VNS Equatorial Mount - is not geometrically correct, right?

[blusky]

Hi all, despite the popularity of the design (especially among some commercial manufacturers) - I understand it it neither precise nor geometrically correct.

The angled but vertical wedges approximate the movement of the slanted circular Northern arc. Therefore, a wider roller is first of all required as the wedge has to accommodate for the North-South range of the (theoretical) slanted arc, and, most importantly, a slightly angled section of a cone is an ellipsoid, so approximating the curved side of the wedge as a perfect circle is crude at our latitude...

Any thoughts?

[blusky]

No VNS users?

[spaceboy]

Problem solved

[blusky]

That's the south sector. If the north sector is still vertical, then the wedges need to be nearly elliptical to track accurately (nearly as the roller makes a contact with the wedge at different width points depending on the angle of the top board). Basic 3d geometry!

The cylindrical platform avoids this problem...

[Moonshane]

I have only made a Poncet style platform not a VNS but as far as I am aware the VNS segments ARE an elliptical curved section?

[blusky]

I have only made a Poncet style platform not a VNS but as far as I am aware the VNS segments ARE an elliptical curved section?

Yes, correct, but these wedges also slide across the rollers (sideways), and one may want to help the thick slice of plywood/aluminium to slide across the roller to reduce the friction...

[spaceboy]

What design did you plum for then Blusky? I can't say I have had any issues with my VNS platform but then it was only ever intended for visual use. I'm assuming your questioning the accuracy as you have imaging in mind ??? There are hurdles to over come with any EQP when it comes to imaging. The number 1 issue being polar alignment. While with some effort it can be done

I would question the point of it. Would it not be easier to get an EQ mount and scope set up ??

My southern bearing is not fixed so it can freely tilt in all directions while the north bearing drops over the bearings (25mm wide BTW). It also allows for adjustment to be made to the height which in turn has some effect on how the segments run over the bearings. This and the adjustable speed motor allows a degree of fine tuning. Again my platform was only intended to save me nudging so how effective the fine tuning would be I don't know as 10-20 nudge free minutes is long enough for me to look at an object. I just can't imagine anyone buying a manual dob with the intension of doing long exposure imaging but if someone did come up with the perfect design that allowed subs of 10 minutes with no star trails then I'm guessing they will be worth a mint.

[Moonshane]

Yes, correct, but these wedges also slide across the rollers (sideways), and one may want to help the thick slice of plywood/aluminium to slide across the roller to reduce the friction...

I am not sure I am understanding your point - probably me being thick.

the wedges don't slide sideways as such but one of them at least is rolled sideways by the friction used and the turning of the drive shaft from the motor.

that said, what does this have to do with geometry of the mount?

all I know is that mine works and Nick's (Spaceboy) works too with the VNS wedges. that's good enough for me

[blusky]

What design did you plum for then Blusky?

I

Yes, saw that, the guy's a genuis. Only look at his cabinet slash trolley with the circular cut for the scope...

The cylindrical (two angled semi-circles), idea courtesy of Don Peckham:

Don's Equatorial Platform with Floating South Mount by Don - Google 3D Warehouse

[spaceboy]

Yes, saw that, the guy's a genuis.

Even with all his kit, looking at the images I'm hazarding a guess that they were captured were with a modified webcam or maybe video and stacked in Registax ?? I'm assuming this is where your concerns for accuracy are coming from ?? If this is the case you may want to consider getting an EQ6 and some OTA rings instead.