Chromatic aberrations or?

[whzzz28 (Nathan)]

Weather been terrible in Brisbane so thought id go through my crappy dust-ridden shots i last took with my new ED127.

I am seeing, what i think is chromatic aberrations. If it is CA then i will be rather disappointed given this is meant to be a triplet APO. Sure it is cheap, but id expect this only from an achromatic, even if it is a bad APO.
The stars are bloated as a result.

Googling has brought two possibilities, if it is not CA: atmospheric refraction (terrible seeing?) or a tilt between optical elements (scope stuffed, send it back... hope not).
Seeing on the first day i took some photos was shocking. It was so bad that i was watching the PHD tracking box twitch a few pixels up/down/side-to-side, so i disregarded most of that data from that day.

Here are a few pictures:
First picture, bottom left of an image. You can see the blue halo, but it doesn't cover the entire star. The orange colored star also has a nice blue halo on half of it and a slightly darker orange tinge on the other.

Second pic, bottom left of another image. Halo still visible but a spike in both images?

Third pic: This is a star from the middle of an image. Halo is rather uniform.

Fourth pic: Bright stars are giving halo's. This is on a DSLR, not sure if this is common on not, but i don't remember seeing any halo's on my ED80.

Fifth pic: Another halo, this star is not in the center, its more towards the bottom left of the picture. The halo is not centered on the star, which makes me wonder if it is an optical issue.


I've checked some of my other images as well, they all have CA on the stars but there does not seem to be a pattern. The CA only normally covers some of the star, half of it, at different areas on the outer regions of the image, in the center it is generally uniform.

Is this likely to be CA, or the seeing was horrible, or am i looking at an optical problem?

Other possible causes that i can think of:
- I had changed the white balance profile on my camera. It was set to AWB, but as it is modified i believed that it may be giving me false colors so i tried to setup a customer white balance by taking a photo of a uniform white piece of paper in direct sunlight. I have since removed this profile and set it back to auto, but haven't had a chance to try it yet (damn Brisbane weather). Possible that seeing was terrible, creating jitter during the exposures and the white balance profile was giving off too much blue?

- Might be normal; but i have a mount/tracking problem. I was using my lodestar for the first time and was having huge troubles getting it to 'stay' tracking. It was tracking really well... too well, so well that PHD complained often that the star didn't move. I thought that was because my polar alignment was spot on, but maybe something else is going on. Dec would often flatline (causing PHD to throw a beep, sometimes only once, sometimes twice). I had the best luck when using 0.5second exposures. 1 second or higher would often throw the beeps. Included screenshots showing this.
Maybe it was just tracking really well, or could it be my DEC axis is binding in my HEQ5? I did have some noticeable slop in my DEC axis so i tightened the worm gear to remove the play. I can turn the gears with my finger easy and the mount doesn't appear to complain about it.

- I am using a Hotech field flattener (not reducer) on it. Could this be inducing the problem? Sadly i never thought to try without the FF in place!

- Am i horribly over-sampling?

Some details:
Camera: Canon 1100d (modified)
Mount: HEQ5 Pro
Scope: Northgroup ED127, moonlight focuser.
FF: Hotech FF

Any suggestions are welcome.

[allan gould]

From a quick analysis.
All stars do not have a blue halo, some have yellow halos. This is the true colour of the stars and the centers are blown out to white with the DSLR chip but hard to state as you don't give the length of your exposures. But i would guess they are about 5 min duration.
Stars in the center have uniform images and halos but stars off center are off centered. This is most probably due to an unflat field ie your corrector is not doing its job. Not all correctors will work on all refractors irrespective of the manufacturers claims.
Your scope appears to handle the colour quite well but it would be better to see the whole image and then all 4 corners and a center of a star field with and without corrector. How are you focusing your scope? That can make a great difference to your image.
What was the altitude of the stars as that can intro Duce colour dispersion seen in some of your images.
The guiding looks ok and I think you need to relax a bit about your imaging.

[whzzz28 (Nathan)]

Hi Allan,

one of these images is with 120sec exposures, the other is 5min.

Focusing is through using the moonlight focusing program (can use ASCOM as well) and i use nebulosity's fine focus command, which puts the DSLR into live view and allows me to fine tune it.
If i remeber correctly the HFU was between 1.5-2.0 during my focusing tests (lower the better), which is the best i've gotten before. My moonlight does have the stepper motor already which allowed me to get these numbers. Normally i only get 2.5-3.0 with my Meade 80mm scope when manually focusing with the knobs.

Ill throw some pictures up later when i get home.
Maybe i am being harsh on myself but to me the stars just seem bloated to the point that it sticks out like a sore thumb.

Cheers.

[whzzz28 (Nathan)]

Pictures. M6 (cluster) is slightly cropped, the other two are as-is.

Leo trio seems to show diffraction spikes on bright stars? maybe pinching? but it doesn't seem to appear in any other shots, and it was the first thing i imaged (or tried to - tree got in the road).

Uploaded to my own host as they are large and IIS is giving me errors when i try to upload (invalid security token?).

http://core-au.net/astro/test

[naskies (Dave)]

Originally Posted by whzzz28

Maybe i am being harsh on myself but to me the stars just seem bloated to the point that it sticks out like a sore thumb.

Nathan,

It looks to me like sensor bloat due to overexposure in the bright star cores, leading to the electrons "overflowing" to the surrounding pixels - i.e. a camera problem, not a scope problem.

The bright white over-exposed cores, and retained star colours in the bloated area, are a bit of a give-away.

DSLRs are particularly susceptible to this, unfortunately. Dedicated CCDs have special anti-blooming features that substantially reduce the problem, and many imagers will also reduce the apparent size of stars during post-processing.

If you try very short exposures - e.g. 15 or 30 secs - I'm sure you'll see that the stars will retain their colours and not bloat.


Cheers,

Dave

[whzzz28 (Nathan)]

Originally Posted by naskies

Nathan,

It looks to me like sensor bloat due to overexposure in the bright star cores, leading to the electrons "overflowing" to the surrounding pixels - i.e. a camera problem, not a scope problem.

The bright white over-exposed cores, and retained star colours in the bloated area, are a bit of a give-away.

DSLRs are particularly susceptible to this, unfortunately. Dedicated CCDs have special anti-blooming features that substantially reduce the problem, and many imagers will also reduce the apparent size of stars during post-processing.

If you try very short exposures - e.g. 15 or 30 secs - I'm sure you'll see that the stars will retain their colours and not bloat.


Cheers,

Dave

Thanks Dave, i thought that might of been a problem. The scope being 1.5x the 80mm is getting much more light, thus 5min exposures on each will vary bloat.

That being said - initial tests of my TV RFL-4087 appear to be promising. Won't know for sure until i process it but stars seem much sharper in the raw ive got compared to what i posted above.

[Visionoz (Bill)]

Originally Posted by whzzz28

... I was using my lodestar for the first time and was having huge troubles getting it to 'stay' tracking. It was tracking really well... too well, so well that PHD complained often that the star didn't move. I thought that was because my polar alignment was spot on, but maybe something else is going on ...

- I am using a Hotech field flattener (not reducer) on it. Could this be inducing the problem? Sadly i never thought to try without the FF in place!

Just a thought Nathan

I also use a Lodestar and have noticed that the ccd is full of hot pixels which might have given you cause for grief IMO!

Also contact h0ughy (sure he is one of the mods on IIS as I've seen him "swing" his axe around on some posts !LOL!) because he uses a HoTech FF on his NG 127mm scope - I also use the same with no probs - note that the FF has a "fixed" ideal distance of 55mm to the DSLR's sensor (if you already know I do apologise for repeating that) and here's a document to read: http://www.hotechusa.com/v/vspfiles/...0sca%20ffl.pdf just in case it might prove useful

HTH
Cheers
Bill