Megalens is one step closer
The 1000mm lens now has auto-focus
It’s been a very slow process getting here. Not helped in the slightest by my procrastination and putting it off for months and months.
When I bought the Megalens, almost a year ago, I intended to use it for astrophotography. I tried it once and discovered that it was almost impossible to focus manually, even during the daytime, and the stars in the corner of the image had massive distortions - they looked more like birds in flight or the letter ‘V’. (Not to mention the insane amount of vignetting!)
I discussed this with the experts on IIS and my options boiled down to:
- Sell the lens on to someone who would only use it for terrestrial photography
- Take it apart and hope I could collimate the optics
- Only use it for terrestrial imaging
I was going to attempt option (2), but quickly realised that I didn’t have the correct-sized tools to do this, and had no idea where to even start with the process. So I parked the whole idea.
Then one day, I realised that the ASI183MC camera has a tiny sensor when compared to my DSLR. So, in theory, the optical defects should be minimised because I’d only be using the centre of the light cone and the worst of the defects would not be captured by the sensor. (I’m probably wildly wrong here, but hey, it seems like a good idea.)
So I started tracking down the pieces to get the Megalens mounted on the HEQ5-Pro with guiding. So I ended up with a long dovetail, a clamp to hold the guidescope, and some aluminium bar to make up ‘claws’ to hold the USB hub and power distribution box.
This initial design had the guidescope mounted under the end of the dovetail, but this didn’t leave enough room for the other pieces. So back in the cupboard it all went. The weight distribution was terrible, too.
The next (and current) attempt has a piece of flat aluminium bar bolted to the dovetail in a cross shape, with the guidescope mounted to one side. To mount the EAF, the cross-piece extended out the other side and a 3D-printed EAF mount was obtained.
After more months of delay and being side-tracked I eventually purchased another EAF (because I sure as hell wasn’t going to take my working setup apart!) followed by a suitable pulley and length of GT3 drive belt.
Some Internet searching turned up a multitude of ways to join the ends of a GT2 or GT3 drive belt and after some careful work I had a custom sized drive belt that allowed me to rotate the lens’ focus movement by the EAF.
The next step was to find out how to create individual profiles in Ekos for the two EAFs. This was surprisingly easy - use the ‘Custom Driver’ option to create two aliases for the ZWO EAF and then use the appropriate alias in the Ekos equipment profile. So I now have 2 profiles - one for the Megalens and one for the SW72ED. Each is fully independent of the other and I can tweak one as much as I want without impacting the other. Yay!
All I need now is a clear night so I can test it all out. I have some hesitation about the guidescope having a long enough focal length to properly guide, and the ASI183 pixels may be too small for such a long focal length. We’ll see how it all turns out…
The images below show just how compact the entire setup is - but not how heavy it is!
Left-hand side, ready for testing
Both the Megalens and the guidescope have their light / dew shields extended. You can see the rotator and filter holder in the optical train. If I can’t achieve focus I have some spacers that can go in the light path to push everything backwards a bit. Hopefully I won’t be needing them, based on my testing with the DSLR.
Right-hand side, ready for testing
In this image, it looks like the Megalens and guidescope are not parallel - they are, it’s just an illusion caused by the camera in my phone.
