Dual Widefield NB Imaging Rigs with Remote Rotatable Cameras - Worm Drive.

[Gina]

Following 3D printing problems with the triple imaging rig, I've decided to design a totally different dual NB widefield imaging rig.  Well, not totally different I guess, it still uses ASI1600MM-Cool cameras, Astrodon 3nm NB filters and vintage film SLR lenses with remote focussing.  The difference is that there will only be two cameras etc. and the rotation system will use worm gear drive rather than spur gears.  This is a method I've been wanting to try, giving easier to calculate motor steps for each angle.  It also allows me to try out worm drive which I've never 3D printed before.

 Instead of using a single printed camera support panel front and back which failed to print properly, this design uses individual camera brackets attached to a U shaped base part.

[Jkulin]

What are you going to use as a guide camera Gina?

[Gina]

The first thought was to arrange the worm drive above the cameras where there is plenty of room, with the drive motor and back end bearing on separate pillars attached to the dovetail bar.

This CAD model shows the worm drive system.  The worm has been extended a bit and and extra conical and cylindrical part added to take a grub screw and the worm gear edited to clamp round the body of each camera.

[Gina]

To make the CAD model easier to understand, I have coloured the parts.  As with the other thread I have shown the cooling vents on the cameras as a hole through it.  The idea was to make sure there was plenty of space for the cooling airflow to escape.

Having thought about the construction I came to the conclusion that the worms would be better between the cameras and the dovetail bar so looked into doing this.  Next CAD screenshots of the assembly show what needed to be done.  Raising the cameras away from the dovetail bar around 16mm provided room for the worms.  The advantage is that the stepper motor could be mounted directly on the dovetail bar and the back end bearing with a bracket on the base part.  Moving the cameras by 16mm didn't seem too big a "price to pay" for the advantage of having the worms easier to mount.

[Gina]

Here are some CAD screenshots of the worm drive system from various angles and some with hidden parts to show the details of the drive system.

[Gina]

A couple of design improvements and changes to make construction easier.  The camera supports have a "rim" added to extend the bearing surfaces and for the gears to rest against to stop the cameras moving axially.  The base back has been lowered to give access to screws to attach the front camera brackets and camera bracket extended to match.

[Gina]

Posted by: @Jkulin

What are you going to use as a guide camera Gina?

Nothing at the moment - concentrating on very widefield imaging which doesn't need it.  Once I get the rig finished and we get some clear nights.  ATM there's no more room on the dovetail bar but I could move the electronics somewhere else and put a guider there. 

Actually, I could think about it and move the imaging rig along the dovetail bar to make more room at that end.  That way I can gain an extra inch.  Yes, I mean inch because the ADM dovetail bar is Imperial!

 

[Gina]

Now to the worm axle, stepper motor and back end bearing.

[Gina]

This shows the rig moved to the right, with the stepper motor moved to the end of the dovetail, to make room for the guider to be attached to the dovetail on the left.

[Gina]

 Changed nozzle to 0.4mm (from 0.6mm) and printed the worms and worm gears.  Went well.  The black thing is nothing to do with the rig - just about the right height to hold the gear in correct mesh with the worm.

Now printing a slightly modified base part :-

 

[Gina]

This photo shows the assembled imaging rig.  There wasn't enough room to get a shaft coupler between motor and first worm so I decided to combine worm and coupler by designing and printing a new worm as shown in the CAD screenshot below.  The worms were made to be a tight fit on the shaft and the first worm was fixed to the motor shaft with a grubscrew.

With the right stepper motor.  The lens etc. for the second camera is yet to be added.

[Gina]

Not much room in here for another A4988 stepper motor driver!

 

[Gina]

There's just about room to get the A4988 driver etc. in the electronics box. The RPi+HAT comes out easily enough so I shall be able to add connections to the GPIO pads on the HAT. Strip board with A4988 can be attached to the side of the box with hot melt glue. Photo shows A4988 stepper driver module and board placed where there's space.

 

[Gina]

https://a.pololu-files.com/picture/0J10073.600.jpg?75d9ca5bb2e095e5c5f64350019e1b81
Minimal wiring diagram for connecting a microcontroller to an A4988 stepper motor driver carrier (full-step mode).

This shows that only two data lines need connecting to the RPi  GPIO.  It does also need Vdd (3.3v) and Data Gnd connections to the HAT plus the 12v and power Gnd for the motor.  However, I suspect that the driver may supply a "holding current" to the motor which isn't required in this case so the SLEEP line can be used to turn the current off.

[Gina]

For the remote control I plan to use similar code to the Astroberry Focuser INDI Driver code but applied to a separate INDI driver as the Astroberry Focuser is already in use for focussing the lens.  Like the Focuser Driver, the new driver can have preset values as well as general control.  These saved presets can hold the angles for a series of panels for a mosaic.  Once set up the driver code in Ekos will retain the camera rotation position just as the focuser retains the focus position.  This should mean that I won't need to provide endstop switches.

[Gina]

GPIO 25 has a pull-down so suitable for the STEP line.  SLEEP could also do with pull-down.  DIR doesn't matter.  These could go on GPIO 20 and 21.

[Gina]

Wiring for the camera rotation A4988 finished.

[Gina]

An update on this project...

Last Thursday (6th May) night was unexpectedly clear so decided to rush reassembling this rig.  Could only manage to get the Ha camera set up in time so decided to try a mosaic of Cygnus through Sagitta and Scutum to Scorpus.  Milky Way.  In Ha.

[Gina]

I had the camera with Ha filter and 28mm f3.5 lens all ready focused and ready to go by astro dark and started imaging the Cygnus region.  Camera angle was not ideal but I pressed on regardless.

 

Over the next several hours up to 03:30 Friday I captured around 50 mins on each panel.  These were 300s exposures and around 10 per panel.  I tried to get a reasonable overlap.

Edited May 9, 2021 by Gina

[Gina]

After some problems with the mount putting me in the wrong area  of sky I eventually zoomed in on Cygnus, plate solved to the appropriate FoV and started imaging, ramping up the exposures until I got a suitable brightness range..

 

This is the first Cygnus sub.

 

As time went on the FoV gradually rose above the horizon and I got more sky in the image.

 

Edited May 9, 2021 by Gina

[Gina]

I then moved on to the next panel, plate solving to the required FoV.

 

Again getting down until the ground crept into the FoV.  This it the first sub in the Sagitta region.

 

Stretched in  PixInsight.

 

[Gina]

On to the next panel, Scutum region.  Chose a frame FoV with an overlap.  Only trouble is that it seems it wasn't enough.  More of that later...

 

Plate solved to position.

 

First sub.  Got the ground back in!

 

Last sub losing the ground.

Edited May 9, 2021 by Gina

[Gina]

Next and last panel is Scorpius.  Well grounded!!!

 

Plate solved.  Note... the reason the Solver FOV doesn't quite match the FOV Symbol is that my guess of the camera angle was a bit out!

 

First sub.  Not just the ground but the bunch of huge oak trees!!!

 

From here the FoV just moves sideways then heads upwards again!!  This is the last possibly usable sub as now getting light.  Imaging rig is now pointing at the observatory south wall!

[Gina]

Here are the results of calibrating and stacking the subs and applying ABE in PI.

 

Cygnus.

 

Sagitta region.

 

Scutum region.

 

Scorpius region.

 

 

[MarkAR]

Great to see you back to imaging the other night Gina, it was a late night but you got some really great shots.