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Fitting a 2" Focuser to SkyWatcher 150PL Newt OTA

johninderby

By johninderby
in DIY Astronomer

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johninderby

johninderby

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Fitting a 2" Focuser to SkyWatcher 150PL Newt OTA

I've just started working on my little 6" f/8 planetary dob project using an OO UK 1/12PV mirror set and CNC mirror cell and have got hold of a 150PL OTA as the starting point. I'm still designing the base and bearing arrangement, but expect it'll be finished by Christmas.

The 150PL OTA comes with a 1.25" R&P focuser. And while the focuser is actually not bad at all really for an inexpensive focuser I wanted a proper 2" crayford focuser. SkyWatcher has now released their " Dual-speed Crayford focuser for Skywatcher 190MN". It's actually identical to their low profile crayford focuser (except for the knobs) but comes without a base plate. However as it's the standard GSO, Antares, SkyWatcher etc. fitting all you need is a standard base plate as is available from Scopes'n'Skies for £2.00 or £3.00 if you're fitting it to an 8" or bigger scope. In other words SkyWatcher is charging you £30.00 for a baseplate when you buy their low-profile focuser. I'm using a Telescope Service baseplate for a 6" scope.

Base Plate

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p3701_Base-for-Newtonian-focusers-for-tubes-with-D--160-205mm.html

Focuser

http://www.firstlightoptics.com/proddetail.php?prod=skywatcher_focuser_190MN

As the standard focuser is only a 1.25" one the hole in the OTA is too small to accommodate the 2" focuser and will have to be enlarged. It's actually quite a simple, but admittedly tedious operation.

Step 1

Cover the area around the focuser hole in the OTA with masking tape.

Draw lines across the outer edges of the existing mounting holes to give you a staring point for locating the new holes.

Measure the hole spacing on the new base and mark the location of the new mounting holes.

Carefully drill either 4mm or 5mm holes depending on the size of the mounting bolts you will be using.

Put the new baseplate in position and loosely fit the bolts.

Now mark the outline of the large hole on the OTA and remove the baseplate.

Now comes the tedious part. Drill a series of 2.5 mm (exact size isn't important) holes as closely together as you can along the outline of the new hole.

Using the drill connect a couple of holes to give you enough room to insert a mini-hacksaw blade.

Now saw through all the holes.

This will leave a very rough sawtooth edge that will have to be sanded down. I used a small sanding drum in an electric drill. BTW don't worry if the hole isn't perfectly round as there will be plenty of clearance when you fit the new focuser.

Step 2

Fit the Telescope Service 6" baseplate.

This base is curved to perfectly fit the OTA and so when tightening the bolts down it will align itself.

Step 3

Fit the focuser by tightening the grub screw on each of the curved sides of the base.

You could use pretty well any Newtonian crayford focusers by SkyWatcher, GSO, Antares, Baader etc. However you will have to get the TS baseplate as the standard baseplates that come with the aforementioned focusers are for an 9" and over OTA (8" mirror).

I have replaced the fine focus knob on the focuser as I simply had a better one sitting around in my spare parts box.

John

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johninderby

johninderby

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That'd be the perfect solution for me.. but, the cost of that baseplate del'd to OZ is $77 !! :) Sofar can't find it available anywhere else, either.

Telscope Service is really helpful so perhaps you could ask them if they could ship it by regular airmail.

John

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Moonshane

Moonshane

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cracking job John!

another option is that you can simply raise the standard baseplate a little with eg flat ali bar cut to size and drilled, maybe painted black. this will mean it will sit high enough off the tube to avoid flattening it and you can then used some kind of foam to create a gasket for the sides.

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johninderby

johninderby

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You do have a very clean and tidy workshop for your projects! My workshop is overflowing with allsorts of half finished/not yet started jobs...

Ah, but you should have seen it last week before I finally got round to a proper tidy up so I could see what I'm doing in there. :)

John

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johninderby

johninderby

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Thats a real neat job. I'm surprised the OO mirror cell fits the skywatcher tube.. fantastic.. wonder if their 9 point 8" mirror cell would fit my 200p ds?

It's not an OO mirror cell as OO doesn't make a CNC one in the 6" size (OO starts at 8"). It's an American made one that came with the OO mirror set when I bought them from another SGL member. http://www.opticalsupports.com/mirror_cells.html

The cell mounting had to be trimmed by 5mm on the outer edge to fit within the standard SkyWatcher end ring and the standard tube mounting brackets were removed. Fortunately three holes in the cell plate matched up with the three large holes in the bottom of the SkyWatcher end ring so I was able to bolt it in place that way using three spacers. Although it took some mods to make it fit the cell now looks as if it was intended for the OTA in the first place.

You'd have to measure your exact tube diameter and check with OO to see if their mirror cell would fit.

John

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step_hen

step_hen

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Thats a super job John. I have been really suprised with the optics in my standard 150PL both on DSO and esp on planets- hence why the 6" Mak is up for sale. 1/12PV optics sounds mouth watering!

One question- why a dob base for planetary? I note shanes planetary scope is a dob too, but i find the tracking of my CG5 a godsend at high mag!

ha ha

hey Stephen - go for it :)

you'll be fine. take your time and there will be no problems. it's only like fitting a new thrust burner to the space station. :D

Shane- You didnt see the damage last time i tried to fit a new thruster

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johninderby

johninderby

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As I'm going to be using tuberings to mount the OTA in a similar way to the OO bases I can then just use another set of tuberings with a dovetail to mount the OTA on an EQ mount later on if I wish. The best of both worlds.

This is going to be a bit of a grab'n'go scope, and with the thin OO borosilicate mirror and a cooling fan it's hardly going to take any time at all to cool it down enough for observing.

The 6" Mak is an excellent scope, very compact and portable but the cooldown time is an issue and optically a 6" f/8 Newt/Dob is more than a match for it.

John

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ahlberto

ahlberto

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THis must be one of the most beautifull and exquisiste planetary newtons ive saw.Believe it or not,another day i was talking to a friend of mine of doing just a project like this.Finding a OTA and getting the 150mm ultra grade Orion Optics set,a good focuser and making the ultimate planetary newton.The mirror cell is so gorgeous...

Congrats mate,you have mounted a scope that is a work of art

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ChrisThePuma

ChrisThePuma

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Hi - this looks fantastic.

Re the "mirror cell" mod. Did you replace the entire mirror with a new one, or just add the fan to it? (sorry - still a noob without scope)

I'm getting a 150PL so very interested in this thread and the mod's available.

Chris

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