Setting circles on a dob?

[JOC]

1 minute ago, Geoff Lister said:

I found that the main OTA tube deflected the compass needle more than the Az & Alt motor boxes.

I hadn't thought that it might be the tube - I wonder what the main OTA tubes are made of to do that?

[JOC]

4 minutes ago, Geoff Lister said:

I was fed up with the handset suggesting a second star, behind a fence, tree or house. So, I spent some time with Stellarium, and produced a table of 14 of the brightest stars visible from my patio, at various times in the year. I then used Stellarium with the time and date for dusk in the middle of each month of the year, and noted the direction (N, NW, W etc.) and the altitude, of 3, 4, or 5 of those stars to give me roughly 90 degrees azimuth and 30-ish degrees altitude difference, to give me good GoTo alignment. I repeated the process for 1 hour before dawn, in case I am up for an early session. Vega, Altair & Arcturus feature strongly, as a first star in my table, each for about 6 (but often different) months of the year.

That sounds like more dedication than I might want to give it!  However, I sit here in admiration of your dedication to the cause.

[Geoff Lister]

My magnetic-base spirit level sits nicely on the top of the OTA's main tube.

[leeseafish]

Here's my Dob Mod - adjustable setting circle, 12v illumination, digital inclinometer.  It all works a charm and was well worth the modest effort.  http://seafoodacademy.org/pdfs/adding-a-setting-circle-to-my-dobs.pdf

[Bobby1970]

46 minutes ago, leeseafish said:

Here's my Dob Mod - adjustable setting circle, 12v illumination, digital inclinometer.  It all works a charm and was well worth the modest effort.  http://seafoodacademy.org/pdfs/adding-a-setting-circle-to-my-dobs.pdf

Glad to see my old 200p still going strong Lee. ?

[tripleped]

Geoff I like the compass idea. Any issues with the optical tube metal interfering with compass at all? Also will a cheaper compass work if not how much should one spend for it to get decent performance for this purpose? Thanks 

[Geoff Lister]

It's difficult to anticipate the effects of stray magnetic fields on a compass - Elmer Sperry developed a gyrocompass system for use on ships, so avoiding this problem.

I bought several, cheap, compasses on eBay, and, by holding one, and, moving it around the assembled mount, found the best position where the OTA had least effect. I don't think it is worthwhile spending lots of money on a compass, provided the needle swings freely around on it's pivot.

Geoff

[tomato]

Here are the setting circles on my 16” SW flextube Dob. The azimuth circle is sandwiched between the baseboards with a hole cut in the top one to read the scale with an illuminated pointer.

For altitude a battery powered inclinometer is attached to an adjustable base on the top tube of the scope, (base not shown in photo).

The alt/az co-ordinates of the object are obtained from Stellarium and the scope moved to those numbers (without delay!) after first setting them up to a reference star in the centre of the FOV.

[Tomatobro]

Here is a similar arrangement as Tomatoes on a 4.5 inch Dob

[Tomatobro]

setting circles can be printed out on an A4 printer using Blocklayer. From memory up to 38 inches in diameter. The larger ones are printed in segments and have to be cut out and glued down which can be fun..........

But carefully done produce excellent setting circles

[bomberbaz]

I was going to do this mod on my dob but I then went the digital setting circles route. 

I think with this option and some practice the results will be just as accurate.