The 500 mm optimum

[Guest]

I'm looking into try imaging. I've got a book called Digital SLR Astrophotography by Michael Covington which is an excellent book but there is one thing I'm slightly confused about.

Right at the beginning of the book he says that for DSLRs the optimum focal length of the telescope is about 500mm. Which is quite small. For an f5 telescope that would give an objective size of 100mm.  I kind of understand the reasons behind the 500mm optimum size which I think is mainly down to resolution. However, if I were to use a telescope with a longer focal length would that make the images worse quality or is it the case that you just don't get any better quality so the longer focal length is effectively 'wasted'.

Also why are many of us chasing bigger and bigger objective sizes all the time? 

Cheers

Steve

 

[ollypenrice]

It's really best not to make FL your primary concern but to consider 1) Resolution in arcseconds per pixel and 2) Field of view. Here's a calculator: http://www.12dstring.me.uk/fovcalc.php

Around 2 arcsecs per pixel is a nice friendly resolution and 500mm will give a good FOV on a DSLR chip. Finer resolution requires finer guiding and better seeing (more stable with less turbulence.) With a good mount and a good site it is worth going down to about an arcsec for small targets like galaxies or planetary nebulae but a 4 inch scope is a great place to start. You could get the same FL with more aperture in a very fast astrograph but 'fast' tends to mean tricky and expensive. I know of no fast, easy astrograph.

You are right that it is possible to image at fine resolutions which will produce no genuine additional detail. This is sometimes called 'empty resolution.'

Olly

[happy-kat]

Chasing bigger objective sizes is perhaps more observing then necessarily imaging, as the bigger the telescope the bigger disproportionately the mount for imaging, plus many DSO are quite big. If you are wetting your toe's could start with the camera and lens equipment you may already own.

[Guest]

Thank you both,

 

Edited October 8, 2019 by Guest