Hey all,

I've had my DSIc for a few months now and all I've really done with it is take lights and darks using nothing but the default settings basically. Like most of you, my imaging time is limited and I rather not experiment and ruin hours of scope time when the weather conditions are good. So, I was hoping I could ask some basic questions on camera function and settings that are unclear to me.

1) Evaluation Count? How do I use this to create better images? What is it actually evaluating?

2) Minumum Quailty % ? Is this used while evaluating?

3) What is Drizzle?

This is my routine with these settings when capturing lights:

1) Set Evaluation Count to 5-10

2) Set Minumum Quantity 30-50%

3) Click Start

4) Pray

Sometimes the image looks good when it's done evaluating, but a lot of times I want to cry. Is there a way to choose the best out of the 5 or 10 evaluation images to start a baseline or when I see a good one during the evaluation, should I crank the min. quality to 100%? I hope these questions make sense...

Oops, I just thought of another question... If I chose 10 for the average exposure, can I get away with 8 or 11 exposures and would this be the same for darks?

Thank you in advance,

Larry

Hi Larry,

Some answers based on my experience...

1) The evaluation count is used to define the number of frames that get averaged up to produce the reference frame that Envisage assesses quality against. You might think that using high value is a good idea, but in practice I've found it seldom useful to go over 5 frames. Also be wary, if envisage picks up a series of "duff" frames during this phase, because it will throw away the good ones later... Moral is to look at what is happening on the screen, and if things don't look right, stop and start again (better still make sure you retain all the data so can process after the imaging session).

2) The minimum quality is an abstract figure of merit for how close the image taken is to the reference image, higher the number the closer it is to the reference image. I'm not convinced the algorithm is that robust though... It does appear to be used during the evaluation phase.

3) Drizzle is one of the many alternative ways of processing multiple image data, originally developed for processing the under-sampled data from the Hubble Space Telescope.. There is a really good description by Stephen Hamilton here: http://autostarsuite.net/files/11/articles/entry4265.aspx. It can give some surprisingly good results with the DSIc, but you never get anything for nothing, and you will end up having to take more sub frames compared to conventional stacking.

For flat frames, I always set the quality to 0% as it makes no odds... you just want an average of a number of frames to reduce the noise in final master flat.  I use a lightbox method with the exposure set to fill about a third of the histogram and take an average of 32 frames in total. Don't forget, with the DSIc you need to get a master flat frame for each colour channel, or find youself a programme that will work with RAW format flats... There's more detail here around processing flats with a single shot colour CCD here: http://autostarsuite.net/forums/thread/17044.aspx.

Hope you find this helps!

JonW

PS: I'm assuming you've set up the tracking boxes for alignment and de-rotation before clicking the start button..?

Hello John-

This has helped, thanks!

A couple of more questions if you don't mind :)

1) What is Average Exposure? Is this a number that represents the amount of darks captured in a specific exposure time and the total of lights should be this amount? I'm confused...

2) What is happening when I take darks, but leave the "Delete Exsisting Dark Frames" box unchecked?

Man, I'm glad you mentioned this one, because I forgot to...Yes, I do use the tracking box thingy, but I'm not sure if I should box in the whole target or just a star?

Thanks you in advance,

Larry

Hi Larry,

>1) What is Average Exposure? Is this a number that represents the amount of darks captured in a >specific exposure time and the total of lights should be this amount? I'm confused...

Do you mean the average exposure box that appears during take darks? I think this is actually just the cumulative exposure time, but I'm away from the observatory so can't check? For info,  I actually use a different piece of software (AIP4WIN) for all my post processing, so just take two sets of 16 flat subs and then use an averaging process to create the master flats for the colour channels (I'd much rather do this with RAW files, but it's not currently supported in AIP4WIN for the DSI III).

Unless you are looking for absolute perfection, I would not spend too much time worrying about the exact relationship between the dark and flat exposure times. As a general rule for darks I use an average of 10 up to 30S exposure time and 5 for anything greater than 30S. The key issues are:

a) with the flat frame is to make sure you've got enough light incident on the CCD to be well over the noise floor, but not so much that you get into the so  called "non-linear" region of the CCD (somewhere between 10,000 to 20,000 units seems to work for me...).

b) with darks making sure you've a sufficient number of subs averaged to reduce the random noise.

This is the best reference if want to follow up on the details on CCD image calibration:: http://www.willbell.com/aip/index.htm.

>2) What is happening when I take darks, but leave the "Delete Existing Dark Frames" box >unchecked?

It will overwrite the only the dark files that you are updating in the "take darks" process and leave anything else intact. If you check the box it will wipe everything out from the darks directory.

>Man, I'm glad you mentioned this one, because I forgot to...Yes, I do use the tracking box thingy, >but I'm not sure if I should box in the whole target or just a star?

I find it's best work with stars (for DSO's at least) as they are more well defined. You just need to make sure that the target won't wander out of view during imaging (more of problem if you're alt-az mounted) and that it shows good contrast against the background. Also keep the two stars as well separated as you can.

Would be useful to know roughly what your set-up is?

Anyway, hope this helps!

JonW

Hello JonW-

Thanks for your Help!

"Do you mean the average exposure box that appears during take darks?"  Yes.

"Also keep the two stars as well separated as you can."

Hmmm...Am I supposed to draw two box's on two different stars and draw a line to connect the box's???  I read the Drizzle Intro Part I and it was mentioned for de-rotation, should I be doing this even if I choose not to use the Drizzle function?? I have not been doing this! How much could this be affecting my images???

"It will overwrite the only the dark files that you are updating in the "take darks" process and leave anything else intact. If you check the box it will wipe everything out from the darks directory."

Will this affect the image if only some of the darks files are updated? In other words, if I update only the 30s files in 30F temps, but the all the other files were previously captured in 20F temps, will that be a problem?

My mount is a EQ-G,  I'm using EQMOD to control the mount and PHD and a web cam to guide.

Thank you in advance,

Larry

Hi Larry,

"Do you mean the average exposure box that appears during take darks?"  Yes

My earlier post on this was incorrect, I looked at Envisage tonight, and the "Avg Exp" setting box determines the number of darks that are to be averaged. The total exposure time appears as text just below where the "delete existing darks" appears. 

"Also keep the two stars as well separated as you can."

>Hmmm...Am I supposed to draw two box's on two different stars and draw a line to connect the >box's???  I read the Drizzle Intro Part I and it was mentioned for de-rotation, should I be doing this >even if I choose not to use the Drizzle function?? I have not been doing this! How much could this >be affecting my images???

All you need to draw is select two stars by drawing two boxes (second box is actually displayed as a cross), Envisage does the rest. With your polar mount set-up, you probably won't get much benefit from the two star approach if you've got good polar alignment, but I always use a two star selection as it makes the stacking process more robust. The instructions are not clear that this function is available in all capture modes, not just drizzle.

"It will overwrite the only the dark files that you are updating in the "take darks" process and leave anything else intact. If you check the box it will wipe everything out from the darks directory."

>Will this affect the image if only some of the darks files are updated? In other words, if I update only >the 30s files in 30F temps, but the all the other files were previously captured in 20F temps, will that >be a problem?

With my DSIc, I keep the darks in separate folders based on the air temperature. That way I always have a set of darks available for almost any temperature range in 5degC steps, and there's a reduced chance of selecting the wrong dark for the temperature. I also tend to always re-shoot my darks and flats for the each new imaging session now. Also, I only shoot the darks for the exposures I need... saves a great deal of time and disk space.

Hope this helps...

JonW

Things are much clearer now. Thanks you have been a great help! I cant wait to try out some of these tools...

Clear Skies

Larry

Hi Larry,

Always glad to help... In my experience the journey to achieving predictable results is not always a smooth one, but there is a wealth of experience in this community who will always help out if you get stuck. Also, don't forget there are other resources available tp you here in the "files" pages. Captured there are a number of issues you have probably yet to see, and lots of good tips...

Good luck!

JonW

Thanks again JonW

There is one other thing thats not totaly clear to me  as well and that is:

1) If I plan to take 10x30s subs "Lights" or what ever exposure length of a target, should I be caputuring the same quantities and exposure lengths of dark files too or will just 1x30s dark do the job? I have started a library of dark files in 5deg. increments and I captured 10 of each exposure length up to 480s or 4hrs. and 33min. worth in each increment. Is that the correct way?

Thank you in advance,

Larry

Hi Larry,

>1) If I plan to take 10x30s subs "Lights" or what ever exposure length of a target, should I be caputuring the same quantities and exposure lengths of dark files too or will just 1x30s dark do the job?

You should always take a number of darks and average them for a given exposure setting. In this case you'd probably want to use an average of 5-10 30S dark frames in order to get reasonable master dark frame. This averages out the random noise in the master dark frame.

>I have started a library of dark files in 5deg. increments and I captured 10 of each exposure length up to 480s or 4hrs. and 33min. worth in each increment. Is that the correct way?

If it were me, I'd just build the library up as I needed the darks. I use the 8" meade with a 6.3 focal reducer, and most of my work is done between 30S and 120S exposure lengths. Nothing wrong with having a complete library, but my advice would be to only take darks for the exposures you're likely to use...

Hope this helps...

JonW