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Paul M

Great sky, shame about the stars!

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Another exceptionally transparent night last night. A pleasure to look up and see old friends in all the splendor! Not that I have time for that now I'm imaging! :)

Back out with the 250 PDS and ASI178MC. Guiding was really struggling for some reason, Might be a setting issue, I can't help myself.... :)

Main issue that I now realise with the scope/camera is bloated stars, really bloated stars. Spent ages on focusing and can't blame that. Also in the imaging train is a Baader MPCC, mostly as an adapter until I get a Click-Lock that'll give me suitable distance without the MPCC. It's correctly distanced(ish) so my bloated stars don't seem to be related to that.

My bloated stars may be down to oversampling:

image.thumb.png.e3e1108bf574296690f05baaa99a568c.png

Many of my subs, even at only 180 sec, have such eggy stars that they just won't stack. Threw away most of my subs from last night. Such is the curve of learning! :)

I did get some images for my efforts, big stars and all.

The Crab Nebula is one of the reasons I chose the 178 camera, it frames so nicely and I love the Crab. Less than half the 30 x 180 sec subs would stack and I didn't knock myself out on processing. Did I mention HUGE stars?

1802820128_TheCrab.thumb.jpg.7f0ca8b1ee177e7676f08cd60a266621.jpg

 

I know M42 is not a suitable target for my combo but I'm like a kid in a sweet shop :)  Had a go specifically for Trapezium. Two versions here. One giving more nebulousity and the other better for Trapezium.

M42.thumb.jpg.0985b26ee3b3ad9e683c597504632f18.jpg

 

Trapezium.thumb.jpg.e9c86bd20c193b88f2449e87134039cb.jpg

No point in working too hard on these targets until I get this bloating sorted. Binning?

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My limited experience is that over sampling won't make the stars bigger.  However,  I spent last Spring imaging at 2000mm with 6nm pixels, which I think was 0.6" per pixel.  After a few images I changed to 2 x 2 binning and the only difference that I could see was smaller file sizes and quicker processing times.  There was no noticeable difference in star size, and to be fair, they really aren't as "pin-point" as images taken at a shorter F/L.

You've hinted at tracking problems.  I suspect that this might be an issue.  If you have PHD2, then what sort of RMS" do you get?  Some nights I have 0.6, and other nights it can be about 1".  The difference in the star sizes is noticeable.

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8 minutes ago, don4l said:

You've hinted at tracking problems.  I suspect that this might be an issue.  If you have PHD2, then what sort of RMS" do you get?  Some nights I have 0.6, and other nights it can be about 1".  The difference in the star sizes is noticeable.

Unfortunately over the last 2 nights I've been struggling to get 1", much more at times.

I'm only just getting going with this imaging lark really, having got stuck in a cul-de-sac of Raspberry Pi problems through the summer, but when I was testing this exact same set up with Ekos/PHD2 I was getting .6" RMS error. So I can't blame either the hardware or the  software. I need to spend an evening on just the guiding. 

I'm back on APT (for now) but guiding is still PHD2. Now I think about it, the other night PHD2 wouldn't calibrate; "star not moving enough" type thing so I "fixed" it in one of the ASCOM settings... 🤔

Would erratic guiding in both axes produce almost round bloated stars? I have visions of trailing and non circular stars with poor guiding.

 

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A general rule of thumb is that your guiding rms should not exceed half your imaging pixel scale. With 0.4" /p, you'd need 0.2" guiding rms if all other conditions where ideal. But if you have 1" rms, that's 2.5 pixels. To get at least round stars, you need to make sure that a) guide ra rms is equal to dec rms, and b) that deviations in ra and dec are truly random.

As @Paul M (sorry, meant @don4l) wrote, seeing will be the most significant limiting factor in how fat  stars look in your images, once you have guiding under control. In practice, your guiding rms needs to be significantly better than your seeing. In your case, your imaging scale is working against you. Even with a premium mount on a high mountain top under excellent weather conditions, 0.4"/p is pushing the limit. I think you can safely bin your images 2x2 or even 3x3, and not see a real difference.

Finally, to get smaller stars in your images you have to pay attention to them during post processing. Especially during stretching, but you can also reduce them afterwards.

Edited by wimvb
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32 minutes ago, wimvb said:

Finally, to get smaller stars in your images you have to pay attention to them during post processing. Especially during stretching, but you can also reduce them afterwards.

I've noticed that these stars of mine are very sensitive to stretching but before I get that far, even in live view for focusing they just aren't sharp although the diffraction spikes do focus sharply the stars are fat and "fuzzy". This is what had me thinking about the MPCC maybe having an input. Right now I don't have the bits to mount the camera at focus without the MPCC acting as a spacer! :) I've got a Baader 2" ClickLock in my FLO basket as we speak https://www.firstlightoptics.com/adapters/baader-2-clicklock-m54-clamp.html. That should allow me to get the MPCC out of the equation not that I need it as a coma corrector with this camera; the considerable coma of the 250PDS is way out of frame!

My next stop RE guiding will be to turn it off and see how the stars are then. I was getting much, much better stars with my EOS1200D DSLR unguided, all be it at a much more appropriate image scale.

If it isn't basic mismatch of camera and scope and it isn't the MPCC, it leaves guiding and generally poor workmanship as prime suspects :)

This problem is now queued, waiting for another clear night.

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13 hours ago, Paul M said:

Did I mention HUGE stars?

1802820128_TheCrab.thumb.jpg.7f0ca8b1ee177e7676f08cd60a266621.jpg

 

In all fairness, it looks to me that the way stars look in this image is determined more by processing than by data capture issues.

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2 hours ago, wimvb said:

In all fairness, it looks to me that the way stars look in this image is determined more by processing than by data capture issues.

Indeed, given my skill set in the processing department that is a good call, but here is a single sub, presented here as raw as I can. Just opened in APT and stretched so you can see it. All 30 subs are there about the same.

image.thumb.png.631be94a6a7d88f2a5dcdf30c83b6c5a.png

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When it comes to reducing star sizes, layer masks are your friend 😊  oh, and noels actions (if you are using photoshop) 

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On 29/09/2020 at 18:58, Uranium235 said:

When it comes to reducing star sizes, layer masks are your friend 😊  oh, and noels actions (if you are using photoshop) 

I am a 'shopper but lack advanced skills.

I once played with layers but nothing fancy. Need to watch some YouTube tutorials.

Maybe wait for google to link Google assistant with Photoshop: "Hey Google, process my images"

:)

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