OnStep on Astro-Physics AP400 (non goto quartz drive)


Gilles Gagnon
 

Well, thanks to this group, I am in the process of converting my Astro-Physics AP400 mount with goto capabilities. I thought it was not possible at first but decided to take the plunge after I took the mount apart to re-grease it and found that I could probably be able to replace the original motors (48 steps, 150:1 reduction) with NEMA14 200 steps motors without too much problems.

My setup is using the MiniPCB board with the following configuration: 200 steps/rotation (OMC StepperOnline 14HS20-1504S), 32 microsteps (TMC2130), GR1 = 2.5, GR2 = 192 and MaxRate = 96. Things seem to be working fine but the real test will be under the stars. I will be controlling the mount using either SkySafari from my android tablet, or kstars and Ekos (indilib) environment with which I have a few minor problems that I hope to sort out soon.

Here are a few photos of the modifications. The prototype motor mounts were 3D printed with ABS. The final mounts will be machined using 6160 aluminium.

I could provide the motor mounts drawing to people interesting in converting their old AP400 to goto capable AP400.

Thanks to all for their effort in this great project.

Gilles




Mark Scrivener
 

Hi Gilles,

Any update on this project? I happen to own an AP 400 non GTO just like yours and would love to convert it. Any additional details or updates you can provide would be great. FYI I happen to own a CNC mill, so if you have a motor mount design that works I'd be happy to make a few extra.

Cheers,
Mark


Gilles Gagnon
 

Hi Mark,

The conversion of my AP400 is virtually complete. I can provide you more up-to-date information as well as the STL file for the motor housing.

The motors I use are the 14HS20-1504S NEMA14 from StepperOnline ( https://www.omc-stepperonline.com/nema-14-stepper-motor/nema-14-bipolar-18deg-40ncm-567ozin-15a-42v-35x35x52mm-4-wires-14hs20-1504s.html , 1.8 degree step, 40N.cm), with a GR1 value of 3.75:1 (GT2 16 and 60 teeth pulleys), GR2 being 192, and 32 microsteps.  RA motor (Axis 1) is reverse direction, DEC (Axis 2) is normal direction.

You need to remove the silver cap on the worm housing in order to reveal the worm shaft and mill a small groove on each worm housing so you can fit the hex Allen key to tighten the 60 teeth pulley on the worm axis.

I do not have much recent photos but I include the STL file as well as a photo of the RA motor housing, FYI. As we've had the worst winter (overwhelmed by snow falls) in many years, i haven't had the opportunity to test it outside yet :-( . 

Let me know if you need more info.

P.S. You can also look at Charles Lemaire recent posting on an AP400 modification that makes use of NEMA17 0.9 degree per step motor.

Cheers,
Gilles


Mark Scrivener
 

Thanks Gilles! All points noted, grabbed the STL file and I'll take a look at what Charles Lemaire did as well. Looks pretty straightforward, but I'll let you know if I have questions once I dig into it.

I totally hear ya on winter and astronomy. I live in Northern CA where we basically have two seasons, wet and dry. Sadly it is a rare year when I manage to get out during the winter. There are lots of winter objects I'd love to image but unless I move it may never happen.

All the best,
Mark


JoAnn
 

Nice. I will be forever kicking myself for not buying an AP400 off eBay for $400. It had no badges but was clearly an AP. I did not know about OnStep or any other DIY electronics. 
JoAnn


Gilles Gagnon
 

$400 for an AP400 mount? Incredible deal, unless the mount was beat-up, or the electronics defective, in which case it doesn't matter anymore as OnStep can save the day! :-) Hope you find another deal like that.

OnStep gave a very much appreciated second life to my AP400 mount, and the mechanical modifications were much easier than I had expected.