Hi there,
during the last two weeks I put together a mobile rig to work with some students from my wifes school. This setup should be used to explain the basic steps of doing astro photography and hopefully attract some young people for the beauty of it
. So I bought a Star Adventurer GTI which feels great for this simple job. The plan was to use my good old Canon EF 300mm f/4 L lens as telescope and pair it with either my QHY 183c or my rarely used Explore Scientific ATR3CMOS16000KPA camera. I own a ZWO ASI T2 to Canon adapter version 1 for years now, but I never used it. However, the building process went really well and I set up a box with power supply (battery powered as well as mains power if needed), Laptop and all other stuff needed to image out in the field. The idea was, to put the mount with the telescope on the ground, place the box beneath it, connect a prebuild wiring harness and this way we are ready to go as fast as possible. I tested everything during the last two nights. Everything works great with one exception.
From experience I know, that the backfocus of the EF-mount is 44mm. This was prooved by a quick web search. So my assumption was to put the astro camera sensor at the same distance. I measured the length of the ajustable adapter to 26.5mm. Together with the 17.5mm backfocus both astro cameras have, I get 44mm (if i calculated correctly). At 300mm focal length both cameras should be undersampled. The QHY183c more than the ATR3 Cmos. So I expected very sharp looking stars. I use a ZWO EAF with a belt to connect to the lenses focus ring. This seems to work really well and the autofocus graph looks very symetrical. Nothing seems to be wrong.
While imaging during both nights (one with the QHY183c and one with the ES ATR3 camera) I realized, that no matter what I do I can't focus to get these really sharp stars I was expecting to see with my undersampling. With the 10 seconds exposure times for auto focus, they look sharper of course than the 3 minute and 5 minute subs show. But this is to be expected. All in all, every image is really soft. There were several autofocus runs and the HFR changes a bit after each run, but all looks quite normal as on every other setup I use. The guiding performance was not the best I've seen, but in average I got around 1 arcsecond error. This should be ok for the small focal length. The error was around 0,2 pixels, which is a no brainer, I think. The images show washed out stars across the whole field. So backfocus should be ok so far? Here is a random frame taken with the ATR3 cmos camera:
So I am not able to improve the sharpness from this point on. Do you have any idea why? Could this be a backfocus issue? But then I do assume elongated stars especially in the edges of the frame, don't I? Do you have any further ideas? It may not be the perfect system, but it should look significantly better than this.
This is so bad. It should be the easiest task and I failed. But that's how life goes, isn't it?
Thank you for your thoughts!
CS
Christian
during the last two weeks I put together a mobile rig to work with some students from my wifes school. This setup should be used to explain the basic steps of doing astro photography and hopefully attract some young people for the beauty of it
. So I bought a Star Adventurer GTI which feels great for this simple job. The plan was to use my good old Canon EF 300mm f/4 L lens as telescope and pair it with either my QHY 183c or my rarely used Explore Scientific ATR3CMOS16000KPA camera. I own a ZWO ASI T2 to Canon adapter version 1 for years now, but I never used it. However, the building process went really well and I set up a box with power supply (battery powered as well as mains power if needed), Laptop and all other stuff needed to image out in the field. The idea was, to put the mount with the telescope on the ground, place the box beneath it, connect a prebuild wiring harness and this way we are ready to go as fast as possible. I tested everything during the last two nights. Everything works great with one exception.From experience I know, that the backfocus of the EF-mount is 44mm. This was prooved by a quick web search. So my assumption was to put the astro camera sensor at the same distance. I measured the length of the ajustable adapter to 26.5mm. Together with the 17.5mm backfocus both astro cameras have, I get 44mm (if i calculated correctly). At 300mm focal length both cameras should be undersampled. The QHY183c more than the ATR3 Cmos. So I expected very sharp looking stars. I use a ZWO EAF with a belt to connect to the lenses focus ring. This seems to work really well and the autofocus graph looks very symetrical. Nothing seems to be wrong.
While imaging during both nights (one with the QHY183c and one with the ES ATR3 camera) I realized, that no matter what I do I can't focus to get these really sharp stars I was expecting to see with my undersampling. With the 10 seconds exposure times for auto focus, they look sharper of course than the 3 minute and 5 minute subs show. But this is to be expected. All in all, every image is really soft. There were several autofocus runs and the HFR changes a bit after each run, but all looks quite normal as on every other setup I use. The guiding performance was not the best I've seen, but in average I got around 1 arcsecond error. This should be ok for the small focal length. The error was around 0,2 pixels, which is a no brainer, I think. The images show washed out stars across the whole field. So backfocus should be ok so far? Here is a random frame taken with the ATR3 cmos camera:
So I am not able to improve the sharpness from this point on. Do you have any idea why? Could this be a backfocus issue? But then I do assume elongated stars especially in the edges of the frame, don't I? Do you have any further ideas? It may not be the perfect system, but it should look significantly better than this.
This is so bad. It should be the easiest task and I failed. But that's how life goes, isn't it?
Thank you for your thoughts!
CS
Christian