Dark Window

Go / no go for deep sky

Field of view calculator

How much sky will your camera and telescope or lens capture, and how fine is the detail? Pick your sensor, enter the focal length, and compare the frame to targets like Andromeda and the Moon.

// Your camera and telescope
Reducer, flattener or Barlow

Picking a telescope fills in its published focal length, which you can still change. Then choose the reducer or Barlow you add, if any. Pixel size is only needed for the pixel scale. If you leave it blank, it’s estimated from the megapixels.

// Your field of view
Width × height
3.37° × 2.25°
Diagonal
4.05°
Effective focal length
400 mm
Pixel scale
1.94″/px
Frame width in Moons
6.5
Planner frame shape
3:2

A good range. Roughly 1 to 2.5 arcseconds per pixel suits most backyard skies and tracking mounts.

Use this in the planner

The planner wants the longest side of your frame, which is 3.37°.

// Frame preview
your frame 3.37° × 2.25°

Too big for one frame: it's about 118% of the frame's height. A mosaic or a wider setup would capture it all.

Rough sizes, drawn as simple ovals. Real objects have uneven edges and fainter outer regions.

Questions

How is field of view calculated?
Divide the sensor size by the focal length, then turn the angle into degrees. The exact formula is 2 × arctan(sensor size ÷ (2 × focal length)). Do it for the width and the height of the sensor. A reducer or Barlow changes the focal length first, by multiplying it.
What is pixel scale, and what number should I aim for?
It is how much sky each pixel covers, in arcseconds: 206.265 × pixel size in micrometers ÷ focal length in millimeters. Most backyard imagers land somewhere around 1 to 2.5 arcseconds per pixel. Much finer than that needs steady air and precise guiding. Much coarser makes stars look blocky when you zoom in.
Why does the planner ask for the longest side?
Dark Window suggests targets that fit your frame. It uses the longest side of the frame in degrees plus the frame shape (3:2, 4:3 or square), so the calculator gives you that number and the nearest shape ready to save.
Do I need to account for a camera lens differently?
No. Use the focal length printed on the lens, and the true sensor size of your camera. Don't use a full-frame-equivalent focal length here, because the sensor size already takes care of the crop.
How accurate is this?
Sensor sizes are the manufacturer's published figures, rounded, and real focal lengths can be a little off the label. The answer should be within a few percent. Plate-solving a test image gives the exact value for your rig.

Got your field of view? Find targets that fit it in the Dark Window planner.