SharpCap Forums

Last night I had trouble dropping frames in LiveStack. After having relative success imaging Bodes galaxy (although I had quite a few dropped frames), I tried the Pinwheel and Whirlpool. With these I couldn't stack at all--just dropped frames! Being the rookie that I am, I tried everything I could think of: increasing gain, decreasing exposure and vice versa. All different combos...(up to about 350 gain and down as far as 15 second subs...and everything in between). I tried the alignment boxes, i.e. increase digital gain, minimum star width, everything. For the life fo me, I couldn’t get a second star stacked in LiveStack.

So, my question is this: is there a general rule of thumb about dropped frames...is it a gain issue? Exposure? Or some other variable. I did try everything and at this point I’m rather stumped.

FYI: I'm in Bortle 7-8; both targets were above 50 degrees and nicely situated...they were centered in the view finder, flat was applied, minus 15 degrees C, PA excellent; display histogram looked good before launching Live stack, i.e., off the left vertical.

Celestron 8" SCT
ZWO ASI294 Pro Mc
f/6
L-Pro Light Pollution Filter

Hi,

if it won't stack then the usual cause is that SharpCap cannot find enough stars in the images to perform the alignment - if you look in the 'Log' tab of the Live Stacking area you will probably see messages going past that confirm this :
If this is the problem then consider the following

1) Turn up exposure and/or gain to brighten stars
2) Check the 'Alignment' tab - turn on the option to highlight stars so you can see what (if any) stars are being detected
3) Carefully try adjusting the settings in the alignment tab - it may be worth doing a 'Reset All' first to remove any previous adjustments.

hope this helps,

Robin

Thank you Robin, you are correct, my log shows numerous ERROR messages "detected zero stars, or detected one star, etc." I'm convinced this must be the problem, but going forward I still have some confusion. Please note the following:

Last night (before seeing you note Robin) to no avail, I tried the following experiments:

1. Tried a Windows 10 laptop fix to disable USB low-power issue (https://www.windowscentral.com/how-prev ... sb-devices)
2. I read that the problem could be the extension cable. I disconnected my repeater cable and tried connecting my camea directly into the laptop, no luck.
3. Tried to use another laptop, no luck
4. Tried long (and short) exposures as much as two minutes (unguided), no luck
5. Tried shorter and longer gain (i.e., less than Unity 120 and as much as 350 gain), no luck

I acknowledge that I am a floundering rookie (capital "R") at all this, but let me ask, could the fact that too few stars were detected be because:

1. These galaxies (i.e. Whirlpool, Pinwheel) are in a sparse star field and my light pollution (New York City light dome) is able to detect them (even at higher gain?)
2. I would love advice on how high can one 'push' the alignment' options, for example, digital gain, black threshold, etc. Saying this differently, since I was getting zero stars, how high can you push those variables. And would it be recommended to push the gain to the max setting for my ZWO294?

Thank you all for any and all suggestions,
Joe

Hi,

I wouldn't turn up your camera gain control much more than half way (the gain gets ridiculously large in the last 25% of the range - not worth going there).

In general for the star detection parameters, set as follows

1) Black level set to about 20 - if you look at your histogram when not live stacking and there is a big peak on the left hand side, pick the % value for the right hand side of this peak - basically excluding all the dark bits of the image from being detected as stars

2) Min/Max star size - set these to the extreme values - 0 minimum and 32 maximum. Turn up the minimum a bit if you have problems with hot pixels being detected as stars

3) Noise reduction - 0 to 2 is typical - keep it low unless you are having problems with hot pixels

4) Digital gain - start with this OFF. Only if nothing else helps, turn up gradually.

If you are still stuck, please capture a single (unstacked, unprocessed) frame of the target with your normal camera gain/exposure and share it here - I'll then try to advise in more detail.

Robin

Much thanks Robin, I now have a plan of attack...I'm hoping to get out tonight.

Joe

Robin,

Indeed, my problem was too few stars. Thank you for your suggestions, here's my image from the other night. Please take a look at the resulting image.

Obviously, the vignetting is horrible!!! I realize the ZWO ASI294 is a problem with flats. (I took my flat at 121 gain, 5.5 second exposures, 25 frames) I'm using an L-Pro light pollution filter (SCT 8", f/6 reducer, ASI294). Any and all suggestions on this rather 294 ubiquitous problem (especially the pinkish-red color of the vignetting) will be most appreciated (I'm not sure if I set the color balance to 50-50...could this have been the issue.

Hi Joe,

good to see that you have the live stacking working now that's a great image ! One thing checked off the list and onto the fun problem of dealing with flats.

To me it looks like your flats are over correcting for the darkness in the corners, leading to the corners of the image becoming brighter and getting the pink glow. The things I'd suggest here are twofold:

1) take your flats at zero gain to avoid being anywhere near the range where these cameras can have their non-linear area.
2) ensure that you are using the option to capture and subtract bias frames when you take your flats and that you are also using dark frames. Skipping either of those can lead to flats that either under correct or over correct depending on which one you skip.

Cheers, Robin

Hi Robin,

Thank you for that great analysis! Can't wait to apply your suggestions. Fingers crossed. I'll check back with my results.

Joe

Robin,
One follow up question if you don't mind. I've read some posts where some say the White balance controls should be set at 50-50 before making the flat. I've used a gray card to get my color balance (while using LP filters), should I use my "corrected" values or go with 50-50 prior to making the flats?

Thank you again,
Joe

Hi Joe,

I don't think it will make a big difference either way unless you are using the white balance to make big corrections (doing that can pull down the maximum image value in a channel or push up what would be an underexposed channel to look well exposed just by applying a boost to each value).

As long as you can get a histogram shape where all three channels are well clear of the 100% (right hand side) and the 0% (left hand side), you will be fine.

cheers,

Robin