Very scared by dovetail fitting on a Nexstar 8i OTA please Help!

[dannydematio]

Hello Folks,

I am quite nervous right now, as I have just fitted a Baader dovetail to my Nexstar 8i OTA, (i.e. last silver version OTA before the orange variety) and then onto the current Nexstar 8SE fork arm.

I had to remove the OTA from my old Nexstar 8i fork arm, because one of the motors has died and there are no parts available. Went and got a new Nexstar 8SE fork arm!

I find that the Baader dovetail does indeed slid into the groove in the SE fork arm, however, it cannot go all the way home due to the design of this universal dovetail. I have a gap of

almost 1/3 of an inch.

Not really that much of the baader dovetail edge is "gripped" by the SE arm. I have tested the OTA/SE arm by putting it through its paces and all was ok, however, it really scares me how flakey this set up could potentially be!

This will be my first time using the new Nexstar SE fork arm. I am not really familiar with it, although it is very similar to the old arm, except for the dovetail way that the OTA is held. I worry that with the Baader in place, if the black securing knob on the new arm ever worked loose during a nights viewing, it would be bye bye to the OTA for sure!

I have a few questions to anybody with experienced of the Baader dovetail and Nexstar 8 OTA's.

Is the Baarder dovetail grip I am seeing really sufficient that once you tighten the securing knob, it will stay constantly secure unless touched again?

is there a better dovetail on the market that will affix properly to the Nexstar 8i OTA?  I was already told that the Celestron dovetail will not fit the 8i OTA.

is there anything I can do to back up the OTA so that if it ever did come loose, it would not so easily slip off. I can imagine the newer 8SE OTA would "sit" in place much better.

I had three screw holes to affix the Baader to the 8i OTA. The screws that came with the Baader were shorted than the originals, however they do hold it ok. I plan to fit longer screws soon.

I must admit that I was very surprised when I mounted the universal Baader dovetail to the SE fork arm and saw how relatively little of the dovetail edge is actually gripped. 

Any guidance here gratefully received.

Thanks

Danny

[Lonestar70]

Hi Danny,

1st thing... welcome to SGL.

One has to ask... What was wrong with the original dovetail fitted to your OTA?

The Baader one is more generally used to convert from Fork type mount to the Vixen/Skywatcher GEM type mount. 

More information required in order to give definate answers... pictures showing the problem area would possibly also help.

Best regards.

Sandy.

[dannydematio]

Hi Sandy,

Thanks for replying, much appreciated.

What I was trying to say is, the Baader dove tail works, however, it seems to me that very little of the DT is gripped by the Nexstar 8SE arm!  This is due to the design of the Baader DT being wider etc.

Now this may be normal and I was hoping somebody would tell me that it is and you just have to live with it, and make sure the tighten control knob is always very tight.

If you look at the two images. You can see the gap where the Baader obviously cannot fit into the groove designed for the thinner Celestron dovetail and also you can see that only a small part (very small) of the tooth in the clamp/grip is actually able to engage with the DT.

I guess I was hoping for reassurance as it all seemed very marginal to me on first mounting. i have the OTA on the arm now and it goes through its paces, holding fine.  However, I know that if the single tightening knob ever comes loose by itself, its bye bye to the OTA for sure!

If anybody reading this has a Baader DT on a Nexstar 8se arm, I would appreciate their feed back as to how secure it all is in the long run!

Thanks and regards

Danny

[Lonestar70]

Hi Danny,

I would not accept that as a safe mounting.

I would be looking at changing to a standard Vixen style bar like this one:

http://www.firstlightoptics.com/dovetail-bars/adm-vixen-dovetail-for-ota.html

The celestron 8" version comes with radius blocks to fit the OTA.

expensive... YES... but better than a damaged OTA due to a disconnection.  

Hope this helps.

Best Regards.

Sandy.

[ChrisLX200]

Have to agree -  that doesn't look safe, it should be a much better fit in the dovetail than that.

ChrisH

[bogdog]

looking @ picture 1 i would say you have not pushed the tube/dovetail flush with the mount before tightening?

loosen the bolt off fully and make sure the dovetail sits flush against mount as tight as possible while screwing bolt tight again

it should be ok at that,you could also add a strip of tin alongside the dovetail to take up the gap a bit

but i think the mount is not flush enough with the dovetail bar yet

[Owmuchonomy]

looking @ picture 1 i would say you have not pushed the tube/dovetail flush with the mount before tightening?

loosen the bolt off fully and make sure the dovetail sits flush against mount as tight as possible while screwing bolt tight again

it should be ok at that,you could also add a strip of tin alongside the dovetail to take up the gap a bit

but i think the mount is not flush enough with the dovetail bar yet

Yes, that's what it looks like to me. I can't quite tell but it also looks like there some sort of obstruction on the none tightening side stopping the dovetail from going fully home.

[Zakalwe]

The shape of the Baader dovetail is preventing it from seating correctly into the fork arm. You can see that in Pic 2...the "wings" on the dovetail are hitting the plastic surround oh the arm clamp.

I wouldn't use the scope in that state. It might be OK, bear in mind that we only have a couple of pictures to go on, but I'd rather get a dovetail that allows it to seat properly.

The ADM type dovetail looks like a better option.

[bogdog]

The shape of the Baader dovetail is preventing it from seating correctly into the fork arm. You can see that in Pic 2...the "wings" on the dovetail are hitting the plastic surround oh the arm clamp.

 it looks that way,but if you try to attach the tube with any part of the screwbolt through that side then it will not seat correctly

if bolt is withdrawn so that the dovetail recess is clear of obstruction then the tube will mate with side of mount then screw bolt in tight

its same design as the skywatcher goto mount i have, if i try to hold tube in the groove and not pay attention and tighten bolt it forces bottom

of dovetail out,the bolt then rides on very edge of dovetail

[Lonestar70]

 it looks that way,but if you try to attach the tube with any part of the screwbolt through that side then it will not seat correctly

if bolt is withdrawn so that the dovetail recess is clear of obstruction then the tube will mate with side of mount then screw bolt in tight

its same design as the skywatcher goto mount i have, if i try to hold tube in the groove and not pay attention and tighten bolt it forces bottom

of dovetail out,the bolt then rides on very edge of dovetail

Zakalwe Is quite correct... the broad flat top plate of the Baader dovetail bar is not designed for that mount.

The width of the top plate prevents the bar going far enough down into the curved faced PUCK... the Baader is designed for FLAT faced PUCKS.

The SKYWATCHER dovetail does not have this wide top plate... it's the same as the ADM... being narrow on the OTA side.

A skywatcher dovetail would be a suitable alternative if the 2 curved spacer plates, provided with the ADM one, can be got or made.

Celestron have made a bad choice with the design of this mounts puck IMHO... it is somewhat restrictive for use with alternative hardware.

Best regards.

Sandy.  

[Zakalwe]

 it looks that way,but if you try to attach the tube with any part of the screwbolt through that side then it will not seat correctly

if bolt is withdrawn so that the dovetail recess is clear of obstruction then the tube will mate with side of mount then screw bolt in tight

its same design as the skywatcher goto mount i have, if i try to hold tube in the groove and not pay attention and tighten bolt it forces bottom

of dovetail out,the bolt then rides on very edge of dovetail

I'm sorry but I don't really understand this?

What's clear is that the DT is not sitting into the DT recess correctly. A properly seated DT will distribute the weight of the OTA along the length of the DT recess. The clamping bolt then stops it from sliding and provides the force to hold the OTA into the recess by friction between the DT recess and the DT. 

In the OP's case, the DT is not sitting into the recess which means that the clamping bolt is clamping AND trying to carry the weight of the OTA. There's no real interface between the DT and the DT recess which means that friction is reduced. 

Its not the proper way for it to work and IMHO it's not worth risking an expensive OTA for the price of the correct DT.

[dannydematio]

Hello Everyone,

Today is my first opportunity to follow up on the situation regarding fitting an older Nexstar 8i OTA to the current Nexstar SE fork arm mount.

Many thanks to everyone who contributed, and to Sandy for correctly identifying the right Dovetail for the job. 

If I had wanted to keep the Baader dovetail, I would have had to remove the forward section of plastic housing around where the SE mount grips the dovetail. This would have allowed an adequate grip, however, cosmetically not that nice and maybe a way for dirt and dust to enter the mechanism.

I spoke to the supplier who agreed to take the Baader dovetail back. I would say that the Baader is very well made and high quality and would be a good solution if the wide design suits your set up.

The ADM dovetail from First light Optics, fitted perfectly and is also very well made. I would recommend it to anybody who still has the older Celestron Nexstar 8i type OTA. The ADM is designed to allow a sweet marry up with the SE fork arm.

Again full marks to this helpful community and forum and here's to many clear nights of safe and steady viewing!

Danny