I think the Evostar 72ed is probably the best value scope for this rig. Although tbh any small apo refractor would do it.

Don't be fooled by the size of these little beauties, as you'll see they're gonna pack quite a punch.

OPTIONAL (but recommended)

0.85x Reducer/Flattener and adapter for 72ED PRO

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The Mount

Skywatcher AZ-GTI Summary

PROs

• Cheap

• Surprisingly good at tracking especially in eq mode (less than 2 arc seconds wobble when guiding).

• Super Portable

• Has GOTO function via phone app

• Can connect to a computer via wifi allowing the use of serious and powerful astrophotography software like Nina

CONS

• Is intended as an ALT AZ Mount. In order to make it work in EQ mode you need to install firmware and cobble together a suitable mount which can be polar aligned. Brief description here

• No polarscope

• Need to acquire counterwieght and counterweight shaft.

• Complicated

• Can only carry small scopes

The Camera

The final and most important part is the camera. I reccomend the ZWO’s asi178mc or asi178mm. This is a very fine lunar, solar and planetary camera (see here) can also be used for deep space astrophotography. To be honest the credit for choosing this camera really goes to the roboscope Stellina  although I had run a few tests on the imx178 sensor a few years before. The colour version is fantastic  but for extra flexibility and performance you'll need the mono version with zwo's mini filter wheel (see here).

Bodes Nebula and the Cigar Galaxy shot with a sony imx178 sensor in a point grey camera back in 2016 through an old televue NP101

The tiny  2.4um pixels in the imx 178 sensor reveal a tremendous amount of detail. This is key for the travel scope design because it means that you can shoot through a small telescope and still get tremendous detail - enough detail to reveal galaxies 30million light years away. Amazing isn't it.  

Evolution of theTravel Rig

In February 2021 the biscuit man decided to take on the awesome but expensive robotic scope known as Stellina. You can watch the video of this astrobiscuit adventure here. I wanted to replicate Stellina's power without the 3 grand price tag. To match Stellina's 80ed optics I borrowed youtuber Astrofarsography's skywatcher 80ed telescope. I mimicked  Stellina's  alt-az fork mount with my Skywatcher AZ-GTI  (in the standard alt-az mode) and I borrowed from FLO a camera with the same Sony imx178 sensor that I though  Stellina was hiding under her bonnet (the ZWO asi178mc)

VS

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

As we were in lockdown the test was done from light polluted London which was a bit annoying but there you go. Stellina has an inbuilt light pollution filter. To match her I plumped for an Optolong L extreme.

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  At f5 Stellina's scope is really fast and gathers light really quickly. For the Evostar 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 hand and to make the reducer fit into the scope I needed  an adapter supplied by Beat from Switzerland (if you buy from him please tell him the biscuit sent you!) 

How did Stellina clone compare?

Stellina

Stellina Clone

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 was too much for the AZ-GTI mount. Overloading the mount accentuated the backlash 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 improving the accuracy of the mounts tracking and at the same time allowing  us to take longer exposures without having to worry about field rotation. To make the mount more accurate you need the mount to be aligned with the rotation of the earth. Now this is a massive pain in the behind to automate which is why Stellina doesn't do it but a geek like me can do it very accurately using sharpcap software's polar alignment tool and some kind of adjustable wedge or a tripod .

Note: Sony's imx 178 sensor handles short exposures very well. That's one of the reasons Stellina uses the sensor. Its a match made in heaven because most alt az mounts are a bit hit and miss.  If you shoot a 100 second long exposure with one of these mounts chances are you'll see that your stars are a bit blurry due to the mount not tracking accurately. Stellina gets around this by taking 10 x10 second exposures instead. One or two of these exposures will have been shot when the mount isn't tracking accurately. Stellina throws these exposures away. Leaving just the exposures where the stars were sharp.

Of course you can use one of the commercial wedges mentioned above but really anything that tilts the mount accuratey will do. I like the smooth control knobs on my old mead deluxe field tripod.

The read noise on the imx 178 is low so stacking lots of short exposure shots isn't too much of a handicap. All the same it would be better to have a more accurate mount, that way a higher percentage of shots would be sharper so you wouldn't have to throw so many away and you could take longer exposures if you wished. If you are using a high read noise camera like a dslr or shooting through light pollution filters longer exposures (like 60s) will produce much better results. I would argue (although some disagree) that even with a low read noise sensor like the imx178 and no light pollution filters upping the exposure length from 10 to 30 seconds makes a big difference.

Turning the AZ GTI into an EQ mount (I talk about this a bit more here)  hasn’t really cost us anything but now the mount has the potential to take 60 second long unguided  exposures. I cannot overstate this enough, 60 second unguided exposures with a little mount like this 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 reasons in the film 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  and very budgety 80mm achromatic refractor. Now the ST80 (mine is actually an old celeston. 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, blue and Ha filters 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  because we would not be limited by the colour camera’s bayer matrix.

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

This shot was taken with the cheap achro telescope on the travel rig from my roof in London. The accurate 60 second narrowband Ha filter shots are what makes this shot stand out.

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.

Now this idea worked perfectly for the red and the green and the Ha 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. One way round this is to cut out the highest frequency blue wavelengths with a baader fringe killer filter... but this solution isn't ideal.

 For the ultimate travel scope rather than settling for a cheap achro your much better off with a small, quality, apo scope like my tak or the Skywatcher 72ED. And if you're really geeky and want the highest resolution, the best Signal to Noise ratio and the ability to shoot in Ha then shoot in mono with a dinky little filter wheel like the ZWO efw mini.

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.

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