Another (very faint) Ghost

[decay]

Hi all,

having seen Stefan’s stitched Iris and Ghost I decided to try the Ghost – in order to get a better feeling what’s possible at my location, now that I’m able to gather some more integration time using my new mount.

So here it is – a pale bit of nothing:

Ghost-1-6-1.jpg (524.36 KiB) Viewed 16822 times

- GSO 8” f/5 / EQ6-R Pro
- Baader MPCC / UV/IR cut
- EOS 2000Da
- 50/200 Guide Scope, ASI 120 MC-S
- INDI / Kstars / EKOS (Astroberry) Raspberry Pi 4b
- ASTAP

- 311 * 49s = 4.2 h
- Moon 63%

Imaging session was from 21:43 until 04:21, but to my surprise the outcome was only about 4 hours of light?! Probably lost due to dithering and/or saving the RAWs. Will have to check that.

@Stefan, thanks for the tip with the dew caps. I made them out of cardboard just for the moment to test how it works. And actually it worked pretty well (Oh my, that’s a lot of cardboard for an 8” Newtonian )

As I read Ron owns a SQM meter and Mike measures sky darkness with ASTAP, what I tried as well. The ASTAP result is as follows:

19.61 Bortle 5 suburban sky (at 72° altitude)

lightpollutionmap.info says 20.33 (which is still Bortle 5) but considering the Moon was shining at 63%, that’s not too bad, I guess. So this will be no longer an excuse for poor AP images for me. (But weather still will be )

As for processing with ST: Due to moonlight and LP domes the stack shows annoying gradients which were not so easy to get rid of. Don’t know why, but Vignetting preset in Wipe module with small corrections worked quite fine. For the final stretch I tried Stefan’s tip to use no ROI and instead I reduced Shadow Linearity to 5%. Then Contrast and Sharp modules using mostly default values. SVDecon (9 samples) worked fine for me, tightened the stars (seeing was average to bad) very well and revealed some small structures near the centre of the Ghost. In Colour module I used default Scientific rendering. Shrink with reduced Iterations (5) only, in order to retain star sizes. (Actually, I like the appearance of the stars in this case.) I did not use Super Structure module, but NR with bumped up (14) Grain Size and Grain Dispersion (9).

As always: Comments, hints, tips and critique are most welcome and appreciated.

Best regards, Dietmar.

[Stefan B]

Good one, Dietmar! Not an easy target with 60% moon.

Very good job on the stars.

The Ghost is already well visible. I usually use the 'No ROI' approach when the FOV contains more nebulosity than background. But here it might be a reasonable approach to get the faint stuff. Maybe we would even see more if more dynamic range would be allocated to the shadows. 5% seems to be a pretty small value. Did you try to push back the noise this way? Maybe this could be rather handled via 'Ignore Fine Detail' in Wipe and 'DimSmall' or even 'Isolate' in SuperStructure.

Is it correct that you used 49 sec subs? Seems rather short for such a faint target. I know that everybody is afraid of clipping signal but I think that clipped star cores are a small price to pay compared to getting small signals above the noise. So I'd aim for 3 min and beyond. This also reduces the amount of dither events and you don't lose integration time (and save disk space ... and let's don't forget the processing time...). Sensor temperature shouldn't be an issue anymore if increasing subexposure length...my scope was frosted over this morning and the OSC was cooling with 10 to 15% power to reach -10°C.

What ISO did you use? With my 1100Da I used ISO 800 for broadband and ISO 1600 for l-eNhance. Don't know how this compares to your 2000D but it might be worth trying to bump up the ISO at least one notch.

Anyway, I'd consider this image a winner as it is. And congrats to your Bortle 5 sky - it's probably Bortle 4 during new moon

Regards
Stefan

[decay]

Thanks, Stefan! Yes, probably not a very suitable target with 60% moon, but the night was quite clear for my location and the moon had low altitude on the other side of the sky and so I decided to try it anyway.

5% seems to be a pretty small value. Did you try to push back the noise this way? Maybe this could be rather handled via 'Ignore Fine Detail' in Wipe and 'DimSmall' or even 'Isolate' in SuperStructure.

Yes and yes. I tried a lot of other versions using Super Structure and the background structures came out more prominent, but the data set is much too noisy and the background looked terrible.

Is it correct that you used 49 sec subs? Seems rather short for such a faint target. I know that everybody is afraid of clipping signal but I think that clipped star cores are a small price to pay compared to getting small signals above the noise. So I'd aim for 3 min and beyond. This also reduces the amount of dither events and you don't lose integration time (and save disk space ... and let's don't forget the processing time...)

Yeah, don't why it is 49 secs. I would bet, it should have been 50 secs. But yes, usually I use 60 sec subs, but indeed, in order to avoid blown star cores I decided to try 50 secs. Background is still apart from the left side of the histogram. Do you think, longer exposure time would gain better SNR? Yes, probable I will go back to 60 secs, but longer seems to be too much for my setup ...

What ISO did you use?

Oh, I forgot to mention. I usually use ISO 800 for broadband and 1600 for l-eNhance sessions. Exposure time with l-eNhance is usually 240 secs.

Best regards, Dietmar.

[fmeireso]

Hi Dietmar,

I fully agree with Stefan. That is a really nice picture. It is well balanced, great starshapes en good color. Nebulae maybe a little bin dim but it still stands out well. And nicely framed!

Well done!

[decay]

Thanks, Freddy!

Nebulae maybe a little bin dim

Yes, but actually I'm quite happy with it. It is really a dim faint object and now I know that even some of the dark nebulae are possible for me. At least on some of the rare clear nights here. More integration time would probably help, but I think I will spend available time for more interesting objects.

And nicely framed!

Yes, I did my best To be honest, that was a bit of a disappointing moment, as I realised, that this object is not available in Kstars. But Kstars offers the possibility to search objects in online star catalogues. Problem was, that my Raspberry was not connected to the internet ... I will have to check that previous to the imaging sessions in future. Or write down the coordinates, as Ron suggested.

Best regards, Dietmar.

[Stefan B]

Hi Dietmar,

seems you already considered everything

But one part of your reply made me wonder...you said more than 60 secs is probably too much for your setup. In terms of tracking/guiding? Or overexposure? Probably not tracking since you use 240 sec subs for the l-eNhance filter, right? And the EQ6-R should be able to perform well enough with a 8'' newt.

But yes, usually I use 60 sec subs, but indeed, in order to avoid blown star cores I decided to try 50 secs. Background is still apart from the left side of the histogram. Do you think, longer exposure time would gain better SNR?

A 5 min sub will certainly yield better SNR than a 1 min sub, right? But the question is if a 5 min sub is better than 5 x 1 min subs. If the histogram is well apart from the left side I guess mathematics tells you that both are equally fine. I don't know if the equation considers everything and I am technically not educated enough to answer that in a profound manner. But I had good results with 5 min broadband subs. I've seen images based on 10 min broadband subs looking amazing. Of course depends also on your skies, temperature, moon, etc.

Regards
Stefan

[decay]

Hi Stefan,

But one part of your reply made me wonder...you said more than 60 secs is probably too much for your setup.

Of course, you're right. Firstly, my statement was worded unclear. I meant that more than 60 secs broadband subs at ISO 800 are too much for my setup. Secondly I'm still thinking like using my old mount. I tried to use as short exposure times as possible in order to maximise the chance to get some more subs not affected by problems with guiding. But of course, now I can And third I was confused by your statement

it might be worth trying to bump up the ISO at least one notch.

And I realised just yesterday afternoon, that I have tom bump down the ISO setting. OR maybe I'm confusing up and down?! And so I already did yesterday evening. Sorry, I should have written that earlier. Now I've used 120 secs subs at ISO 400.

Thank you!

Best regards, Dietmar.