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Adding a focuser to a SkyWatcher Explorer 200P

Chris Reynolds
 

Hello everyone,
I'm new here, so a quick intro.  I dabbled with Astronomy as a teenager with an old telescope that my QuarterMaster grandfather had liberated from the British Army at some point.  The highlight was a fuzzy view of Saturn and its rings.  Now I'm approaching retirement I'm dabbling again and got hold of a second hand telescope.  So far I'm enjoying myself and appreciate the work that has gone into the OnStep project.
I've just finished my OnStep build for my 200P on an EQ5 mount which is working well.  I'm writing up my experiences (pretty positive overall, pro-tip don't try widening the bore on a wrongly ordered GT2 pulley with a blunt general purpose drill bit) in a yet to be finished post but I did want to ask a quick question.
I've got the stock Crayford focuser on my 200p.  I can see how I could use another stepper motor to turn the existing focusing system and I have an LV8729 in my axis 4 slot on my STM32 based OnStep box.  Is that likely to give satisfactory results with a pair of GT2 pulleys?  I'm assuming that it is better to get the 0.9 deg steppers for the focussing?  Any thoughts, words of caution or recommendations are welcome.  At the moment I'm experimenting with photographing DSO's but who knows where the hobby will take me.
Many thanks
Chris
P.S. Obligatory first (well third maybe, let's gloss over the first two) light picture

Howard Dutton
 

On Thu, May 21, 2020 at 05:34 AM, Chris Reynolds wrote:
I've got the stock Crayford focuser on my 200p.  I can see how I could use another stepper motor to turn the existing focusing system and I have an LV8729 in my axis 4 slot on my STM32 based OnStep box.  Is that likely to give satisfactory results with a pair of GT2 pulleys?  I'm assuming that it is better to get the 0.9 deg steppers for the focussing?  Any thoughts, words of caution or recommendations are welcome.  At the moment I'm experimenting with photographing DSO's but who knows where the hobby will take me.
This covers the topic fairly well I think:
https://onstep.groups.io/g/main/topic/74226797

Chris Reynolds
 

Thanks Howard, that proves my search skills aren't all they could be.

Dave Schwartz
 

The focuser project I've been working on is also on a single-speed Skywatcher Crayford focuser - on an ED80, but it may be pretty much the same as you have. They tend to use the same upper body and just change the lower end to either fit in the end of a tube, like mine, or into a radius adapter to fit on the side of a Newtonian.

The point of this is that I have 3D printed a direct-drive bracket that fits in the four socket head screw recesses, uses longer 3mm screws to fix the thing down and has a hole for the pressure adjustment too. I have it mounted and think I have the stepper mount centered (I made a jig that has a disk on the end of a 4mm i.d. post that goes over the shaft... rotates within the motor hole so seems to be centered).

The thread Howard linked to has my reasoning why a direct coupling should work. My calculations show this should have a motion of .51 steps/micron or, inversely, 2 microns per step. The table show that your F/5 scope should have a depth of focus of 72 microns so you should have the range covered.

I bought a NEMA14 motor (https://www.amazon.ca/gp/product/B00O0BMM2E) and a coupler from the 5mm motor shaft to the 4mm Crayford shaft (https://www.amazon.ca/gp/product/B00OK6DV2Y) from Amazon. The coupler is yet to arrive so I don't have final confidence that it is precisely centered.

If you have access to a 3D printer, I can send you the FreeCAD source (or just the STL file).

On 2020-05-21 9:15 a.m., Chris Reynolds wrote:
Thanks Howard, that proves my search skills aren't all they could be.

Chris Reynolds
 

Hi Dave, that definitely looks worth a go, I hadn't thought of direct drive.  Yes please to the files, buying a 3D printer was one of the side effects of this hobby as I used it to print the case and motor mounts for my set up.

Dave Schwartz
 

Howard... can I get file upload privileges? I've got a number of 3D printable designs like this I'd like to place on the onstep.groups.io website that I'm not really interested in putting on Thingiverse.

On 2020-05-21 1:21 p.m., Chris Reynolds wrote:
Hi Dave, that definitely looks worth a go, I hadn't thought of direct drive.  Yes please to the files, buying a 3D printer was one of the side effects of this hobby as I used it to print the case and motor mounts for my set up.

Howard Dutton
 

Try now.

Dave Schwartz
 

On 2020-05-21 2:00 p.m., Howard Dutton wrote:
Try now.

Serge CLAUS
 

Hello,
I have the same telescope.
I reused an original stepper motor supplied (the AD / DEC tracking kit).
I bent the square (2 hacksaw cuts) and glued the motor with double-sided tape for bathroom mirrors.
I added a sleeve bought on Aliexpress.
It works very well with a 1/4 step TMC2100.

Chris Reynolds
 

Dave, thanks for the files much appreciated, I'll try a test print later and see how well it fits mine - they do look very similar.
Serge, thanks for the photo and the idea, great to get another approach.  Sadly mine didn't come with motors so I'll have to buy one but using that tape is something I wouldn't have thought of.

Dave Schwartz
 

When you print that bracket, you have to use supports. At a minimum 'from the build plate' so that the bottom of the main plate is supported because the side braces are extended down, 'everywhere' to support the top of the large motor hole too but I have found that not to be necessary as long as you don't mind a little sanding to round it out.

On 2020-05-21 5:52 p.m., Chris Reynolds wrote:
Dave, thanks for the files much appreciated, I'll try a test print later and see how well it fits mine - they do look very similar.
Serge, thanks for the photo and the idea, great to get another approach.  Sadly mine didn't come with motors so I'll have to buy one but using that tape is something I wouldn't have thought of.