Extra long "vixen" dovetail

[MjrTom]

Hello

Sorry for having to start a thread on this but I can't find for the life of me a retailer that stocks extra long dovetail bars.

Currently I am using an ADM 11 inch dovetail on my 102mm f/10 refractor however ideally something around 15 inches would be far better for balancing the telescope and also to space the tube rings further apart to help with vibration and flexure.

Looking for something with similar build quality to the ADM ones if possible.

Thanks in advance!

Edit: an image of the current spacing

[Merlin66]

If you can't find one you could consider getting a length of aluminium section say 20 x 40mm the length you want, mount the rings to the ends of the bar and then fit the existing dovetail to the underside???

[Psychobilly]

FLO Sell a 14" one by ADM...

First Light Optics - ADM Vixen-type Universal Dovetail bars

[MjrTom]

Thanks guys.

Yeah I saw that one Peter, I got the 11 inch one from FLO but was looking for something a bit longer than 14 inch if possible.

Should have got the 14 inch one to start with and it would probably have been enough just about. 15 inch gives me that little extra to play with

Currenlty when rebalancing various eyepieces, finder scopes etc i find that I am using most of the length of the dovetail. With a long dovetail it would allow the scope rings to sit more centrally on the optical tube assembly and therefore push the balance point closer to the objective lens end of the telescope.

Had looked at maybe a counterweight system but looks more complicated than just extending the dovetail and therefore the balance point.

Hope that makes sence

[Psychobilly]

I just use a set of adjustable ankle weights from Tescos on my Vixen Frac .. cheap as chips...

http://direct.tesco.com/q/R.212-3496.aspx

Well maybe not as cheap as chips but a lot cheaper than an extra long dovetail...

[barkis]

I sent you a PM Tom, did you get it?

[MjrTom]

I just use a set of adjustable ankle weights from Tescos on my Vixen Frac .. cheap as chips...

Actually thats not a bad idea, might have to consider this

Thanks

[John]

I'd avoid using the standard long dovetail bar - I got more vibrations when I fitted one of those which I eventually traced to the bar twisting slightly. The temporary cure was a shorter bar followed by a solid CNC one which is very solid. Not quite as cheap as the standard long bar of course

[MjrTom]

Hi John,

Thanks for your input

To be honest I recon a few inches longer is all I need and with the CNC aluminum ADM ones they are about the best made Vixen size dovetails I have seen and have no flex at all.

Think I shall source the 14 inch ADM dovetail and see if that meets my needs

[Moonshane]

would a standard 33cm dovetail to and botton not provide adequate stability for visual? much cheaper than one of the ADM or WO ones surely?

[MjrTom]

would a standard 33cm dovetail to and botton not provide adequate stability for visual? much cheaper than one of the ADM or WO ones surely?

Thanks Shane,

The standard 33cm dovetails are only pressed/cast aluminium by the looks of it and not upto the job of mounting an optical tube thats 1100mm long and weighs 6kg in my opinion.

The current ADM 11inch one I use currently is quite a step up in rigidity.

I have included a more recent photo (today) to show what I mean with regards to the ring spacing, saddle location and amount of travel available.

[Caldwell 14]

I'd avoid using the standard long dovetail bar - I got more vibrations when I fitted one of those which I eventually traced to the bar twisting slightly. The temporary cure was a shorter bar followed by a solid CNC one which is very solid. Not quite as cheap as the standard long bar of course

Thats very interesting John, do you mean more scope "wobble" when using higher mags with thiner dovetails, aka Skywatcher ones?

[MjrTom]

My experience is yes.

The ADM ones and Altair Astro and a few others for that matter use aluminium ~4 times thicker which removes pretty much all flex and wobble

You get what you pay for for sure.

[Moonshane]

Thanks Shane,

The standard 33cm dovetails are only pressed/cast aluminium by the looks of it and not upto the job of mounting an optical tube thats 1100mm long and weighs 6kg in my opinion.

The current ADM 11inch one I use currently is quite a step up in rigidity.

I have included a more recent photo (today) to show what I mean with regards to the ring spacing, saddle location and amount of travel available.

fair point Tom. i had a 120mm Celestron f8.3 which was pretty heavy and always seemed OK but you know a lot more about fracs than me!

[John]

Thats very interesting John, do you mean more scope "wobble" when using higher mags with thiner dovetails, aka Skywatcher ones?

Not wobble as my mount was pretty stable. This was a higher frequency vibration which took a while to trace. Once I suspected that the long standard dovetail bar was the culprit I experimented by using a bar around half the length and the vibrations were noticeably less.

[MjrTom]

As John says its a kind of resonance.

This is especially noticeable as I manually track objects using the Alt/az mount.

[Caldwell 14]

uummm right, wobble was perhaps a little strong, slight tremmer might be a better way of putting it, something I would never have considered, very interesting.

[MjrTom]

Just a quick update from me

I managed to pick up a 14 inch ADM dovetail bar and it has rectified the balance issue and makes it far easier to rebalance the scope when switching from the 35mm Panoptic to a much lighter eyepiece like a BGO for instance.

Thanks all for your input

[John]

Good result Mark