265 × 3.76 ÷ 960 = 0.81 ″/px.
Common questions
How do I calculate the field of view of my camera and telescope?
Enter the Telescope’s focal length and the Camera’s pixel size and resolution, press Add to view and read the field of view in arcminutes, with the diagonal beside it. By hand, divide the sensor’s width and height in mm by the effective focal length in mm and multiply by 57.2958 for degrees.
What is pixel scale, and what should mine be?
Pixel scale, or image scale, is how much sky one pixel covers, in ″/px. A smaller number samples finer detail. A common rule of thumb is two to three pixels across a star’s blur, so with 2″ Seeing about 0.7 to 1 ″/px is enough. The tool gives the number and does not rate it.
Does a focal reducer widen the field of view?
Yes. A 0.8× reducer turns 1200 mm into 960 mm: the field widens 1.25 times in each direction, the focal ratio falls from f/6.0 to f/4.8 for a 200 mm aperture, and each pixel covers more sky. The tool uses the factor you pick; a real reducer’s depends on its spacing.
How accurate is the framing?
The field of view and the pixel scale are as accurate as the numbers you enter, and the Barlow or reducer factor is the nominal one. The Outline is placed by angle, so it is a reliable guide to composition, not a preview of the image.
Do I need an account, and is my equipment saved?
No account is needed. What you enter lives only while the page is open, and the tool stores only your Theme Mode and language in the browser. It reads the equipment catalogue and the survey imagery from the network, so it needs a connection.
Related tools
In the store
Equipment this tool helps you choose, at David Astro: