Analyzing tilt and curvature in Focusmax
Posted 21 March 2014 - 05:30 AM
I've just written a small article about collimation, tilt and curvature issues. Over the years I've had a lot of headaches, and I would like to share this with you, if that helps somehow.
Any questions, comments are welcome.
Posted 21 March 2014 - 07:10 PM
Thank you for this. At the moment I am at the edge of selling all my photography gear due to funny shaped stars in the corners of my images.
I will check out focusmax. Thank you
You mention that tilt can also be adjusted by collimation? Is this by collimation of the primary or secondary on a Newtonian?
Posted 22 March 2014 - 02:52 AM
Tilt can be corrected if its small enough by secondary collimation.
Posted 22 March 2014 - 06:36 AM
My tilt is not too big, but still enough to ruin a big enough part of my image, especially when the focus lies on rich starfields. Its just about 15 -12 percent towards the outer top left corner that is elongated and funny looking.
If you do not mind, I would like to discuss your first steps that you give in a bit more detail.
This is what you give on your page:
1) Collimate with a wide field eyepiece, at low power. Then use a high power eyepiece, about 50x per inch. A last round with more magnification to leave everything perfect is better.
Is this collimation done by moving the primary or secondary? I assume it is a star test you are talking about, so then you are meaning to adjust collimation by the primary mirror? (In a newtonian) In your previous post you specified the secondary mirror?
Further, I suspect that you analyze this tilt with a star at the center, then taking the star to each of the four corners? How do you allow for coma?
And this is all done near the zenith?
2) Slew to other stars. Does collimation hold?.
In other words, move to other parts in the sky?
3) Measure optical defects that will impact image:
- Turned down edge
- Spherical aberration (1/4 for deep sky imaging is ok)
I will try to find more info of the above to get a better idea on how to evaluate it. But in my experience and limited knowledge, there is not much you can do about these as they are problems caused by the primary surface. Not sure though.
4) Put the same weight in the focuser as the whole image train. Does collimation hold?.
I am guessing this is to eliminate focuser sag to a good extent? Should this also be done at the Zenith?
Posted 22 March 2014 - 06:39 AM
Is this collimation of the secondary done by first adjusting secondary tilt, or is it more of a secondary rotation and offset issue?
Posted 22 March 2014 - 09:33 AM
I think this is a good approach because you are actually measuring the surface of best focus - which can't be determined by analyzing a single image. You could have a perfectly flat field, and still have the stars get larger as you move away from the center - and your method would correctly measure it as a flat field. Your method is also less affected by seeing and guiding.
You could take it a step further and actually calculate the field curvature as the radius of the sphere going through the best focus surface. This would be a number independent of the detector size and would capture the inherent field curvature of the optics. You would have to correct for the motion of the primary mirror if you use the primary focuser, however.
Posted 24 March 2014 - 04:01 AM
You are focusing using FWHM, even if the star is comatic, it will hit the best focus no matter what.
Yes, it's done at zenith from the start, the point where gravity does not affect. Then, you point to other parts of the sky to see if gravity is ruining your optical train.
If you suffer from TDE, SA or Astigmatism, your image will suffer. You are looking for the best star profile. If you have just bought an instrument, analyze it and return it to the manufacturer if it does not fit what's supposed to be. Astigmatism can also be a problem with your focuser alignment.
Putting the focuser at the zenith only guarantees that it will not slip under load. Try to put weight looking another part of the sky to test for sag. Sag should be near zero.
Posted 26 March 2014 - 08:51 PM
Posted 31 March 2014 - 02:42 AM
This is a straight forward technique, thank you for sharing! Have you compared the results to those given by CCD Inspector? Do you focus multiple times in the center and each corner to come up with an average number?
CCD Inspector uses a quite different technique, it tells you where your FWHM is higher, but does not give info if it's negative or positive focus shift. I have had more sucess with Focusmax, but I've to admit that it's a tedious process.
If you have to average, then the focus process it's not repeteable. It's important to have sucess. Wait for cool down, use a night with good seeing, or calibrate as good as you can your Focusmax!.
Posted 31 March 2014 - 04:10 AM
Posted 31 March 2014 - 09:19 AM
I have been using a couple techniques you mentioned. On the Bhatinov mask, I learned you can do the same star in each corner focus and check if it is OK, but remember to rotate the mask 90 Degrees in each position.
I have been pretty much relying on CCDInspector though. I have put up a series of images from my efforts:
One key with this (or any easy pitfall to fall into) is to test your collimation in a field in the Milky Way with a lot of stars (>2000). results with fewer stars are not reproducible.
Anyway, the Tak is a bit special insofar as the secondary is off-axis (not centered) and requires a special mask to do real-time collimation with CCDInspector. I am awaiting the mask.
I know one thing, my results will vary over an evening with the seeing and with a wide-angle scope like the Tak any atmospherics seem to show up.
In any case, I have FocusMax installed and am awaiting the adapter for RoboFocus so I guess I can try your technique also.
Thanks for the post.
Posted 31 March 2014 - 02:17 PM
Collimation should be done with eyepieces only, your eye is king here. If your focuser is orthogonal, free of play and does hold collimation, you are done. No need for CCD miscollimation