Choosing a telescope for galaxy astrophotography

Step 1: work out the image scale you need

Most galaxies worth photographing are between 2 and 11 arcminutes across. The question is how many pixels that spans.

arcsec/pixel = 206.265 × pixel size in µm ÷ focal length in mm

With 3.76 µm pixels, an 8-arcminute galaxy spans:

Below about 700 mm you are photographing a smudge, regardless of how good the telescope is. That is the floor.

Step 2: find your ceiling, which is the atmosphere

Seeing sets a limit that no telescope penetrates. Under 2-arcsecond seeing, detail finer than about 2 arcseconds is smeared away.

Sampling at roughly half the seeing value captures everything available. In 2-arcsecond seeing that is about 1 arcsecond per pixel — which 800 mm already provides with typical pixels.

Sampling much finer gains you noise, guiding difficulty and longer exposures, not detail. This is why plenty of imagers get better galaxy results at 1 200 to 1 600 mm than at 2 800 mm, and why reducers are so widely used on long Cassegrains.

Find out what your site’s typical seeing actually is before buying focal length you cannot use.

Step 3: be honest about the mount

At 1 600 mm, half an arcsecond of guiding error is a visible smear in every frame.

• Load an equatorial mount to well under its rated capacity. Ratings are visual ratings.
• Use an off-axis guider beyond about 1 000 mm. Differential flexure between a separate guide scope and the main tube becomes larger than the errors you are trying to correct.
• Expect to spend more on the mount than on the telescope, and to be glad you did.

Step 4: field correction

Galaxies sit in star fields, and misshapen corner stars ruin an otherwise good frame.

• A Ritchey-Chretien has no coma at all, but its field is not flat — a flattener is worth having for APS-C and larger.
• A standard Schmidt-Cassegrain has coma and needs a corrector; an aplanatic one (EdgeHD, ACF) does not.
• An imaging Newtonian always needs a coma corrector, and at f/4 the spacing tolerance is tight.

Step 5: plan for multiple nights

Faint outer galaxy structure needs total integration measured in many hours. That makes two unglamorous things valuable: repeatable framing between sessions, and a setup reliable enough that you are not troubleshooting instead of collecting.

The honest summary

Galaxy imaging is the most demanding purpose on this site. It rewards a good mount, decent seeing and patience far more than it rewards an exotic telescope — and a modest 8-inch reflector at 1 600 mm on a mount that guides at 0.5 arcseconds will beat a far more expensive telescope on a mount that does not.