Update…
Managed to do a quick test last night.
Flipped the filter over so that the filter glass was now about 3mm further back from the rear lens element.
Attached pic shows the effects of the filter in its 2 different positions.
Left hand pic – filter closer to lens
Right hand pic – filter further from lens
The vignetting/reflection effect seems to be less defined and demarcated in the right hand pic – looks more like a vignetting effect that could be dealt with by flats.
I’m too lazy for flats though!!!
That's a very big difference in the observed effect.
Don't you already use one for your modified 40D? You should be.
The simple way is to put in the filter and take an image of white sheet of paper under direct sunlight. Set that image as the CWB. The camera will do the maths between that image and the image your taking. The real way is to get a gray sheet from a photo shop and do the same.
I'd like to say it works like a flat but I don't know enough about the workings inside of a canon camera to say that and the guys at canon are very reluctant to tell you about it.
Don't you already use one for your modified 40D? You should be.
The simple way is to put in the filter and take an image of white sheet of paper under direct sunlight. Set that image as the CWB. The camera will do the maths between that image and the image your taking. The real way is to get a gray sheet from a photo shop and do the same.
I'd like to say it works like a flat but I don't know enough about the workings inside of a canon camera to say that and the guys at canon are very reluctant to tell you about it.
Got it!
Yeah Robin I use CWBs regularly for daytime stuff - I'll try that.
I use CWB for astro shoots as well with a modified camera else your white balance will be set to the cameras default unmodified setting. However with astro shoots remember that most stacking software can be set to ignore the cameras CWB so make sure it is using the cameras white balance.
On a side note, I notice there seems to be no option in maxim to ignore the cameras WB. Here is a shot processed in maxim after someone reset my CWB to a normal daytime image. This had me stumped for ages.
Took this really rough flat tonight to see if the vignetting was just that or an internal reflection.
To my surprise, I got the image below (stretched to highlight)...
Looks like I may be able to sort this out with decent flats perhaps?
Doug ()
ps...now I need a decent step by step for taking good T-Shirt Flats
Your light box looks just what I need to standardize the flat taking process.
So, you get a number of bright, white images with the histogram roughly 1/3 from the left - what next?
Use them as-is in calibration software? (ImagesPlus in my case)
Sorry to be a dumbass - I've childishly avoided flats
I screw around with exposure time to get the histogram away from the left hand side and then runoff a 1/2 dozen images. The scope has already been focused on a star. Made a big difference to post processing for me when I was using the 8" dob.
That's unfortunate. What lens was giving the trouble?
Canon 70-200mm f4L - my main imaging lens unfortunately!
As you can see, the rear element is pretty reflective!!!
Cheers for all your advice Robin
Doug
No problem. I managed to get all my stuff setup tonight and I have a new counterweight arm camera mount and a junk 90-300 lens to try out later on. I'm wondering if I'll have the same trouble with this front filter.