Pre-Processing....????

[Shelster1973]

Is this possible and is it done?

Reason for asking is due to the joy and frustrations that is AP.

In a move that deies all new equipment purchases, my LP filter and T-Ring holder / adaptor turned up and it was full clear skies last night.

So to make the most of the opportunity of both, got set-up, did 2+4 align on my mount and wasa ll ready for some increased exposure times.

First target was M10 as was nice and high, so limited LP for the filter.  Decided to go for a 120 sec exposure at ISO400 captured with BYN (BackYardNikon).....nervous wait as the first exposure captured and downloaded and was very happy to see the beginnings of a nice looking M10 when it did pop up.  Watched the second one come through and that was similar.  Happy with that let it carry on for the full 15 exposures I had set up.  Took obligatory darks and bias at the end.

Next target was M92 which was followed by Deneb and then finished off on Whirlpool Galaxy.

All these looked really goodo on the images that were downloaded.  Did drop the expousre times down for subsequent imaging (M92 to 60 sec, Deneb to 25 sec and Whirlpool to 45 sec all with a 30 sec pause between exposures)

First image of Whirlpool looked good.  Could see it clear but a wee bit faint so was feeling a happy bunny..........pride, fall and all that.

Thought I would throw the RAW's into DSS and see what the images looked like briefly before heading to bed (was 0200 by this point).

This is were I got frustrated.......

Thought I would try Deneb first.....loaded in the lights, then the darks and bias...select all, go to register and go on teh advanced to see the star level it can find...........and the survey says..........4........        and this is at 2%.  Persevered through and it says that it is only going to stack one image.  And the same thing happnes for the three others too.

When you open the image in DSS preview screen (ie click on it in the list) it does appear very dark and you can only get detail out by moving the slider in teh top right of the screen towards the left edge. 

Finally gettign to my original point (many thanks for sticking with it if you have) can you increase the brightness of the RAW image either in DSS or elsewhere prior to stacking? and if so will it have any negative impact on the final stacked image quality?

TL;DR - Can you bump up brightness of images prior to stacking???

Not got any images up from last nights effort yet...will put up some unstacked subs later this evening when I get back from work...and hopefully some stacked images too......fingers crossed.

[kalasinman]

DSS will stack what you give it. If you load the lights to something like Lightroom, check all, import, under settings "synch settings" and bump up the exposure, export to original folder as TIFFs, then load to DSS, they should be fine. As you've already run them, use your master dark and master bias in next DSS run. They are already saved as TIFF.-Jack

[steppenwolf]

My first thought on this is that a stretched image shouldn't make any difference to the stacking software from the point of view of stacking the images. If the 'additional' stars can be revealed by stretching the image then they must be there in the original data so the software *should* be able to identify them.

On the second point, pre-stretching an image to death in a 'pre-processing' session would be a bad idea as you will permanently burn out fine detail that you can't recover later. The key, as ever with deep sky imaging, is to capture longer exposures.

[Shelster1973]

DSS will stack what you give it. If you load the lights to something like Lightroom, check all, import, under settings "synch settings" and bump up the exposure, export to original folder as TIFFs, then load to DSS, they should be fine. As you've already run them, use your master dark and master bias in next DSS run. They are already saved as TIFF.-Jack

Cheers for that....wil have a look at that later on.  Being as a cheap sakte, are there any free alternatives to LR that I can use?  Have got Paintshop Pro X5 installed, could that be used?

My first thought on this is that a stretched image shouldn't make any difference to the stacking software from the point of view of stacking the images. If the 'additional' stars can be revealed by stretching the image then they must be there in the original data so the software *should* be able to identify them.

On the second point, pre-stretching an image to death in a 'pre-processing' session would be a bad idea as you will permanently burn out fine detail that you can't recover later. The key, as ever with deep sky imaging, is to capture longer exposures.

Really love to take longer exposures, but at present am unguided so am sitting on teh raggedy edge of what I can expose to in my current setup.  Hopefully have an adaptor coming that will allow me to start guiding....just need a bit of fettling on the webcam and should be away.

[carastro]

 If the 'additional' stars can be revealed by stretching the image then they must be there in the original data so the software *should* be able to identify them.

You would think so wouldn't you Steve, but I have had the same experience as the OP at times with DSS and the only way i could stack them (assuming I had tried reducing the star detection threshold in DSS and that still didn't work) was to do what the OP suggests and it seems to work then.  I think it's one of the odd things with DSS.  Maybe a different stacking software would work when DSS doesn't.

Carole 

[steppenwolf]

You would think so wouldn't you Steve, but I have had the same experience as the OP at times with DSS

Have to put my hands up here and say that I don't use DSS myself so perhaps it is an issue with this particular stacking software! I don't really like the idea of forcing a strong stretch so early in the processing but if it has to be done, it has to be done.

I remember when I first started using MaxIm DL for autoguiding many years ago I couldn't get it to calibrate so stretched the image for a second attempt and low and behold it worked. I asked the software writer about it and was adamant that it was pure coincidence (which I have subsequently proved to be the case) and it was he who explained that the data was either there or not. MaxIm perhaps uses a 'behind the scenes' non-destructive stretch on dim images when stacking perhaps?

Really love to take longer exposures, but at present am unguided so am sitting on teh raggedy edge of what I can expose to in my current setup.

I've been there and persevered like you but in the end, we all succumb!! Don't let the lack of guiding put you off though.

[lensman57]

You would think so wouldn't you Steve, but I have had the same experience as the OP at times with DSS and the only way i could stack them (assuming I had tried reducing the star detection threshold in DSS and that still didn't work) was to do what the OP suggests and it seems to work then.  I think it's one of the odd things with DSS.  Maybe a different stacking software would work when DSS doesn't.

Carole 

Hi Crole,

At times if you disable the " reduce noise by applying a median filter " in the advance dialog and then adjust  the star detection slider then DSS will work just fine.

A.G

[dph1nm]

I would try without bias or darks (you don't need both if you are not doing flats).  The only time I have ever had a problem with DSS is if the stars are out of focus (and I only take 30sec exposures). I am wondering if something hasn't been oversubtracted resulting in a negative background  ...

NigelM

[Shelster1973]

I think it may be something to do with my warped stars..had a closer look last night when noght as tired as a tired thing and they are looking rather elongated.  Outer edges deffo due to coma but the others are obviously down to tracking errors.

Wjhen I next get out am going to drop my exposure time down to 30s and up the ISO to compensate for lower exposure times.

Also have adaptor coming so as to get my webcam onto the finder scope and wil try some guiding on it too.  Have managed to get PHD2 talking to the mount, so just need the camera mounted and linked to that and should be away.

Also updated the firmware on teh Nexstar+ handset last night.  Not sure if that will help me any but has to be of some use....

[kalasinman]

I would try without bias or darks (you don't need both if you are not doing flats).  The only time I have ever had a problem with DSS is if the stars are out of focus (and I only take 30sec exposures). I am wondering if something hasn't been oversubtracted resulting in a negative background  ...

NigelM

I don't understand this. If bias frames are to eliminate read noise, and darks heat related noise, and flats remove gradients, dust bunnies and vignetting, how can you come to such a conclusion. Please explain.

[RobH]

The bias signal is contained in your darks, so if you don't use flats then a dark subtraction only is fine. Flats need to be bias calibrated in order to work properly. In a lot of ways though, flats are the most important of the calibration processes, as any other noise can be removed by using an outlier rejection algorithm ( sd mask etc) during stacking, as long as there's a bit of movement between subs.

[kalasinman]

Thank you Rob.

[Thalestris24]

I think it may be something to do with my warped stars..had a closer look last night when noght as tired as a tired thing and they are looking rather elongated.  Outer edges deffo due to coma but the others are obviously down to tracking errors.

Wjhen I next get out am going to drop my exposure time down to 30s and up the ISO to compensate for lower exposure times.

Also have adaptor coming so as to get my webcam onto the finder scope and wil try some guiding on it too.  Have managed to get PHD2 talking to the mount, so just need the camera mounted and linked to that and should be away.

Also updated the firmware on teh Nexstar+ handset last night.  Not sure if that will help me any but has to be of some use....

Yeah, I think DSS likes stars to look like stars and be in good focus... Also I'd recommend using a higher ISO if you're limited to short exposures - 800 or even 1600. I think 800 is fairly normal for AP anyway.

Good luck!

Louise

[Shelster1973]

Well coma corrector now here so that will help eliminate one issue. Tried the autoguiding with webcam and it was an unmitigated disaster. Could not get any stars to show no matter what I tried so next saving item will be a qhy-5 cam.....can also use that for some planetary imaging too. Will be going mono on that one.

Last time out had upped the ISO to 800 but was having a few tracking issues. Did a 2+4 alignment and that was pretty bang on. Did an aspa after that and things went a bit awry after that. Did a second 2+4 but was not as accurate as the first time.

Will keep persevering as may not have chosen the best star to carry out an aspa on.

We live and learn......

[dph1nm]

Wjhen I next get out am going to drop my exposure time down to 30s and up the ISO to compensate for lower exposure times.

It doesn't (compensate). Changing ISO does not alter the sensitivity of your camera to light (which is a fixed quantity), and so it doesn't affect the required exposure times. 30s at ISO400 will collect the same number of photons as 30s at ISO1600.

NigelM

[Superdavo]

I did it once, using Camera Raw in Photoshop. Unfortunately, I have never compared those results to results without pre-processing, but it turned out pretty much ok.

David