Swapped focuser on my 190MN

[wimvb]

I took the plunge and replaced the stock focuser on my Mak-Newt. I wasn't pleased with the low profile Crayford focuser because of the floppy extension tube. While I was able to fix it in place by locking the thumb screw, I almost had to use pliers to do so. Imo, this is a poor design for an imaging rig. On the other hand, replacing the focuser on the 190MN is a bit of a gamble as people have failed before, and messed up collimation beyond all hope.

There are two focusers that are "drop in" replacements for the stock focuser, that I'm aware of, MoonLite and Feather Touch. Both can be mounted on the stock base plate without modifications, although they both do need adapters. I went for the more expensive of the two: Feather Touch FTF2020BCR with 2" barrel and 2" travel. I ordered the focuser in August of last year from Astroshop. The delivery date for the focuser was pushed further and furher into the future, and after reading on CN that the founder had passed away, and that the second person was no longer with the company, I decided to cancel my order and go for another focuser alltogether. The Omegon Steeltrail 2" is a very low profile focuser that fits in the baseplate of the 190MN without any modifications. Or so I thought. The diameter of the dovetail is 78 mm, the same as that of the stock focuser. No problem there. But the Omegon has such a low build height (46 mm), that the focuser knob's edge is below the edge of the housing. This means that when the draw tube is all the way out, the focuser can't stand flat on a surface. It will rest on the knobs and the edge of the base that is furthest away from the knobs. Since the focuser will be mounted on a tube, that may not be much of a problem, but unfortunately, the dual speed knob is closer to the focuser housing than the singel speed knob, and it doesn't clear the stock focuser's base plate. In order to be able to mount it, I had to cut a ring from gasket paper to get the clearance I needed (2 x 0.8 mm gave me just shy of a mm clearance).

Because the Omegon is a lot (!!) cheaper than a Feather Touch (290 € vs 900 €), I could afford a focus motor and a Pegasus Powerbox

Focuser with shim between housing and base plate. Note the clearance of the focus cube over the tube. Phew!

Focuser orientation needed to get clearance. (Btw, the pads on the camera are vibration dampeners. The camera fan on some ZWO cameras is known to be a source of vibration). Never mind the cables, I will clean that up before the start of astro season in late August.

To get the focus cube bracket where I wanted it, I had to use washers. Without the washers, I'd have to use the last slot in the bracket to attach the focus cube to the focuser. But then the axle of the motor wouldn't fit all the way into the coupler.

Thumb screw clearance. When the focuser is all the way in, this screw pushes against the bracket. I may have to replace it at some point, but for now, I have set the zero position of the focuser to this distance.

Pegasus Powerbox Advanced

The old "power boxes" and usb hub. To the right, the 12V distribution box, and in the middle, the 5V distribution box. The powered usb hub is also no longer needed. When I used a Rock64 SBC with Ekos, I had to connect my imaging camera through this usb hub, or it would crash the system. With the RPi 4, this hub was no longer neede, but I kept it so as not to alter balance.

[Robindonne]

Maybe swap the problemThumbScrew for a cheap alternative?   Who needs 3 screws all the time?

[wimvb]

I did a bit of testing this night. Collimation is as good as it will ever get. Slightly defocused stars look very symmetric. Also, the background is more even before any image calibration than it was before. No more dodgy stars in one corner. A FWHM/Eccentricity map still shows a very small amount of tilt, but I can address that tomorrow, and it isn't noticable in an image.

The focuser and power box do their job. Next step is to dial in the best settings for the autofocus routine in Ekos.

[wimvb]

5 minutes ago, Robindonne said:

Maybe swap the problemThumbScrew for a cheap alternative?   Who needs 3 screws all the time?

Thanks for the tip.I had already thought of replacing the one that is in the way with a hex bolt. I just have to check the size/pitch.