Return of the Mak

New scope alert! I’ve purchased a secondhand Maksutov Cassegrain...and my wife’s gonna kill me. 

My first scope was an ETX-90, purchased from a photography store in Bath’s Podium Shopping Centre - now long gone and overtaken by Waitrose. 

The ETX-90, along with ETX-70, 80 and 127 are all Maksutov-Cassesgrain scopes, fork-mounted into a GOTO base. They are a great all in one starter scope and are small, portable and battery operated. With minimal aperture this type of telescope delivers good magnification due to the folded optics. It’s focal length is 1250mm at f/13.8. I enjoyed mine for a few years before selling it and buying a small refractor. Doing so meant I gave up the ability to view the planets (even if very small) in favour of wider star field views. 

My current large scope is an 8” Ritchey Chretian astrograph, which I have used for visual use but it’s not optimised for this purpose. This has a focal length of 1624mm at f/8. Compared to my small refractor’s 430mm at f/6 this is a vast improvement for high power use on small targets, but still not ideal for visual observation.

My observatory was built with deliberately high walls to keep out stray city light, but this limits my lowest altitude to around 35deg above the horizon when due south. Often the planets and moon sit below this level so they can’t be seen from the BLT. As such I’ve done little lunar or planetary observing for a while and have missed it. 

My external pier has better views at lower altitudes but is not strong enough to support the weight of the 8” scope. However, a smaller, long focal length scope would fill this void; light enough to mount on the pier where much lower altitudes can be observed. This new Mak gives me a planets & moon visual scope. 

OMC-140 deluxe 

Let’s check out the stats: 140mm aperture (or 5.5” in old money) with a whoppping 2000mm focal length. It could be used for photography but is ‘slow’ at f/14. The catadioptric light path uses a lens and two mirrors to pack high focal length with minimal aperture, folding the light a number of times within the tube. The 2000mm fl is greater than my 8” RC.

The scope has a 1.25” eyepiece holder and a single rotating dial for focussing. It came with a 50mm finderscope which I’ll probably swap out for a red dot finder as I find these easier to use. It has a vixen dovetail bar across the bottom edge for mounting. The OTA is carbon fibre making it light and even suitable for taking away on weekends. 

First Light

I set the scope up on a manual alt/azi mount in my living room, added a William Optics 45deg prism and 17mm eyepiece. I was testing focusing on external object when I spotted....stars. Impossible I thought, given that new equipment normally means 2 weeks of solid downpours. I set up quickly outside and there were indeed star twinkling out through gaps in the clouds. Accepting that the Mak wouldn’t be cooled down yet I still wanted to give it a spin. 

I ran through a few objects including Venus, M42, M35, M44 and M45 to see how familiar objects appeared and fitted (or not, for the larger objects) in the eyepiece. I had to swap the finder scope from one side to the other (holes are provided on both sides of the tube) due to the way it is side mounted on the alt/azi mount. 

So far I’m really pleased I made the jump and am looking forward to seeing how I can make use of my visual colour filters to explore the Moon and planets. I will also investigate camera setups as I have never had the focal length to do high frame rate capture for planetary imaging. There’s going to be a good apparition of Mars later this year from UK skies so that will be what I’ll work towards. 

Camera Test

Second night, there was a crescent moon hanging onto the western horizon when I got home from work. There was hazy cloud, so the moon was certainly not crisp. Still, I thought it would be a good opportunity to stick in a camera and test out how it will focus (if at all). I found I could very quickly achieve focus and grabbed a few snaps to illustrate the focal length. These small B&W images were taken through my guide camera an ZWO ASI 120 mini which has a small sensor and pretty low resolution. These were also taken from a static alt/azi mount, not tracked, so will never be perfectly sharp capturing a moving target. However, it gives me confidence that with a higher resolution camera on a tracking mount, some imaging of lunar detail will be possible. This will be good practice for the planets later in the year.

First Impressions

Pros

• longest focal length of all my scopes

• Perfect for hunting planets and exploring the lunar craters

• light enough to use on my pier with a mid- low range mount head

• also fits on my manual alt/azi mount for Grab n’ Go 

• Can use for high frame rate photography 

• Transportable 

• Looks cool (like my RC’s little brother)

Cons

• as these are sealed scopes they famously need a long cool down time (c. 1hr anecdotally)

• Long focal lengths require good seeing and steady mounts

• 1.25” back limits which accessories I can use

• it would benefit from a top vixen bar or handle for easy of carrying 

Camera Test 2

Wow. This was a great decision!

Planetary Imaging

Oct 2020: I’ve been getting more confident with planetary imaging too although taming the long focal length is part of the challenge.