Flats and Offset Question

[GaryS]

Hi All:

I captured my first Flats yesterday and noticed that the various scope coverings I tested each created a Flat with a different 'offset'.

So first, what is 'offset'?

Second, offsets with the L-eNhance filter in place ranged from 1.039 - 1.051% while offsets with no filter in place ranged from 0.44 - 2.17%.

Can someone help me understand what the offset is all about or provide a reference or 'link' to somewhere that presents an explanation of the offset concept. If there is a 'typical' or 'ideal' range that offsets should have, it would be great to know what these are.

Thank you!
Gary

[turfpit]

Gary

What camera are you using?

Dave

[GaryS]

Hi Dave:
I'm using the ASI 294 MC and ASI 178 MM - both are uncooled and the offsets I noted are for the ASI 294 only.
Thank you,
Gary

[BlackWikkett]

Hi GaryS,

In your question about offset are you referring to the suffix "offset=" that SC adds to the title of the master dark when it's created with the flat creation tool? If this is your question this value is used by SharpCap if you used bias frames while using the flat creation tool to make your flat. I'm not sure there is a "correct" value for this. Main thing to note when you create you flat is to aim for the peak of the histogram (white channel in case of color camera) to be around 50% and nothing at saturation past 100%.

-Chris

[GaryS]

Hi Chris:
Yes, its the suffix that SC appended to the Flats I created yesterday. The 'offset' numbers varied all over the place without a filter in place but were closer together when the filter was on.

So, can you steer me to a link that explains what the 'offset' percentages mean and why the presence of a filter creates such a wide range of values??

Also, I tried applying the Flats last night and noticed no improvement in the vignetting I get on my 142mm F3.6 Schmidt Newtonian.

Thank you for helping me out with this!
Gary

[BlackWikkett]

Unless you're changing exposure, gain, brightness etc settings the offset= should be pretty much the same.

I posted on the other thread the process I use.

-Chris

[admin]

Hi,

The 'offset' number at the end of a master flat filename is a crude attempt to improve flat correction for people who are not using dark frames with their flat frames.

In order for flat correction to work properly, you should really use a dark frame so that you can separate the dark frame signal from the signal caused by light falling on the sensor and then only correct the latter portion with your flat frame.

If you're not using dark frames then you would correct the whole of the signal (including the dark/offset/bias part of it) which would lead to over correction.

In order to try to get around this problem when darks are not being used, and to eliminate the bias signal from the flat frame itself, SharpCap captures two sets of flat frames – one at the exposure chosen by the user and one at a much shorter exposure (typically 1 ms or less). From this information, SharpCap can calculate a true flat frame without any bias signal (by subtracting the shorter set of frames from the longer ones) and also an estimate of the percentage signal level that the camera would have with no signal (i.e. in the bias frame). It's this percentage signal level that is recorded in the file name and used by SharpCap if (and only if) you subsequently use that flat frame without using a dark frame.

So, long story short, if you're using darks the value of that number doesn't matter.

Cheers, Robin

[turfpit]

Gary

Consider starting a new thread whose title accurately reflects the problem you are experiencing. The responses are now split across 2 threads - one in Bug Reports but it is not a bug you are experiencing.

Elsewhere you said:

Sorry to be still so far down the learning curve on SC after 9 (!) months trying to learn...

9 months at this game is nothing. Have a look at my viewtopic.php?f=16&t=596 particularly the dates on the images. It has taken me 3 years to get to be able to consistently achieve this https://www.astrobin.com/1graw1/?nc=user and I will be looking to improve on that in year 4. 3 years elapsed time is not actually that long when time lost to weather and a > 50% moon is taken into account.

I would agree with the capture strategies laid out by Chris (@BlackWikkett) in the other thread. My advice is don't try to understand everything at day one. If someone gives you advice then look at their images - if you think they stack up (no pun intended) then follow the advice but don't expect instant success. Information without supporting images I tend to ignore these days but YMMV.

I think your main issue at the moment is removal of vignetting. I had that once and it turned out I had spacer tubes between the camera and a 1.25" filter drawer - well nobody said 'don't do that'. First step to avoiding vignetting is make sure that from telescope to camera is always a reducing diameter. If the diameter of the optical train decreases and then increases you will get vignetting that is difficult to remove.

If you do create another post then stick to one camera only (reduce the complexity) and include:

• details/images of the optical train

• capture settings

• links to light/dark/flat frames, then we can see what is actually going on (images from a 10MP camera will be too big for forum uploads

• workflow (the steps you carry out in the exact order you carry them out

Provide the above info and the advice/suggestions you get back will help solve your problem and accelerate your learning.

Dave

[GaryS]

Thank you everyone!

I’ve just returned from 6 weeks away and am picking up the flats issue which, at this point is really a matter of understanding the best process for capturing the Flats. I’ve heard so many variations in the process here and on CN forum that I confess to longing for an outline of a process that most experienced folks would agree on. Lacking that, right now here’s what I plan to do:

1. Follow the general steps that Robin listing in the manual regarding the capture settings and histogram shape
2. Utilize 2 layers of white pillow cases below my illuminated panel which will be set to its lowest brightness
3. Perform the Flats capture at night just as I set up for observing
4. Apply both master Dark and Master Flat.

If I missed something, I’m hoping for a correction. If this procedure doesn’t work, I’ll be back with a separate topic with data and images to illustrate any problem.
Cheers
Gary