Target framing calculator: will it fit my camera and telescope?

What the calculator compares

Every target in the list has a catalogue size: a long axis and a short axis in arcminutes, or a single diameter. Galaxies often also have a position angle (PA), the direction of the long axis measured from north towards east. The calculator compares that outline with your field of view, computed from your sensor and focal length with the exact 2 × arctan formula used by the field of view calculator.

It reports three things:

• Frame used: the larger of (long axis ÷ long side of the sensor) and (short axis ÷ short side), with the camera rotated to suit the object. Above 100 % the object does not fit.
• Fits north-up: whether the outline fits with the sensor’s long side east–west and north up, which is how many imagers leave the camera. This needs a PA; when the catalogue gives none, the answer is “unknown”.
• Focal length for 50 % and 80 %: the focal length at which the object’s long axis spans that share of the sensor’s long side.

The margin setting keeps some sky around the object: with 10 %, an object “fits” only if it uses at most 90 % of the frame.

The geometry

For north-up framing the calculator turns the object’s ellipse to its position angle and measures the box around it. With semi-axes a and b and the long axis at angle φ from the horizontal (φ = 90° − PA when north is up):

box width = 2 × √( (a cos φ)² + (b sin φ)² )    box height = 2 × √( (a sin φ)² + (b cos φ)² )

The focal length for a chosen fill solves the field-of-view formula backwards:

focal length = sensor long side ÷ ( 2 × tan( object size ÷ fill ÷ 2 ) )

Worked example: M31 at 400 mm

The SEDS catalogue gives M31 as 178′ × 63′, and SIMBAD gives its long axis a PA of 35°. An APS-C IMX571 sensor (23.49 × 15.70 mm) on a 400 mm telescope sees 201.8′ × 134.9′.

• Frame used, camera rotated: max(178 ÷ 201.8, 63 ÷ 134.9) = 88 %. It fits within a 10 % margin.
• North-up: the long axis sits 55° from the horizontal, so the outline’s box is about 114′ wide and 150′ tall. That exceeds the 134.9′ height: it does not fit north-up.
• Focal length for 50 %: about 227 mm; for 80 %: about 360 mm.

So at 400 mm the galaxy fits only if you rotate the camera to follow it. The M31 framing guide covers the same numbers for other sensors.

Why catalogue sizes are a starting point

Catalogue sizes come from different surveys with different depths and different ideas of where an object ends. For bright galaxies the values usually agree within 20 %; for large emission nebulae they can differ by a factor of two or more, because one catalogue measures the bright core and another the whole ionized region. The targets section lists every catalogue value we found for the featured objects and says which one each framing guide uses. Deep exposures also show faint outer structure beyond most catalogue sizes, which is what the margin setting is for.

The chart in the calculator draws objects without a published PA as circles of their largest dimension, so it never implies an orientation the catalogue does not give. Your own sky charts or planetarium software show the real shape; this calculator is for checking the numbers.

When it does not fit

If “frame used” is above 100 %, you have three options: a shorter focal length (a reducer, or another telescope), a larger sensor, or a mosaic. The mosaic planner counts panels for the same targets and gear.