MyTelescopes

My Telescopes

First of all: I learned the hard way that USB cables are not made for the harsh conditions of a cold winter night. Beside of getting stiff and lose contacts they have often too high resistance for providing a stable supply voltage to the Raspberry Pi. Multiple times per night the Pi crashed or rebooted. Therefore I supply with 12V (anyway needed for the mount) and added a 12-to-5V regulator directly at the Pi. The 5V go with short and thick wires to the Pi's expansion header, pins 4, 5 and 6. For the 12V wires I prefer silicon rubber cables which stay flexible until very low temperatures. Since that all my Pis are running stable.

I own 2 telescopes:

1. Skywatcher N 150/750
2. MEADE LXD55 series telescope with SC-8

Both have GoTo mounts.

I made my first trials with the Skywatcher, Raspberry Pi Zero and HQ camera.

In this setup my Laptop runs KStars/EKOS and the main INDI server. This INDI server loads the INDI mount driver which sends commands to the SynScan app on my mobile phone. The SynScan app controls the mount. The main INDI server also connects to a second INDI server with indi_pylibcamera driver running on the Pi. All these devices (laptop, mobile phone, mount and Pi) must be in the same WIFI. This sounds complicated but it works. It is even not needed to sit with the laptop aside of the telescope: after alignment you can enjoy the rest of the night in a warm room. But there are some limitations:

• The Pi Zero has not enough memory to process images without swapping to the slow SD card. Together with the slow WIFI of the Pi Zero you waste a lot of time between the exposures.
• During exposures the laptop must stay connected to the Pi. After every WIFI breakdown you need to restart the INDI server and EKOS.

With the Skywatcher telescope and a Raspberry Pi HQ camera I already did some cool photos. I tried that with the MEADE too but the large focal length made it more sensitive to imperfections in tracking. That made me belief that I need auto-guiding.

The first assembly I made was more or less to proof feasibility. Instead of spending money in a guide scope I mounted a Raspberry Pi camera behind my search scope. With an OTA focal length of 2000mm I need about 200mm focal length for guiding. My finder scope has a magnification of 8, so I bought a 25mm tele-objective for the Raspberry Pi camera. All that came in a pice of plastic pipe to sit stable on my finder scope. The guiding camera still sees the crosshair of the search scope and adjusting focus is difficult, but PHD2 was able to guide.

Excited by results I bought a finder scope with 1.25" ocular tube and added a HQ camera. For easy focussing I use Bahtinov masks on the finder and main scopes.

The same finder scope with camera can be used on the Skywatcher. Unfortunately the Skywatcher Alt/Az mount does not support guiding commands. Likely this is a limitation of the SynScan app. I believe auto-guiding can work on Alt/Az mounts. This will become my next project. In the meantime I use the Skywatcher scope on the LXD55 mount.

The Pi 3 of the guiding camera is strong enough for graphical login and running PHD2. So the guiding pictures do not need to be transmitted over WIFI anymore. The main camera has a Pi 4, runs KStars/EKOS, controls the mount and does the exposures which are stored on its SD card. With my laptop I remote login to both Pis and start everything. Once guiding and exposures are running I can disconnect my laptop. Running everything locally made a hughe improvement in stability.

An interesting alternative to an expensive telescope are old photo-lenses. I got one for free, bought an adapter in internet for small money and connected a Pi Zero:

This is pretty cool when you need shorter focal length. The focal length printed on the photo lense is calculated for the original film size. But the sensor in the HQ camera is much smaller. The full image size of the HQ camera is only a fraction of the image size you would get on the chemical film. Therefore the photo lense has 5...6 times higher focal length than printed on it. My zoom lense has an effective focal length of about 150 ... 450 mm.

The right back focus (distance between last lens and camera sensor) is very important when you use a photo lense. In most cases you will be able to focus the sky. But when you read something else than $\infty$ (infinity) on the focus ring, your back focus will be wrong. Than you will not get good focus over the whole image. Either you see stars as dots in the center and as ellipses in the outer regions of the image, or opposite. It is worth to spend time in adjusting the back focus: look on a region of the sky with many stars, set the focus ring on $\infty$ (infinity) and adjust the back focus ring on the HQ camera until the stars are focussed. Maybe you need additional distance rings between the lense and the HQ camera.

I am looking forward for the next nights with clear sky!