Hello! I notice from time to time that B-filtered star images get light diffusion halo around them as I get down to about 35 degrees while my I and V images are not affected. At least, I do not notice any equivalent star bloat till I get to 30 degrees and under. I am in southern New Mexico, about 4000 feet elevation, and relatively dry conditions.
For example, as I follow the short period variable SZ HYA (taking images every two minutes in a different filter), B filtered images get a diffuse halo around them as altitude goes below about 36 degrees while I and V filtered images do not show anything down to about 30 to 32 degrees degrees.
The manual says to stay above 30 degrees.
Does anyone else find that B-filtered images start to halo at altitudes above 30 degrees? Best regards.
Mike
Hi Mike,
Can you attach an example of a good image and a haloed image? I'm not sure exactly what you are describing, so there could be multiple answers. Thanks!
Arne
Thanks! I did not save the B-filtered images from the recent runs. I'll save them from a run in the next couple of days and upload them. Best regards.
Mike
Hello! Here are a images from SZ HYA taken a couple of days ago in a Dropbox folder. There are tracking problems, which are more common as my LX200 points to the horizon. I'll try to get examples with better tracking. But, it shows haze around the stars in the B image the do not show up in the I and V images. At times, this light diffusion halo is much more significant.
Perhaps high cirrus clouds that scatter more blue light than other wave lengths at low altitudes? Best regards.
https://www.dropbox.com/sh/27wu3383oxnxu2b/AAAEYrxEnyH3H4WcQ7Ujk0Ifa?dl=0
Mike
Thanks, Mike! I'll look at these images later today.
Arne
The flattened image looks like aberration (chromatic?) Refocus could help. Going through focus may bring it to focus, then out of focus with vertically flattened images.
Also, if you are using a mirror, it might flop, out of focus. You may not see it in more in B than the V or red filters if the filters don't all change the F.L. by the same amount.
Ray
Thank you for your note.
I have an 8" Meade LX200. Mirror flop (of some sort) is a problem with my fork. At least, when the scope slews from overhead to the horizon, focus can change so that the stars become noticeably bloated, and when I slew from a target I've finished imaging in the western horizon to one on the eastern horizon, there can be a significant focus shift.
There is a small amount of focus change between filters as well, and I compensate for this with focus position change in SGPro.
Could dust on the front corrector plate cause light scattering at low altitudes in different amounts for different filters? However, the effect seems to vary between imaging runs. It is a dusty environment here in southern New Mexico, so perhaps a combination of air dust, corrector plate dust, and mirror shift? Thank you and best regards.
Mike
I was able to look at the BVI imageset you placed on your dropbox. Of course, V and I are trailed, and so I can't compare the star profile of those images with the B image. At airmass 1.7, you shouldn't be seeing any horizon effects like dust or distant cirrus. If at airmass 1.7, you are having focus shift and the stars are becoming bloated, then I would correct the focus change problem before worrying too much about anything else. I would also be sure that you have correctly calculated the focus offset between the B,V,I filters. B will often show a bigger offset, and the seeing is a function of wavelength, so that B is almost always poorer fwhm than other filters.
If you are able to get a two-image set some night, with B near the zenith and at higher airmass, showing the bloating, then I can examine them a bit more.
Arne