Field Flattener and ED80

[luka]

Hi

What performance should I expect from a field flattener? Perfectly round stars?

I recently got a Hotech SCA field flattener for my ED80 + Canon 1200D. I thought I would get perfectly round stars in corners but this does not seem to be the case (see the croped corner image below). It is MUCH better than just ED80 but not perfect.

Now, I have noticed that my T-ring is 10mm "thick" and with Canon's 44mm flange-to-sensor distance I have 54mm total distance to the flattener. This is 1mm shorter than the recommended 55mm. Could this 1mm be the reason for non-perfect stars? Hotech does not give a range for the filter but only specifies it as 55mm.

Thanks
Luka

[Atmos]

That 1mm shouldn't really make a difference. The picture crop hasn't been attached, without seeing it I am wondering if it could be slop in the focuser causing the problem. Is it the stock focuser on the ED80?

[luka]

Thank you for a reply Colin.

Crop attached this time. I forgot to do it before. Ignore the noise and fluff on the sensor :-)

It is only the crop of the bottom right corner, is it worth putting pictures of all 4 corners? The stars are always elongated towards the centre, like without the flattener but it is much better now.

It is a stock ED80 focuser.

[Atmos]

Is it to the same extent in all four corners? I am also wondering whether there may potentially be extra distance in any threads or parts? Someone that's connected a DSLR to a flattener may be of more help than I :lol:

[janoskiss]

That does not look bad at all for the very extreme edge of field. It's not unreasonable that you should need to crop your frames some if you want pinpoint stars with a relatively inexpensive setup. How do the stars look at "12 o'clock" edge of a landscape frame?

[luka]

Colin, the extra distance in threads would only increase the distance closer to the required 55mm. But I am sure everything was as tight as it gets, I am always worried that something will fall off and hit the ground :-)

Steve, at the top (12 o'clock) the stars are elongated but less than in corners.
I am guessing less distance from the center = less elongation than in the corners?

Thank you for your replies.

[troypiggo]

I'd recommend playing with the spacing. The theoretical distances don't always agree with testing/trial and error results. I found that with my combination of flattener, scope, camera, I needed 10mm more than what the theoretical distances were calculated to be needed.

[janoskiss]

Yes, that's a given. But I'm more curious about how the distortion depends on distance from centre of the image. You probably find a central region that is virtually free from aberration.

From the image you've posted, the star elongation looks like something that could be tamed in processing. I don't know what off-the-shelf software that could do that, but I've seen a lot worse aberrations in consumer DSLR lenses that the manufacturer's software is able correct.

[luka]

It may be possible to fix this in software but I thought that the flattener is supposed to correct this. But I am not sure what amount of correction to expect from it. Are "perfect stars" in corners is just a marketing gimmick or if I am doing something wrong, like the missing 1mm in the distance between the flattener and the sensor.

[janoskiss]

You won't get perfect results till you spend a lot of money, perhaps ~10x the cost of what you're using now. (Note: "perfect" = limited only by the fundamental laws of physics). If you were promised "perfect stars in corners": yes it very much sounds like marketing hype. Re the 1mm mismatch: experimenting with the placement of the optics is worth doing even if only to learn how sensitive the setup is to that.

Bert (avandonk) knows a lot about these scopes and how to make the most of them. If he does not read & respond to this thread then it'd be worth browsing his old posts and maybe firing off a PM to him for further info. Bert's brain is definitely worth picking on this topic. (Sorry to sort of speak on your behalf Bert, but I've seen you do amazing things with Synta/SW ED80s.)

PS. Re your other thread about switching to a Newt: a fast 8" Newtonian would be trickier still to make near perfect off axis.

[luka]

I dug through the pile of "spare parts" and found another Canon T-ring. This one is 11mm thick but the thread size is 48mm and the flattener takes 42mm. Clearly not all rings are made equal. Using 11mm would give me exactly the needed 55mm distance.

I also found two Nikon rings (48mm and 42mm threads) and they were both about 8.5mm thick. For Nikon we have 46.5mm + 8.5mm = 55mm. Spot on as required by the flattener.

I will try my Nikon D90 once the weather clears up. It has been cloudy for days and the forecast does not look good at all :mad2: