Calculating finderscope field of view

[David P Osborne]

Apologies if a) this has been covered elsewhere and I have missed it and/or this should be in observing rather than equipment but how do I work out FOV for SW 9x50 RACI finder scope to then make a cut out to place on star charts for star hopping when planning a session.

[Grant]

Is it this one? 

http://www.firstlightoptics.com/finders/skywatcher-9x50-right-angled-erecting-finderscope.html

If so, the FOV is listed in the specs as 5.6°

You would then need to know the scale of your star charts to make an appropriate cut out - you could always lookup the distance between two known stars in your chart and then use that as a ratio to create the cutout...

Hope that helps.

Cheers,

Grant

[glowjet]

Although true FOV is often published, an interesting practical way of finding actual FOV, is to find how long it takes for a star, located on or near the celestial equator, to cross your field of view.

Choose your star, switch off any drive, and wait for it to show on the Eastern edge, time how long in seconds it takes to cross through the centre of the field and disappear at the Western edge. If you divide the figure by 4  the resulting field diameter will be in arc minutes, however, you may find for most stellar objects measurements are given in arc seconds, you then just simply need to divide by 60 to give the result in arc seconds.

If you do the exercise three times and take the average, your results will be pretty accurate. So you can now check all you optical pieces and make templates to suit your star maps, either by using thin clear plastic sheet and drawing circles in water proof ink, cutting out circles or simply using formed soft wire circles, the choice is yours, enjoy