Building a STELLINA clone

If you are willing to operate a mount and do all the geeky things nerds do then you can build a Stellina replica for less than half of what Stellina costs...

To mimic Stellina’s design I employed my sky watcher Alt Az GTI mount  (I talk about this mount here). You can control the via wifi through an app on your phone (just like Stellina!) In alt az mode this mount is able to handle a Stellina sized 80mm aperture ed refractor.  YouTuber Astrofarsography very kindly lent me his Skywatcher Evostar 80ed scope. You can check out his YouTube channel here.

astrofarsography%20text_edited.png

Thing is the Skywatcher 80ed is too long. Or another way of putting it is that it is  too slow. In fact at f7.5 I’d say the Evostar 80ed is disappointingly slow (I think at F5.9 the skywatcher 72ed, or a Williams Optics 80mm is a better scope for deep space astrophotography). Stellina's scope is really fast. For the 80ed to catch up we need to reduce its focal length to about 500mm I used an old borg 0.7x super reducer. You can’t buy these anymore. I got mine for about £200 second had and used an adapter supplied by Beat from Switzerland (if you buy from him please tell him the biscuit sent you!) to allow the borg reducer to fit into a standard 2” focuser.

I think the Evostar 72ed is a better scope than the 80ed

The final part of the clone is the camera. I used ZWO’s asi178mc. This is a very fine lunar, solar and planetary camera which (as mentioned here) can also be used for deep space astrophotography. Oh and I nearly forgot. I employed an absolutely brilliant light pollution filter, the optolong L enhance. Turns out this light pollution filter is more aggressive than Stellina’s but hey ho, I had no way of knowing what Stellina was using beneath that futuristic looking plastic exterior.

optolong%20L%20enhance_edited.png

The optolong L eNhance filter performs as well as filters I own which cost 3 times the price.

How did Stellina clone compare?

Stellina

Stellina Clone

Compared with the real Stellina the Stellina clone is a faff to use. The main source of faff is the dark, flat and bias frames that you need to collect in order to remove the amp glow from the camera. Other sources of faff are that the relatively heavy and long Evostar ed80 telescope accentuated the backlash in the mount to the point where my software (APT) could not centre the rosette nebula in the frame… and because the nebula was so dim I failed  to centre the nebula manually. This problem is easily resolved by adjusting the backlash setting in the app. I set the backlash to 10minutes in both RA and DEC and the mount behaved very well after that. In fact it behaved better than Stellina, allowing me to shoot 20 second long subs. Stellina is programmed to shoot no more than 10 second long exposures. Stellina’s 10 second long exposures are fine for most purposes but in the city with an aggressive light pollution filter 20 second long exposures are much better. So despite being a faff if you get everything working properly the potential for longer exposures means the quality of the images you can get with this Stellina clone set up are better than those you can get with Stellina. The difference between the two set ups will be less noticeable in a dark site where longer exposures are less critical.

Improving the design 

The redesign of Stellina clone was based on increasing the set up’s maximum exposure length (and exposing Stellina’s 10 sec max exposure length weakness). To take longer exposures you don’t want your mount to be level with the ground you want it to be aligned with the rotation of the earth. Now this is a massive pain in the behind for a robot to automate but a geek like me can do it very accurately. using sharpcap software's polar alignment tool with some kind of adjustable wedge or a tripod .

skywatcher_star_adventurer_wedge_green_1

Skywatcher has a little adjustable wedge which isn't very precise but with a bit patience it does work

The Skywatcher AZ GTI mount doesn’t have a polar scope, so after an initial rough alignment - simply by angling the mount to 52 degrees( which is my latitude in London) and pointing it North - I then use sharpcap software's polar alignment tool along with an adjustable wedge or tripod of some kind to align the mount to the rotation of the earth to with an accuracy of less than 1 minute.

I like the smooth control knobs on my old mead deluxe field tripod but tbh do anything you like, just get the mount aligned to the rotation of the earth and you are good.

Turning the AZ GTI into an EQ mount (I talk about this a bit more here)  hasn’t really cost us anything but it’ll make a massive difference because now we will be able to take 60 second exposures. I cannot overstate this enough, 60 second unguided exposures is a big deal. The fantastic performance of theAZ GTI in eq mode is one of the most surprising discoveries I had whilst making this video. Its just a shame that for budgetary and focal length reasons I wasn’t able to pop a nice  little 72ed scope on this fantastic little goto eq mount. Instead I plumped for the very good (but not quite as good as an ed scope) and SUPER SONIC value 80mm achromatic refractor. Now the ST80 (mine is actually an old celeston. FYI don’t buy the new celestron. See here for which 80mm achro scope  to buy) is exactly the same fatness and length as stellina’s scope but it isn’t as good bc the cheaper glass can’t focus the red and the green and the blue light in exactly the same spot which results in your stars having blue halos.

To get around this problem I decided to try and use a filter wheel loaded with red, green and blue filters (link) and the  mono version of the asi178 camera.  The idea was to photograph just the red light through a red filter and then refocus and photograph the green light, refocus and photograph the blue light etc… In theory this would give us even more resolution than using a colour camera without filters because we would not be limited by the colour camera’s bayer matrix.

affordable_LRGB_dichroic_filters_edited.
five-position-filter-wheel-details_edite

You don't need to spend a lot on RGB filters. Better to save your money for the narrow band filters!

In order to achieve the precision focusing required to focus each channel seperately I unscrewed the focus knob and screwed it back in with  a massive jar lid wedged between the focus knob and the screw.

178mm_edited.png

ZWO ASI178mm - a great planetary and deep space camera

Now this idea worked perfectly for the red and the green channels. The problem comes with the blue channel. I discovered the really high frequency, almost ultraviolet blue wavelengths are not controlled at all by the cheap glass and the blue filter by itself is not enough. So I employed the baader fringe killer which blocks the highest frequency blue wavelengths. Now the blue channel produces fairly tight stars but because a chunk of the blue wavelengths are  blocked by the baader fringe killer filter the blue channel is now dimmer than the red and the green channel. To make the blue channel as strong as the red and the green channels I doubled the blue channels exposure length. Which was ok bc the mount can handle up to 1 minute long exposures.  (Note: If I’d have used an ed scope I wouldn’t have needed the fringe killer filter and the blue channel would have worked just as well as the red and the green).

baader%20fringe%20killer_edited.png

These cheap achro telescopes are generally faster than their more expensive apo cousins and this allows the achros to outshine the apos when it comes to narrow band imaging. I actually use achros for narrowband imaging from my roof in London. When I started I never dreamt such images were possible from the big smoke. This was taken with a skywatcher st120 and my borg 0.7x super reducer. Its as good as any shot taken through a telescope that cost 10 times the price.

Narrow band imaging relies on the fact that clouds of gas in space glow in very specific wavelengths. Hydrogen for instance emits 656nm wavelength photons. Narrow band filters work by blocking all light apart from the specific wavelength that the gas emit. In doing so they block out roughly 99% of London’s light pollution, and 99% of star light too which makes them very dark. In order to image through these dark filters you need long exposures.  The second amazing discovery from making this film was that the uncooled 178mm camera is sensitive enough to work with a narrow band Ha filter with just a 60 second long exposure. Normally narrow band astrophotography relies on even longer exposure lengths (I often shoot 1hour long exposures with my narrowband rig!)

Of course the filters  for narrow band astrophotography aren’t cheap. I recommend the astronomic 6nm filter. The 12nm filter is cheaper and would work well if you live in a dark skies location. There are even cheaper narrow band filters but many  cheaper filters will produce annoying halos around bright stars. ( Note; if I had to buy cheaper narrowband filters I would choose the optolong filters bc the optolong LeNhance filter is so good but I’ve not tested optolong narrowband filters yet).

Conclusion

 I reckon this roboscope inspired set up has potential to make a fantastic travel rig. Key to its brilliance is the sensitive small pixeled camera paired with a fast small refractor ( I would combine it with a small ed refractor like this one rather than the achro refractor I ended up using to save money). BTW having a fast refractor is key. Only fast refractors can make good use of the small pixels. The icing on the cake of this rig is the mono version of the asi178 camera. Mono cameras allow you to shoot Ha (hydrogen alpha) in the city and luminance in a dark site. A luminance channel gathers 3 times more light than shooting with a colour camera so although using filters is a faff, mono cameras do rock! I think this idea is going to be the subject of a future video. Perhaps when lockdown ends I can take a rig like this somewhere dark.

 
ak_ha-ccd_12nm_1_edited.png