Mounting OTA on dovetail bar

[wasyoungonce]

I have a Celestron C8 OTA that was originally a fork mounted tube.

I want to mount it to an equatorial C5 mount with a dovetail. The C8 Tube has 2 machine screws on it's rear cell housing and 1 on the front of which none line up with the dovetail bar.

Thus I have to drill and tap the front and rear housings to attach the dovetail bar.

I see that this type of fitment is common place on all OTA's like this but i have a few questions I'd like to ask anyone who has been in a similar situation:

The housings are around 3mm and I'm worried the shear torque will pull the threaded screws out! Should I make a threaded metal doubler out of steel to fit inside the tube to spread these forces?

It appears that this type of dovetail fitment is normal to many SCT OTA's. Has anyone else done this?

Rather than all the above get tube rings instead or am I a whimp?

I have to pull apart the OTA to do all this should I start drinking now?:lol:

I'd like to hear experiences from anyone that has done this as I'll only get one go at this. Here are a few pic's of the OTA, I have marked the existing screw points and the places where i need to drill with crosses.

Any advice gratefully accepted

[Dennis]

Hello WYO

I have a Celestron C9.25 whose rear mirror cell has x2 holes (8x32) and front corrector cell x1 hole (10x32) and I used to mount it on a Vixen style dovetail rail before I re-fitted the original CGE wide rail.

I cannot comment on the thickness of the metal in the C9.25 cells vs. your C8, but so far I haven’t had any problems, even thought like you, I think this is a puny way of fitting an OTA to a dovetail rail.

I think my screws only insert for approx 4 to 5 full turns?

Cheers

Dennis

[JohnG]

Whenever I mounted a dovetail onto a C-8, it required 2 radius blocks, the rear one was held by the 2 wide screws and the dovetail was attached to it, the front radius block had a long screw that went through the dovetail and the radius block onto the corrector ring

Cheers

[Astrod00d]

I also have an older C8 tube that was once attached to a Nexstar single-arm mount. My problem was the same as yours, none of the OTA holes line up with that aftermarket dovetail. The newer C8 tubes have narrowly spaced holes in the rear cell to match the narrow dovetail.
I didn't want to drill and tap my OTA. Too much risk of damage.
Fortunately, I was able to drill and tap the existing Nexstar single-arm mounting bar to match the dovetail eg I used the original mounting bar as an adapter.. This gave me a useable but not perfect dovetail attachment. Maybe it's possible to get a 6mm thick aluminium plate from an engineering supplier and drill it to match your application. May be better than drilling the OTA.
I finished up getting a good strong C8 dovetail from Ken Dauzat in the USA
http://www.kendauzat.net/other.htm
I haven't attached it yet becaude my existing solution is adequate and I'm thinking about a move to wider Losmandy-style bars.

Cheers,

Rob

[Dennis]

Good pick up John. Both my Celestron OEM dovetail bars have the radius already machined into the dovetail bar, so they do not require radius blocks like the aftermarket dovetail bars.

Cheers

Dennis

[wasyoungonce]

Thanks gents. I had looked at Kendazut's dovetails but noticed that his CG5 one had holes for the 2 rear screws and 1 for the front screw...not ideal. In general there must be many OTAs out there with this similar dovetail fitment (read not much holding them to the dovetail).

I see that the wider dovetails (re:Losmandy) allow fitting of radius blocks which I see as a good thing but alas the wider dovetail won't fit on a GC5.

I like the idea of radius blocks and would be comfortable drilling and tapping knowing there was better support on the OTA. If only the wider dovetail would fit the CG5!

Decisions Decisions!

Thanks for your input guys it's given me some ideas

edit:
Ken does have adaptors but they are a bit pricey.

http://www.kendauzat.net/other/1021051b.jpg

Then you need a Lomsandy dovetail on top of that.

[casstony]

I've never liked the way celestron fits their dovetails to the ota's. Another idea is to make a semi-circular ring for the rear that would reach the side mounting screws and use the single front mounting screw (most of the weight is at the back of the tube). Then you could either screw this ring to your dovetail or replace the dovetail with steel flat bar 40mm x 6mm, welding the ring to the bar. I've done something similar to this with an old Meade ota, but it looks rough as guts due to my poor trade skills and lack of proper tools. It works well though.

[JohnG]

The other alternative is to use a set of Parallax Rings:

http://www.parallaxinstruments.com/ring.htm

Not cheap but a much more secure way of mounting a C-8 or similar, only drawback is that you need probably a Losmandy 'D' series plate.

I don't know if this is any help or not but Losmandy now have what is called the "V" series Dovetails, they are based on the Vixen saddle dimensions, have a look on the Losmandy website for details.

Cheers

[g__day]

I must state I view the standard CG5 dovetail to OTA attachment for the Celestron SCT is the most unimpressive part of the entire set-up!

The CGE attachment is much better, but Celestron should have also provided a CGE width attachment - that use the 40mm separated holes not the 20mm separated ones - than has the CG5 width flanges - also 40mm width (vs the CGE's 75mm separated flanges). The 20mm simply put isn't rigid.

[wasyoungonce]

I totally agree.

I'm thinking of going to the local metal basher's and getting them to roll some 5mm or so mild steel to act radius blocks. Drill tap and bolt these to the dovetail and OTA.

Any misalignment would be corrected by mount alignment.

[wasyoungonce]

Thanks for all the input gents.

Just one more stupid question! Can the CG5 saddle plate be replaced with a Losmandy saddle plate...so i can use the Losmandy dovetails?

I'm looking at:
http://www.losmandy.com/access.html

Item GM8 SP

GM-8 saddle plate http://www.losmandy.com/gm8sp.gif
Can be mounted to other mounts for use with GM-8 dovetail plates.
Wt. 10 oz. Size .75"x 3.5"x 3.5" Two mounting holes 3.0" sep. for 1/4" shcs
Any thoughts on this type of fix?

[Dennis]

Careful – there are 2 GM8 saddle plates; one (GM-8 SP) seems to accept 3.5” dovetails whereas the other (GM-8 SPD) seems to have been designed for the Losmandy D series, which I think is their 4” system?

Cheers

Dennis

[wasyoungonce]

Thanks Dennis, I emailed Losmandy about their GM8SP saddle plate.

I saw that they had a vixen type dovetail, VCM8 and VC8B adapter, which had radius blocks and i believe screw holes to fit the std OTA holes. The Radius blocks would help alot on distributing forces.

So I asked about those as well.

I'll await their reply

[JohnG]

Dennis is right about the 2 types of saddle, the GM-8SP is designed for the older and now defunct "C" Series dovetail plates, the GM-8SPD is now the standard saddle for the industry standard "D" Series plates.

Be aware the Losmandy is pretty tardy in answering emails so it might be a while. :)

This is the new "V" Series page. http://www.losmandy.com/v-series.html

Also, if you want to go from CG5 to Losmandy, ADM has a saddle that will apparently do the conversion:

http://www.optcorp.com/product.aspx?pid=439-735-216-217-9704

Cheers

[wasyoungonce]

Just an update:

I have the Mount, with GOTOSTAR fitted, thanks to My Astroshop and Steve. It works a treat..indoors of course. I will endeavour to do a write up of this system once I get it all together for others to peruse.

Anyway, you cannot replace the CG5 saddle but there are Vixen to Losmandy adaptors from ADM and Kendauzat each around $110 USD.

Couple this with a Losmandy D series dovetail...$199 Aust this setup is looking pricey.:sadeyes:

Alternately I have asked a ATS if they can get the Losmandy radius blocks for the D series dovetails (they are available in the US) and am thinking of attaching a 10mm alloy plate on top of my vixen dovetail (via screws and tap) and attach the radius blocks a la Losmandy style. Bingo! Poor man's Losmandy. There is nothing wrong with the vixen dovetail...it's just the method they employ to attach it to the OTA re: screw spacings that is wrong/bad/poor.

I also asked about the Losmandy V series dovetail as it also utilises a radius block (albeit somewhat smaller).

If it all goes south I'll maybe try making my own radius blocks..breakout the file! I can feel a blister coming on.:doh:

Anyway I'll await the replies and take it from there. Radius blocks are available from the US for around $55 USD. Using my own vixen dovetail, attaching a 10mm plate to this and Losmandy radius blocks appears at this stage to be the cheapest option...for a good solid OTA mounting.

As I resolve this I'll keep this thread updated...someone might find some of the info useful.

Thanks to all those so far, especially John G.:thumbsup:

P.S:

I thought I'd post this Australian link:

http://www.starstuff.com.au/index.html

This guy appears to be making dovetails and other stuff right here in Melbourne. I've contacted him by e-mail. Others may find this link informative.

[wasyoungonce]

Problem solved. Went and spoke to Luke at Starstuff and he's making a customised 100mm wide plate and dovetail block with radius blocks to suit the OTA and mount.

A very nice and accommodating gent with some very nice handy-work, willing to accommodate my wishes.

Nice to keep the money in Local manufacturing as well.:thumbsup:

I'll post pics when it's finished.

[wasyoungonce]

Just an update. Luke Bellani of Star Stuff, a member of these forums, machined me a custom dovetail bar with a 4 inch wide platform and radius blocks to suit. He made the dovetail so I can use it as a saddle plate later on.

Now I'm happy with the way the OTA mounts, much better than the standard dovetail. Here are some pics:


Luke can be found at http://www.starstuff.com.au/index.html

His work is just perfect! Costs: $215 Aust...cheaper than Losmandy and ADM saddle plate adapter setup.

All I have to do is learn how to use the Gotostar!

[JohnG]

That looks nice and solid, a very nice piece of machining. :thumbsup:

Cheers