mike1485

Hi Matt There is the Image Inspection dialog which gives a preview with the ability to use zoom/pixel readout/etc - but of course it's not real-time. If all you need is a visual check of the image then I suggest applying the stretch by clicking the "tick" icon on the main window in the usual way. If you don't like it simply hit the undo button. You can compare before and after using "undo" and "redo". If you don't want to do this on your main image just clone it first by applying an identity stretch (Stretch Factor (D) = 0) with the create new image checkbox checked. I hope this helps. Mike

You can also click and drag the image to move it around as well.

This is a really interesting set of images. Thanks for sharing them.

Enjoy! Nothing but thick cloud down my way unfortunately .

Some great advice as usual from vlaiv. I am not a great fan of complete removal of stars using Starnet++ - as you have observed it tends to leave plenty of blotchy patches. To some extent these can become less noticeable when the stars are replaced. One little trick using Starnet++ is to apply an inverted Luminance mask to your image and then apply Starnet++. This has the effect of reducing your stars, making them a little less prominent in the image. In your image it looks like many of the brighter stars have become bloated and this may well have resulted when stretching the image. If you have been using Histogram Transformation for your stretching this process can be heavy on stars - ie, to get the result you want in the nebulosity you end up over-stretching the stars. I see you have posted in my recent posting on the GeneralisedHyperbolicStretch script, this script can help achieve a better stretch that will protect those stars.

Hi "Lazy" (if I may call you that!). Thanks for your interest and comments.

Hi 69boss302. Thanks for your comments. My collaborator in producing this script, Dave Payne, has recently shared "before" and "after" images of SH2-155 The Cave Nebula on Astrobin. I have copied his brief comments, which include links to the images, below: "One of the first things I wanted to do upon the release of the script, was to re-process some older images of mine that I though the script would allow me to do a "better" job on. My first choice was of SH2-155, The Cave Nebula. The original was stretched primarily with the CT (Curves) process, and my notes have me being very careful on the star -stretch -> https://www.astrobin.com/w0ojfx/0/?r=0 Using the script, I was able to attain better stars, and at the same time, really bring out the background nebulosity, particularly towards the top of the image. In addition, the nebulosity on the RHS of the image looks more three dimensional now -> https://www.astrobin.com/w0ojfx/B/?r=0 As an added bonus, the actual stretch took far less time the second time around." Dave Payne

Hi Phillyo, thanks for giving the GHS script a try and posting your feedback here - I appreciate your time and interest. I am sorry that you got "bogged down" in the documentation. I would hope the script should be pretty self explanatory without even opening the documentation and for many just the tooltips in the script will be plenty to get going and explore the benefits it can bring to your image processing. I do believe that, for many images, the additional control, flexibility and analytical tools provided by the GHS script, compared to a something like EZ Soft Stretch, can significantly improve the final result. Certainly not everyone will want to read the documentation "cover to cover" but it is there as a resource to come back to as you wish. If you feel there are some steps we could make to improve the accessibility of the GHS script please let me know. Thanks again for your feedback.

Hi Ouroboros - not a numpty question at all! Copy the javascript file to your Pixinsight/src/scripts folder. Then copy the GeneralisedHyperbolicStretch folder (containing the html and images folders) into your Pixinsight/doc/scripts folder. Once you have done this fire up Pixinsight and go to the Scripts>Feature scripts... menu item. Click the Add button and select your Pixinsight/src/scripts folder, hopefully it will report one new script found, namely Utilities>GeneralisedHyperbolicStretch which should be ticked for inclusion. Click Done and now go to your scripts, GeneralisedHyperbolicStretch should appear in your Utilities category. It should work fine on a Mac - I developed it on a MacBook Pro. Any problems post back here and I will try to help further. Clear Skies Mike

Hi Scotty. In the preferences dialog you can change the behaviour as you prefer. And your preferences are remembered between sessions.

I have been working over recent weeks on a new Pixinsight script designed to significantly enhance the image stretching functionality already available within Pixinsight. The script provides an integrated environment within which to design, appraise and apply stretches to astronomical images. It incorporates a new set of stretching equations, known as Generalised Hyperbolic Equations, that were initially proposed by Dave Payne. These equations have been found to provide a wide range of controllable and repeatable stretches that often facilitate significantly improved results compared to the traditional approaches. The script also incorporates the traditional Histogram Transformation and Arcsinh stretches and extends their functionality too. Dave and I hope this will be a useful additional resource for Pixinsight users and we encourage you to give it a try. The script, together with detailed documentation, is available from here: https://github.com/mikec1485/GHS/releases/tag/v1.0. We would welcome feedback from anyone who uses the script, including bug reports or suggestions for future development. Happy image processing and clear skies!

This is awesome - well done and thanks for sharing!

Thanks Alan. All good points and advice. I had noticed Starnet++ has a hard time with halos!

Thanks, this sounds like sensible advice.

I am taking my first tentative steps into narrowband imaging using ASI2600MM and Baader narrowband filters. The exposure calculators that are based on swamping read noise give very long exposure times for Narrowband imaging - understandable given the very small bandwidth being captured. However, even at 300 seconds I am finding that the centre of bright stars are completely filling the pixel well leading to clipping. This would only get worse as I increase exposure. It seems I have various options; Keep exposure time to the maximum that does not lead to clipping in the centre of stars - this leaves the bulk of the histogram well to the left (albeit separated from zero so not clipping at the low end) but this would seem to me to be leading to a very compressed dynamic range in the area of principle interest (nebula) and while multiple exposures making up equivalent time should compensate on SNR it feels like the dynamic range issue would remain and I am not really utilising the strengths of the ASI2600MM. Expose long, eg based on swamping read noise, and aim for histogram perhaps 25%/35% from the left - this is likely to give better SNR for the nebula but the stars will be clipped badly leaving all bright stars as clipped pure white and probably bloated. Expose long and then replace the stars from a separate RGB capture - replacing bloated stars with unbloated may be a processing challenge and furthermore it seems a little odd to be replacing with natural star colours when the rest of the image is probably combined based on a false colour eg SHO combination. Something else I haven't thought of that maybe someone here can enlighten me on!

Thank you both. Experience from the field sounds reassuring. I guess I am over-thinking it.

I have been thinking about getting a filter wheel to automate my image capture routine but one aspect concerns me. If I use just two filters over a night I can see that the flats for the first filter can be taken before the capture sequence starts and for the second filter can be taken at the end. But if three or more filters are used surely i would need to intervene and take flats when the filter change takes place? This would seem to rather negate the automation. If I leave it and take flats at the end doesn’t the motion of the filter wheel (potentially dislodging/moving dust specks) plus any slightly different alignment potentially cause an issue?

Older drivers did support offset. Discussion on ZWO forum is here: https://bbs.astronomy-imaging-camera.com/d/12956-latest-zwo-ascom-driver-isnt-supporting-offset-correctly

So, using SharpCap I have the following results. ASIAIR. Nebulosity SharpCap(Ascom) SharpCap(Native) Median: 2800 2856 2800 2827 Std dev: 6.5 1073.6 6.4 1081 So it appears Vlaiv has hit on a very plausible explanation for the different StdDev figures. Thanks Vlaiv. One thing I note with the Ascom driver is that there is no way to change the offset value. Apparently this is a known issue with the current version of the driver as distributed by ZWO and results from a problem with compatibility with the ASCOM platform on some older computers. I believe 70 is the Ascom default used for this camera (this is the value in the fits header for ASIAIR fits files). So this is the value I used for the "brightness" parameter in the Sharpcap native test and in the Nebulosity "offset" parameter. I am not sure what all this means in practice or why one driver should result in such different values from another. Any views/thoughts welcome!

Thank you. I will give that a go later and see what difference it makes.

Results from SharpCap consistent with Nebulosity. Median 2827, StdDev 1081. Still haven’t worked out how to use Ascom Driver in SharpCap.

Just to check - you are running the same offset setting in both trials?

Interesting results. I didn’t realise the driver could give such different results. I have not managed to figure out how to switch drivers to test my camera. I think it is currently using the native driver in sharpcap. I’m not sure what the ASIAIR would use as that is Linux based I think. Presumably something different again.

Thanks. That sounds like a useful avenue to check. I am not a SharpCap user but I can probably figure it out! Do I need SC Pro or does the free version give me the necessary functionality?

I am pretty new to this hobby and trying to gradually improve my understanding of what is going on "under the bonnet" (or "under the hood" for our cousins across the pond!) I have taken single bias frames with my ASI533MC camera, setting the gain at 100 and the offset at 70 (this value is set behind the scenes in the ASIAIR Pro). One bias frame with exposure time 0.001s was taken using the ASIAIR Pro and one using Nebulosity. So far as I can tell all settings are the same the only variation being the software used for capture. Analysing image statistics in Pixinsight shows: ASIAIR. Nebulosity Median: 2800 2856 Std dev: 6.5. 1073.6 Subject to some random variation I would have expected these parameters to be very similar irrespective of the method of capture. That seems to be the case for the median value but the standard deviation is very different. Can anyone shed any light on this to help me understand what is happening? Many thanks