NGC 3628 Sarah's Galaxy

[rubecula]

This is an RGB rendition of NGC 3628 Sarah's Galaxy (aka the Hamburger Galaxy)

R, G and B each  = 10 x 600s binned 2x2

I also took 15 x 600s Lum binned 1x1 but adding them to the RGB spoilt it so I have just left it as RGB.

Captured with SGP and PHD2.  Stacked in APP, rest of processing in PS.  Sadly no sign of the long tail, but I guess revealing that would require much, much more data.

It looks a bit fuzzy to me, I think that's due to some high level very thin cloud around during capture (or poor focus - I'm still struggling to get SGP autofocus to work accurately all the time).

I recently saw a thread that said using a Bahtinov mask to determine offsets for all filters and then only focusing with the L filter worked well.  I'm going to try that when we get some darkness back

Thanks for looking

[DaveS]

A wee bit blue in the core perhaps, but otherwise good detail in the dust lane.

The Tidal Tail is exceedingly faint, you need a lot of data from a dark site to reveal it

I took 15 hours RGB and 8 hours L from a nominal 21.66 location and still barely recorded it.

[Kinch]

Not sure if this was one of the images that you thought had fat stars - they appear normal to me for this field of view.

Anyway - just wanted to get back to you about using offsets for doing focusing in SGP. That would not be necessary in an LRGB image. One really only needs the offstes for narrow band filters.....to avoid taking 30 or 40 seconds to do each single focusing shot.....making a focus run through maybe 9 focus positions an extremely long, time consuming, exercise.  I think with LRGB you should be able to work fine focusing through each individual filter. For me, my LRGB focus shots are each 5 secs binned 2x2.

[x6gas]

3 hours ago, Kinch said:

Not sure if this was one of the images that you thought had fat stars - they appear normal to me for this field of view.

Anyway - just wanted to get back to you about using offsets for doing focusing in SGP. That would not be necessary in an LRGB image. One really only needs the offstes for narrow band filters.....to avoid taking 30 or 40 seconds to do each single focusing shot.....making a focus run through maybe 9 focus positions an extremely long, time consuming, exercise.  I think with LRGB you should be able to work fine focusing through each individual filter. For me, my LRGB focus shots are each 5 secs binned 2x2.

Nice result!  As Brendan says nothing wrong with those stars, especially at that focal length!  I wonder if the sub length is a bit long for good star colour?  Do you have a lot of saturated stars in your unstretched data?  Also the sky does look a bit dark as I think you alluded in one of your other posts.  Many of the great and the good recommend a background sky colour of 23/23/23 when sampled in PS (though I tend to leave mine at 30 during processing and almost the last step is to drop to 23-27 depending on the image - I'm probably guilty of leaving my backgrounds a bit too light...)

You can do the work to sort out the filter offsets if you wish if you find that focus shifts materially between filters... but you might be surprised and not need to.  Baaders are supposed to be parfocal (though I can't see to what f-ratio) so the difference might be small enough for you not to need to bother.  I just focus for every 0.5 degree temperature change using the L filter and I'm good.  If you do go down the Bhatinov route then I thoroughly recommend Bhatinov Grabber which, if you have not come across it already, takes the guesswork out of focussing and shows you the latitude you have across acceptable focus.

 

[rubecula]

Brendan, thanks for your input.  I've tried letting SGP autofocus do the work with LRGB filters, sometimes it works, other times I get an out of range response and have to intervene manually.  I've had some help from the SGP forum but am still struggling.  It must be me and/or the settings I have.  A highly respected member of this forum (not sure whether it's appropriate to name them) stated they use offsets for all filters so I thought I'd give it a go.

Ian, you've given me a lot to think about, thanks:

Star saturation - I'll go back and check that,

Background colour - I presume you mean R/G/B 23/23/23?  Not sure how you check and change that.  I tried Image/Adjustments/Replace Colour on the whole image but that made a mess of the DSO.  Then I tried deselecting the DSO but that produced a discoloured border between the sky and the DSO.  Ho Hum.

[x6gas]

2 hours ago, rubecula said:

Background colour - I presume you mean R/G/B 23/23/23?  Not sure how you check and change that.  I tried Image/Adjustments/Replace Colour on the whole image but that made a mess of the DSO.  Then I tried deselecting the DSO but that produced a discoloured border between the sky and the DSO.  Ho Hum.

Hi Robin,

Yes that's right, R/G/B 23/23/23 (if all three numbers are the same the 'colour' will be monochrome ranging from black - 0/0/0 - to white - 255/255/255).

The way I do it is to place "Color Sampler Tool" points on background sky in the image.  You can place up to 4 in my version of PS so I put one towards each corner.  The colour sampler tool is one of the tools under the eyedropper circled on the left in red in the image below.  Note that there is a tool setting (yellow arrow - apologies for my rubbish drawing!) where you select the sample size - it's best to avoid a single pixel and go for 5 by 5 or 11 by 11 average.

Each sample point will give a reading under the info tab (if you can't see the info tab you can activate it from the "window" menu or by pressing f8).  The readings for sample point #1 are circled in red on the right below, with the other sample points right and below.

 

To get them all reading the same, I use Levels (Image>Adjustments>Levels or ctrl L) and then select the individual colour channel (see screen shot below), adjusting the black point slider to get the reading I want for that channel before moving on to the next.

I still stretch my data in PS and I tend to do a relatively gentle stretch, then check the levels and adjust the R, G, and B channels to 30 if they are above that, then repeat, and repeat, and repeat until I get the stretch I want.

Then, as I said above, once I've finished processing I adjust the levels to somewhere between 23 and 27 - in the example above I've chosen 25.  Note that if you have a gradient across the image that will show up as different values in the different sample points.  I've used Gradient Xterminator on this image but there is often still a little variation across the sample points (you can see my values range from 24 to 26 in fact).  A difference of 1 or 2 across the points looks fine in my opinion.

I hope this makes sense - please do shout if not!

Best wishes, Ian

Edited June 14, 2020 by x6gas

[rubecula]

Wow, that's so good of you Ian.  Thanks for that illustrated explanation.  You've added a whole new avenue for me to explore.  I had a quick go on the image and as well as lestening the blackness it revealed the small distant galaxy that is just to the centre left of NGC 3628 and also (if I'm not fooling myself) a very feint hint of the NGC3628 tail.  It has however made the background a bit blotchy so that may need more work.

I had a problem with the captured images.  The camera dessicant seemed to have suddenly given up and there were what looked like little drops of water and some icing in the images. I've had to clone them out so that may contribute to the blotchiness. The folks at Atik kindly sent me some new dessicant tablets so it's all ok now.

I'll have a play tomorrow and put up a copy when it's as good as I can get it.

Once again thank you, that was very generous of you. You're a 🌟