Resolved persistent residual charge?

I have owned my STT-8300M for about a year. All of my narrowband images, mostly Ha and OIII, have looked fine but I have had a band of illumination that takes up the central ~2/3 of the FPA that is particularly noticeable in one of more filters of any LRBG sequence. No matter what I have tried -- flat box, sky flats, nearby vertical wall in twilight, Alnitak flip flat -- I have been unable to produce good LRBG flats, where good is defined as producing an acceptably flat calibrated light frame. The problem has not been a show-stopper for several reasons: I have been focused on narrowband imaging, I am not yet trying to make "pretty" color pictures and, when I do an LRGB sequence, I have been imaging from dark (21.5 - 22.6 mag/arcsec**2) sites and typically a single filter per night.

What's changed...?

I added the AO-8T recently, which brought about all kinds of new challenges. My old methodology was to use Sequence Generator Pro and PHD2 for automation and guiding; however, those tools cannot take advantage of the AO unit, so I switched to using Maxim DL manually for a few nights in order to shake out the AO unit. In another thread, I just posted a very successful first light image taken in H-alpha.

I am now playing around with an evaluation copy of ACP in order to achieve the same level (or more!) of automation with MaximDL that I had with SGP. One of the things that I have been doing is running many auto-focus sequences just to exercise the software (ACP + Maxim DL) and hardware as well as to set up the various configuration files (e.g., filter focus offsets) in ACP. All of this recent activity under a bright (~17.4 mag/arcsec**2) sky.

A few mornings ago -- notably, after many turns of the filter wheel during the evening -- I noticed the following in all of my bias and dark frames, another 36 x 4-minute sequence taken as dark, bias, dark, bias, ..., taken immediately after a few hours of RGBL auto-focus and other calibration runs.

My filter wheel currently has seven filters and the eighth slot is completely empty. I am likely going to put a NII filter there as I am interested in planetary nebulae. FWIW, the order is: R, G, B, L, SII, Ha, OIII, empty.

Final notes: The humidity that evening was near 100%, outside temp was ~12°C, and I ran at -20°C. I have regenerated the desiccant plug tonight but have not yet had a chance yet to power the camera back up and take some darks and bias from inside in a dark and dry (~40% humidity) room.

Is it indeed persistent residual charge from the .940-um opto-sensor reaching the focal plane due to an empty filter slot, as described in the attached pdf? What else could it be, especially to show up persistently for ~2 hours in dark and bias frames?

Thanks for your help!

 

Ah, fascinating, I hadn't noticed that ACP was defaulting to readout with RBI pre-flash. That explains the "persistence" over two hours of dark and bias frames.

Thanks!

I will update this thread next time I have the camera powered up.

 

ACP has controls for readout mode, as well as ways of defaulting the readout mode for various types of frames. If you have questions feel free to post to the DC3 Dreams Communication Center.

 

Bob, I have only used ACP for a few nights but I am already very impressed with the configurability and scriptability -- I am still on the steep part of the learning curve but you've already earned another ACP customer.

 

I have set up the camera and scope and played around with some additional ACP configuration files and with Maxim DL. I have been able to confirm that the RBI pre-flash is the cause of the illuminated area / residual charge.

 

Do you mean that the pre-flash made things worse?

 

Bob, that is correct, RBI pre-flash was making things worse. RBI pre-flash was turned on in ACP, until I correctly configured RoModeInfo.txt. I will be performing some additional experiments as I am still surprised that it could be such a large effect.

 

Ok thanks. I learned something new!!

 

How does one know whether or not to use RBI pre-flash? I have a new STXL-16200 and a new STT-8300m. Should I use RBI pre-flash? If so, are the defaults of length of 4 and flush count of 3 ok?
I assume all lights, darks, bias, and flats should be taken the same way, either with, or without pre-flash?
On the STXL-16200 (haven't tried the STT-8300m yet), the Expose tab of the Camera Control box has "Readout Mode" settings of either "RAW" or "RAW w/ RBI pre-flash". If "RAW is selected, are the RBI length and flush counts used? Or are they only used if "RAW w/ RBI pre-flash" readout mode is used?

 

Thanks for the quick response Doug.
To turn RBI off is it sufficient to set the "Readout Mode" to RAW, or do the RBI lengths and flush counts need to be set to 0?
I am using Maxim 6.13