Mak cone error

[dawson]

My 180mm Mak is mounted on the standard Skywatcher dovetail, like this http://www.skywatcheraustralia.com.au/wp-content/uploads/BK180MAKSPZ.jpg (I can't find 216mm ID rings anywhere).

 

Anyway, I checked for cone error in accordance with Dion's video ("hello again....").

 

There is loads. If I get Polaris in the centre of the field of view in the finder scope (9x50) and in a reticule eye piece (12mm) in the OTA, then flip the OTA 180 degree on the RA axis, and adjust the dec axis, Polaris is miles off; it's no where in the FoV in the scope, and roughly in the position shown in my diagram in the finder scope.

 

Can I reduce the cone error given how my OTA is mounted to the dovetail? I'm still hunting for tube rings which I think would make reducing cone error easier.

 

Thanks for any replies.

 

James

 

[Brantuk]

There's a procedure on this on CN:

 

http://www.cloudynights.com/ubbarchive/showflat.php/Cat/0/Number/1597637/page/127/view/collapsed/sb/6/o/all/fpart/1

 

And this article on differential flexure may help as well:

 

http://astronomy.mdodd.com/flexure.html

 

[dawson]

Strips of foil under the dovetail then. That's a good idea. The screws are so short that they only just really get a good grip on the nuts inside, so I need to be careful. I also think i'll struggle to undo the screws enough to slid some foil in, and be able to tighten them up again without taking the corrector plate off again; last time I had to hold a socket set on the nut inside whilst screwing from the outside.

 

Rings would make my life easier!

 

I might leave this little project until the weekend.

 

Thanks for the links Kim.

 

James

[Teslar]

James if you can find 220mm rings then do what I did and fit cork spacers. I've got some spare cork if you need it

[dawson]

That's kind. I'll keep an eye out for them too!

Hen's teeth, either of them.

James

[T A WOW]

I can get you some proper engineering shim if you want any. In 0.001" 0.002" 0.003" 0.004" 0.005" and 0.010".

[dawson]

Thanks. I need to invesitage how much i'm out by.

Do you think i could calculate how thick my shins need to be give by how many arc minutes i'm out?

[T A WOW]

Probably. But i couldn't tell you how to do it.

[xanthic]

You could calculate it, but it'd probably be wrong by the time you have mounted the scope and tightened it up (though you might be able to work it out roughly). First you need to work out if it's out positive or negative then add shims to the front or back accordingly. 

[Guest Kheldar]

I'm sure the handset compensates for cone error as part of the three star alignment process?

[dawson]

No, the manual says if there is cone error, do the two star alignment on the same side of the meridian as you want to observe. All other references to cone error say to fix the cone error before doing the alignment.

 

I found a guy at the IAS today who makes rings and I say his rings and they are lovely. So going to email him with the dimensions and he's going to send me a quote; I suspect circa £100.

 

Speaking to various other dealers of various things, most of them don't really understand cone error, which was interesting; not that I fully understand it either.

 

James

[dawson]

Said bloke who I met at the Astronomy Show back in May is making me some rings.

 

James