Skywatcher 8" Dob Setting Circle

[sunshine185]

jflowers, thnx for the link, but i measured it out and its only 19" diameter, i think its a little small, could be wrong though!

[sunshine185]

blah,the link i posted printed a circle 22" dia, its too big, mines 21.5" i think. oh well ,I'll keep searching.

[Carbon]

Confused now I thought I would align 0 to Polaris as close as possible , set my adjustable pointer to say 3 degrees if I was 3 degrees out then add 3 degrees to everything I wanted to see , is that right

Sorry, don't mean to confuse anyone.

I drew up a quick sketch illustrating what I do for alignment.

The upper board has a cutout displaying part of the degree circle mounted on the groundboard.

When I take my scope out I orient the base so that the 90 degree reading points roughly north using a simple compass. Or if you know where Polaris is you can just eyeball this orientation.

Then I carefully level the base using shims or whatever.

Then I sight Polaris or any other known target through the eyepiece, find its current degree coordinate (I use Sky Safari) and quickly move the pointer to the correct degree reading. That's it.

Also make sure your Wixey is calibrated to true level and you're good to go.

[scuffer]

Thanks carbon , good easy explanation , sunshine I think the base is 520mm which is more or less 20.5"

[sunshine185]

thx! now does some kind soul have a link to a 20.5" diameter setting circle pdf?

[sunshine185]

I tried printing a few of but it was a disaster, so I downloaded the protractor program from startrak and printed off 4 lots to make a full 360 degree, fastened it together, cut it to size and marked the base with a thin permanent marker.

[stevetynant]

looks good mate

[stevetynant]

heres what I did - easy (and I'm rubbish at DIY as you can see) but all works great out in the dark

Setting Circle

Then made an adjustable base to make sure everything was perfectly square when I set up (and no fractured ankles as I walk around it in the dark)

Then a small moveable marker

Put the Dob with the setting circle on top of the Dob Base and position the moveable marker wherever you want it (between the base and the dob plate)

Job Done

You don't necessarily have to line up with Polaris - line up with any prominent star or planet is easily visible at the time just make sure the moveable marker is also moved to the Azimuth position shown on Stellarium or your Phone app. Wixcey on and goto - cheap and works suprizingly well

PS -If you think Polaris will be your goto star for polar allignmet I found the best position for the setting circle is by making the 0 or 360 at 9 O'Clock I would suggest - if you put it either at the top end of the OTA or the Bottom end - the OTA Itself will be above it an obscure your view- just a thought.

[sunshine185]

Agree Steve,I put 0 at about 9 o'clock, I got quoted 20 quid to print and laminate a setting circle to fit the base, can't afford it so I went with the permanent marker job. The phone inclinometer I have isn't accurate so I will be on the lookout for a wixey. I am also contemplating lowering the 250px a little further down the side of the base, my 200p was at the perfect height for seated viewing, the 250px is a bit higher, there's a good six inches our so between the base nut and the mirror cell, I'm sure I could lower it about 3-3.5 inches.

[sunshine185]

P.s. Your setting circle looks the business!

[stevetynant]

cheers Sunshine - I think I'd be tempted to get an adjustable chair rather than move the OTA South - wouldn't that mean drilling into the sides of the wooden housing?

Their only @ £25 from Amazon (I think a Wixeys about the same price) I know it all mounts up but once you've got them they shouldn't need changing.

Good luck anyway - really interesting post

cheers

Steve

[scuffer]

Sunshine when did the 250px come along ? , good pics , I feel like I'm being left behind , have just ordered some adjustable feet , am gonna try my levelling base slightly different so will post when its done , and thats a very patient way of doing a setting circle but effective, both look very clear.. I remember when my dob was just drop and go , now its mod and mod and put back in the garage.

[Carbon]

One thing I need to do is to add leveling feet directly to the groundboard. I intend to replace the three stock rubber/plastic feet with three adjustable feet of some sort. ....Off to Home Depot's plumbing isle after work....

[knobby]

I just love Dob modding

Nice jobs guys ...

[scuffer]

Finally got my setting circle done .... not that there was any hurry !! . Had to wait to get it printed as it was being done as a freebie . I used a circle created by abberation so thanks to him , perfect size and the lines to cut around the dob mount are spot on. I decided to turn it round 90 degrees so that zero isn't under the primary mirror - won't know what difference this makes till I use it, also made the line thicker with a marker pen every 5 degrees , then held it down with some perspex sheet ... tried foolishly to try and cut this out in 1 piece , it's screwed down in 5 seperate pieces now but looks ok. Thinking of re-doing my marker with a red LED on the top. pics below

[Carbon]

You degree circle is looking good!

[sunshine185]

Hopefully some clear weather will be here for you to give it a test run, I'm delighted with the accuracy of this cheap Pushtu mod.

[scuffer]

looking forward to testing the accuracy , I do only plan on using it for hard to locate objects , I want to learn the skies but it has all changed since I last saw it , just checked the weather and it says Friday will be clear ..... but I want a beer on Friday ....decisions decisions

[Carbon]

The accuracy is dependent on how well you level the base.

[stevetynant]

Excellent job Scuffer - interesting that you cut a square inside circle to go around the Dob Housing - I just took mine off and screwed straight through so didn't then have to screw into the Dob base at all.

Looks good though - get yourself a Wixey and the weathers due to change for the weekend and your away.

Nice one

[scuffer]

Got my leveller ready to go , had my Wixey months and it has only had 1 quick test . . . Not so easy lining up to that .1 of a degree . . . How many millions of miles is .1 degree ? , didn't realise how barren the summer months were going to be , still , i got years to learn his stuff just need to keep patient

[Demo johnny]

Why do some setting circles go 0 -360 clockwise or counter clockwise?

[stevetynant]

I think it depends on whether you make the setting circle static by fixing it to the lower base plate then cutting a hole in the upper plate to view it or you fix the circle to the upper plate so the circle actually moves and the pointer is fixed to the base plate. Certainly if you fix the pointer and the circle to the upper base plate it needs to run anti clockwise

[Demo johnny]

I think it depends on whether you make the setting circle static by fixing it to the lower base plate then cutting a hole in the upper plate to view it or you fix the circle to the upper plate so the circle actually moves and the pointer is fixed to the base plate. Certainly if you fix the pointer and the circle to the upper base plate it needs to run anti clockwise

I am being thick today but if you turn the top section to 90 degrees on a clockwise scale whats the difference if your scale is on the top or on the bottom section?

[sunshine185]

i have my circle on the top, going anti-clockwise, so i put my marker on the bottom base then find polaris, for arguments sake let`s say polaris is 0 degrees. i adjust the marker so it points at 0 on the circle, then i look for something else, for example, some object in the east is 60 degrees, so i rotate the dob anti-clockwise until the marker points at 60 degrees, if i had put the circle on the top, in a clockwise direction, the marker would be pointing at 320 degrees. its the way you set the setting circle up which decides if it should be clockwise or anti clockwise.