The lowest cost field flattener / corrector that I could find for my SW 80ED refractor was the Starguider 2″ Field Flattener sold by Sky’s the Limit on ebay.co.uk. They have a £124 buy it now price currently. According to the seller (who is well-trusted in the UK) this is the same as the TS 2″ Field Flattener (TSFLAT2), although the latter (if you order directly from TS) comes at about £167 for the corrector only – adapters are extra.
So, I took a chance with the low cost Starguider one, which comes with adapters (so at a savings of well over £50 overall compared to TS) and should be good for f/5-f/8 refractors – with the f/7.5 of my 80ED falling into that range.
It comes with a T-adapter that adds 38mm to the light path and also a 15mm 2″ filter thread extension. If you use a regular T-ring with your DSLR, the sensor should lie at about 55mm from that, hence with the setup as is you get the sensor at 55+38+15 = 108mm from the Field Flattener thread. According to the TS listing the correct sensor to flattener distances are:
— Focal Length < 450mm: 128mm
— Focal Length 450-490mm: 123mm
— Focal Length 500-550mm: 118mm
— Focal Length 560-590mm: 116mm
— Focal Length 600-690mm: 113mm
— Focal Length 700-800mm: 111mm
— Focal Length from 800mm: 108mm
Hence, for a Focal length of around 800mm or more, you are all set. There is some tolerance (5% according to the instructions), so I guess you could try even a 700mm OTA like that.
But I have a 600mm 80ED, so I would need a little extra length. Luckily, the thread is the common 2″ filter thread (48mm), so my Orion Imaging Filter was just right for the job, adding 7mm to take it to 115mm. Pretty close. So here is the setup I used:
The UK weather of course did not come to my aid. Between the clouds I managed very few shots, but it was enough to be delighted with the results. In fact, before seeing the difference I did not consider my refractor as having that much coma, so now that I’ve “seen the light” I don’t think I will use it again without the corrector. Here are two comparison shots (50% resize), first without then with the flattener – at first glance you might not even realize it is the same starfield:
Although the much more pin-point stars are clear in the second photo, here is a 100% crop from about the middle of the 18MP Canon 550D frame to the left edge, showing about the same stars so you can clearly see the difference:
I would say over 90% of the coma is gone. I am not sure if adjusting the sensor distance by a mm or two would get even more pin-point stars at the edge. It is already pretty good, but perhaps I will try a small adjustment. It is not too hard to make small adjustments to the distance given that this is a 2″ filter thread, so things like a 2″ UV filter (which many people have to protect their optics from dust anyway) can be added or removed for these adjustments.
Overall, a very pleasant experience. Skywatcher ED owners do have the option of going for the matched reducer/corrector, but this one works as well. It does not reduce (I checked, it gives you just a marginally larger field – like if it was a 0.993x reducer), but it does a good job at flattening the field. And I guess it is even more relevant to owners of other refractors who don’t have the matched choice.