Ghost stars in ST8-XME

[rogerg]

Hi all,

Does anyone have advice on dealing with ghosting in CCD images? My ST8-XME exhibits significant ghosting.

For example, I will take one 60 second exposure, slew to another object and take a second 60 second exposure. It is common for the residual image of a bright star in the first exposure to still be present after 3 subsequent exposures.

It's a real pain. I'm hoping others at least have the same problem and perhaps have a workable solution?

Thanks,
Roger.

[strongmanmike]

Hi Roger

Yes that is Residual Bulk Image or RBI and it is a problem with some chips including the KAF.

Modern cameras deal with it by flushing the CCD with an IR flash inside the camera before each exposure

Not sure there is much you can do about it other than warming the camera up and cooling it again and taking some dark frames before moving on..?

Mike

[gregbradley]

High cooling keeps the ghost image trapped for longer. Either try not to image a bright object just before an imaging run or warm it up and recool. It will eventually fade over more images.

Some sensors are more prone to it than others. I have only seen it on my 16803 chip twice and it was minor on both times.

FLI has an RBI annihilator which consists of an IR flush several times of programmable length before you take an image. The downside is it increases noise as that flush bleeds out slowly during the subsequent image. I think you have to take darks using the flush as well to match at a temp. So I have never used that feature and have not noticed any problem. Its more for the now unpopular KAF09000 chip which has it bad. I believe the KAF6303 has this problem as well at times. KAI series chips work differently and do not suffer from this effect. Its only KAF series sensors. Not sure how Sony sensors fare with this. I have only heard of it connected with Kodak chips.

High cooling here is an advantage as it reduces the rate of bleed from the flush during the subsequent light exposure.

As far as for your SBIG simply be aware of it and don't image something bright before an imaging run and do darks on a freshly started up camera not one after an imaging run which may still have ghost images in it.

Greg.

[rogerg]

Thanks for the replies guy's.

It seems yet another reason I should have chosen a different camera to the ST8 when upgrading from my ST7. The preflash sounds attractive.

I typically cool with the cooler power at 80-95%. I will do some testing to see what results are like if I drop it back to 60% or so.

I rarely photograph bright stars, I'm finding most of the ghosting is coming from stars fainter than magnitude 6 in these exposures of 60 seconds. It's surprised me just how faint a star can cause ghosting which lasts for at least one additional exposur.

I'm seeing the problem a lot in my supernova imaging. Time between the start of each exposure of a different object is approx 78 seconds (including slew etc - 60 sec exposures, 18 seconds slew and other delays).

I can pretty much guarantee every photograph has ghost stars, and considering I'm working at a narrow field of view on mag 10-15 galaxies you can imagine most fields don't have particularly bright stars. It's a shame the problem is so persistent even under these conditions.

[clive milne]

http://www.narrowbandimaging.com/images/rbi_nasa_tech_briefs_april2009.pdf

http://www.narrowbandimaging.com/images/RBI_presentation_crisp_7249-22_with_speaker_comments_paper.pdf

[PeterM]

Hi Roger,
My Starlight Xpress SXVH9 & SXVHR9 are perfect for the job, no RBI. Since the newer starlight cameras came out you occasionally see these at bargain prices especially the SXVH9. Maybe an add in the wanted section might flush one out. I do have a friend Jeremy (germ on IIS) who has a SXVH9 and he maybe willing to sell at a fair price. Just a thought.
Good luck.
Peter

[rogerg]

interesting thought but I'm unlikely to switch camera right now .... wait and see :)