Gunshy

Hi, I know this is a dated post, but I have had the exact same problem, In the middle of the night I had my A-P mount stop tracking, and NINA started throwing all kinds of errors related to ASI, etc. In fact to get going again I have to disconnect and reconnect the camera - but I don't think the issue is really camera related. I think this is actually related to median flips and tracking limits.... Here is what I surmised happened. My mount started the target (second target for the night) in a counter-weights up position with the telescope on the East, so all should be good and no flips should be required. In APPC I had both "East Limits" and "West Limits" selected. Judging timing from where the mount was when the problem occurred (I am surmising this because I was asleep), the mount stopped tracking, and all the problems/errors were thrown, just when a meridian flip should have taken place if the session had been started with the telescope on the West side. I am not sure who actually stopped the tracking, NINA or APCC, whomever did not know that the telescope was actually already on the East side and did not flipping, not tracking stopped with the limit was reached. Further to this supposition was the fact that, in this particular sequence/target, I had the the following commands in which I was awake to observe Slew to RA/Dec: - the mount slewed to the target with the telescope on the West, counterweights (CW) down Perform Autofocus: - did the autofocus on the target - worked fine Slew, Centre and Rotate - the mount essentially first did a flip, putting the telescope on the East with CW up I wondered at the time, why the original Slew did not go to the CW up. Now I think that there is something fundamentally different between the two Slew commands in NINA. Although I don't think the problem was caused by APCC, I think the solution actually is to change my APCC settings. I notice that my "newer" version has been NINAfied with a check-box to "Send Limit with offset to NINA". I had this checked, but I also had both "East Limits" and "West Limits" checked as well. Toggling the "East Limits" box indicates that it should only be checked when SGPro V3 is being used and doesn't mention NINA at all. Tonight I will be repeatiing my sequence, this time with the "East Limits" box unchecked, and will further post if this was the issue after all..... but I have convinced myself that this is the problem. At any rate, my USB connections and electronic hardware all seem to be fine and this is an alternative solution. Cheers and Clear Skies, Dave

Folks, Mike did all the work in porting this over to a compiled process - Mike just used me as a guinea pig, but am I ever glad he did. As a process, I essentially keep GHS open all the time now - monitoring the histogram and often tweaking the stretch ( for example after HDRMT, or after LRGB combination, or after adding back in the stars, etc. etc.). Thanks so much Mike for all your work. I am loving the process/module version - and that's not stretching the truth!. Dave

Budgie! Thats a pretty spectacular image! Well done! I am glad the script helped out. Dave

Thanks Mike for making an excellent well thought out environment to apply GHS stretching. The first thing that strikes you is the preview, which is great, but there is so much more that you have done here. My favourite new thing is the "Col" option. What this option does is use a modification of same technique as the "Arcsinh Process" to enhance the colour saturation of images as you stretch - only this time using the GHS to do the stretch. That way you can have the best of both worlds - colour saturation enhancement of AP + the benefits/control associated with GHS. In practice, the GHS actually also produces much less frequent and severe colour artifacts in the brightest stars. Try the "Col" option rather than "K/RGB" for your colour images. If it is too much, then use the blend option to find a balance of images stretched with "K/RGB" and "Col". Mike will be posting a video about this on the ghsastro.co.uk site.

Hi "Lazy". Go to Feature Scripts and uncheck the box that points to the directory where you installed v1

I just wanted to show a revision I made to some year old NB data I had on NGC2337 (Rosette). I was able to really bring out the nebulosity around the Rosette and control stars using GHS. To avoid bringing out noise as well, Mure noise reduction helped tremendously. (The handling of the stars was my own experimentation with Starnet to substitute RGB and I used the method of Cuiv the Lazy Geek to achieve my take on the Hubble pallette)

Thanks so much for that great answer. I know the question was a little off-forum topic, but I appreciate the answer. I will definitely be looking for these "dust lanes" in my globular images.

First off, Fantastic Image! I love images that I get lost in, and this is certainly one of them. Astronomy question - Are those dust lanes in the foreground of the cluster are within it itself? If within the cluster, than what is the difference between a cluster and mini/dwarf galaxy? Thanks also for the star analysis. That helps me a lot! Mathematically, the GHS stretch is most like the masked stretch as well (and I believe is the actual limit if an infinite number of mask-steps were taken, and an exponential stretch were employed). As for the HT, I have to say it would be difficult to make a better all-round stretching transform with essentially one input control parameter, but can struggle with star bloat and bringing out dim features. The ASH is the equivalent, in terms of brightness, is the equivalent of performing the GHS stretch with a negative b-factor, so if you like the star shapes that it yields you can use either method. The colour enhancing magic that ASH provides actually has little to do with "arcsinh" itself, but has more to do with the manner in which the stretch transform is applied. @Mike1485 and I (well, mainly Mike) is thinking of a way to either put a similar formulation into GHS, or just the ability to stretch saturation / luminance as an alternative to R,G,B. Currently, I think GHS is good a preserving saturation or separately "stretching" a colour, but lacks the enhancing ability of ASH (I actually enjoy the colour enhancement in your image). That's all fine and well, but your posting makes it real, and much appreciated. Those stars are a tricky business! Thanks again for sharing!

That's spectacular! I'm off to visit your Astrobin site now. Thanks so much for sharing, and I'm glad you had such good success. Any tips, tricks, or difficulties you encountered along the way to producing this would be greatly appreciated.

Nice work, Vineyard, well done. Lots of detail in the nebulosity. Additional star protection, if desired can be had by moving the HP slider down. HP is also most helpful when doing subsequent GHS stretches to add/adjust contrast within an image. LP can similarly be used to avoid darkening the background too much. I'm very glad you find the tool useful. Keep me posted! Dave

Hi Vineyard, One of they keys I have found, aside from D & b, is the SP point. For the initial stretch @mike1485 and I thought about setting up a button that would "automatically" choose a D, b, and SP to use. We abandoned this approach because it was dependent on both the image/subject and the goals of the image processor. I find myself changing my mind on where exactly to advise placement of SP, at least for the initial stretch. So my final answer on this, important parameter, is to try a few different ones on our image at first and see, while keeping it within the histogram you can see : too low an SP and background noise will be apparent, too high and only the highlights will be emphasized. Somewhere in between will balance highlights with background according to your taste. Of course, subject stretches of the image can be made to adjust this balance further. As for b and D, in the documentation I say to place the histogram peak somewhere between 0.2 and 0.25 after the initial stretch. Even this advice may not be best for the image you are processing. For galaxies or subject matter that only occupies a minority of the total image FOV, a much lower histogram peak may be desirable. If you have access to the histogram information, take a look at the histograms that go with the images you like on either on a sharing website, or by loading the images into your own image processing software. That can give you a sense of where to put the histogram peak, and just as important what shape it should take. On subsequent stretches, SP remains important but also the levels of all the the other parameters. One thing of note is that subsequent levels of b should be much smaller than on the initial stretch (where b>8ish is recommended to protect stars). Too high a b here may make the image to the right of SP stand apart from the image to the left of SP and also exaggerate the noise or "graininess" within the pixels that exist close to SP. With a smaller b, the precision with which SP is place is less important than on the initial stretch. Use SP, LP and HP here to control the overall brightening and dimming of the portions of image itself. For example you and dim the background while brightening the highlights (or not) by manipulating these parameters. One trick I have learned, is when going to apply HDMRT for "localized brightness and contrast (or dynamic range) manipulation" is to somewhat overbrighten the highlights, or the portions of the image that you want the HDRMT process to take effect. Other than that, by trial and error you can develop your own "tricks" and if you do, please continue to share them! Cheers, Dave

Actually no, the video isn't really a one off. For the video, I didn't know what the subject matter of the video really was until it was stretched. However, as I am playing with the script I am learning things myself, so I would like to modify my instructions somewhat, to : Place SP somewhere within the histogram near the peak, move the peak up and test via the preview. Then adjust to taste, keeping SP within the histogram range that is above "0" in the linear view. Here are my refined guidelines, but please realize that most of the people I have seen replying and reading these posts are far more skilled and talented than I at actual imaging and image processing than I am. I am fearful that those just starting out will take my advice as gospel, when that is not my intent. All of these stretches are reversible (via undo, image deletion, or the "undo" button), so the best advice I can give is to play with it, see what you like best, and make that your individual style of data interpretation: The further left you put SP for the initial stretch, the more you will bring out the background. However, if it is too far to the left, you may bring out low level noise too much. Then try moving it towards the right. If you put SP on the right hand side of the peak, then the background will tend to be left more dark. So if this is your goal, ie leave. the background dark then it may be best to be on the RHS (Right Hand Side) of the peak. As I say the video - in general the whole curve on the LHS of the histogram of the linear image - will generally contain the background noise, the background itself, and the subjected matter. The histogram peak might be in the background or the subject matter and which of the two depends on the image, and how much you want to stretch it depends on what you want to show. On an STF view, check how much of the image is covered by the subject matter itself. If the subject matter covers more than half of the total image, the peak likely represents subject matter and you should place SP on the LHS of if. If less than half, you can move SP more to the right. On the video a got a bit "lucky" on the SP position. A little less lucky, and I would have had to go back, reposition SP, and conduct another initial stretch. Clearly the peak in the Helix nebula image represented background. Check the image after a "test" initial stretch and matching parts of the histrogram (including where the peak is) to the image will help you design a better initial stretch SP position as well as guide your follow-up stretches. Cheers, Dave

Sorry, My first video posting on Youtube. Should be good to go now. Dave

Thanks everyone for the kind comments. Here is a half hour video tutorial on getting started with the GHS script. It was taken from a Zoom meeting where I was showing some fellow members of my local centre (Victoria) of RASC how to get started. I have put the video on Youtube. Please excuse the videography and elementary editing. https://youtu.be/ke5B8XQBwVc I hope this helps. Thanks, Dave

A fantastic image, of which the GHS script played a small role can be on Astrobin,: https://www.astrobin.com/lzexnj/ Thanks for sharing, Randy!

Hi Sp@ce_d I am glad you are finding it useful and stuck it out. It is a bit of different approach - but once you get used to it, I find it really grows on you. The way that Mike has put the whole script together with the interaction of the histogram and the previews, really makes it feel like you are in control of what happens to the image. By including the HT transform and Arcsinh transform as selections too, even adds functionality to these stretches too. Cheers and Clear Skies! Dave

Thanks for keeping with it. The GHS script is meant as a multistage process, that is meant to be applied a few times to an image to control the stretch as you go. What I was trying to describe was just the initial stretch (or two if you include the black point correction. After the initial, highly focused stretch, you really have three options, 1) repeat the initial stretch, using a different SP or b, to see if a "better" focus point and/or degree of focus emphasizes the parts of the image you like more. This is the "design" feature of the script. Use the histogram and preview to guide you. 2) continue to apply GHS to the image - you have complete control over where you put the brightness and contrast. Again use the histogram and preview to control. 3) After the initial stretch, then go to STF or your other one-step stretcher to finish it off. You will get some of the benefits of the GHS script this way, but not all, because you are relinquishing control to the automatic stretcher somewhat. The GHS script should only be compared, when finished stretching and is designed to not be automatic, or a single step solution, ie. Free your inner artist. Having said that, from the posted images, it seems that your initial stretch might help from a slightly higher SP, or focus point to hide the background noise a bit more and stretch the brighter nebulosity more but that is a matter of taste. Hope this helps

Your images are looking great! Technically, you can the blackpoint adjust before the first stretch, but practically this is hard to do. Generally I do it after the first big stretch. There are multiple ways of doing this, the easiest is with the linear stretch (I think it is call linear-prestretch), where you simply change the stretch type, set the blackpoint and then execute. The blackpoint will be substantial in most cases, and execution will shift the histogram significantly back to the left, and dim somewhat, the image again. Then it is time to use a much smaller, or negative b (much less stretch focus) and adjust your image. This part is quite initeractive and much to taste - but basically you can put the stretch and additiona contrast addition/subtraction wherever you want;. The documentation tries to give some hints of what to look for to avoid artifacts and highlight what you want. Otherwise, play a bit. Avoid creating addtional peaks and valleys in the histogram, or fix them if they appear. What the stars, and the previews and play. Soon you will get the "knack". C.S. Dave

Thought I would add a bit of a "quick" guide that should get you started - the primer is admittedly a bit of a book. For most deep sky, you just need fix three parameters to design the initial stretch, using the histogram as a guide. Put the focus point (SP) between where the histogram just leaves the bottom axis and the histogram peak, ie. to the left of the histogram peak. You will have to zoom in on the LHS of the histogram to pick this point and you will end up with a low value (<0.01, 0.001? maybe even 0.0001) This is where you will want to add the most contrast. Set the local stress intensity (degree of focus, b) value between 5 and 10 (you need a high degree of focus for the initial stretch, because most of the data is tight in the histogram peak - try 10!), then adjust D (the stretch amount) to put the stretched histogram peak somewhere about 1/5 to 1/4 of the way across to the right (you will have to zoom out of the histogram again) and then preview or execute! Now, the HT process only requires you to adjust one slider (the middle one) to control the amount of stretch, while the GHS script asks you to adjust three parameters, this is what allows you to "tailor, or design" the stretch for your image. Behind HT/STF is a great all-round stretching function, but is an "off the rack" fit. Setting three parameters should not be too onerous, we hope, and once you get the "knack" of it.... If you look at your stars in the stretched image, I would bet that they are less impacted by the stretch (more Gaussian in shape, less bloated) and dimmer nebulosity has been made visible. Do you like it? If you want to further adjust the stretch, I suggest trying a different SP (only fine edits of approx. 0.0001 are likely needed), you will see how this transforms the resultant histogram - you likely want the resultant to be as wide as possible. When changing SP look for a wider histogram result, with less shift to the right or back to the left. b (and D) will be too large if the peak separates into two peaks, but other than that, go for the maximum focus and stretch amount to taste. Use the histogram result and image previews as your guide, and then execute again. In a couple of tries you would have mastered this important initial stretch. You may then want to adjust the black point - this is deliberately put as a separate step in the GHS - using the linear stretch selection, which will end up shifting the histogram somewhat to the left again, dimming the background. Finally you can manipulate the stretch further (using smaller b and D factors) to make the stretch focus and amount less. Hope this helps.

Thanks Mike I also just posted an image of The Pleiades, trying to bring out the Merope nebulosity. https://astrob.in/sn56r7/0/ M45 is a tough nut because the stars are so bright and generally come with halos. Using the GHS Script, I am able to highly focus the stretch just behind the histogram peak, but above the noise (not at 0 or the black point), and watch all the faint nebulosity come alive. Because of the focused nature of the stretch, the stars (away from the focused stretch) remain intact. I would love to see what others think of the script too, but for me, it is my go-to tool now.

If you can browse the Pixinisght documentation, you will find the equations / methodology behind the arcsinh stretch. There is nothing particularly magic about the arcsinh stretch itself - just that it is a gentle, logarithmic stretch, that for modest stretches, distributes contrast evenly enough that artifacts don't arise in colors when it is employed. What the arcsinh stretch process iin Pixinsight does, is to stretch the luminance channel, and normalize the stretch factor (by dividing the actual stretched luminance by the unstretched luminance). Then this normalized stretch is applied across the individual colour channels to increase the standard deviation (saturation) of the individual channels to enhance the colors. If you do this with something harsher than logarithmic stretch, you will likely encounter colour artifacts, which is why the arcsinh function itself is used. If you keep repeating a very soft stretch, such as a logarithmic stretch or an arcsinh stretch on an image - you will get star bloat because while it appears gentler on the stars, repeated use or a very large stretch factor is way harder on the stars than even a true hyperbolic stretch, performed 1-3 times to get your viewable non-linear image. This is because the arcsinh stretch isn't particularly focused on your linear histogram peak. The arcsinh stretch process is indeed wonderful at enhancing colors, when not overdone, but you have to know its limitations. If you are a Pixiinsight users,, you can tailor make your stretch to best preserves your colors and avoid star bloat, with a new script shown here https://stargazerslounge.com/topic/387548-pixinsight-improved-image-stretching/. You can use this script to either alone (it is really good at preserving colors) or with a couple of shots of the arcsinh stretch process to further enhance them. Even if you don't use Pixinsight, you might find the documentation helpful. Clear Skies!