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Imaging with a Celestron 130


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On 17/09/2021 at 12:37, AstroMuni said:

Did you notice a big difference when you swapped mirrors? I had toyed with this idea before the pandemic hit us but wanted to understand the full potential of my scope the way its built - spherical mirror and all.

Not sure that anyone would put spherical mirror in F/5 newtonian.

Although it won't make much difference when imaging (seeing will mask any aberrations on that level) - it would seriously impact visual performance on planets and moon.

Spherical mirror won't suffer from coma - parabolic mirror will.

You should probably concentrate on collimating your scope first. Second thing to try to sort out would be focuser. Focusers on such scopes are rather poor. I've seen people 3d print replacement focuser adapters because original are not quite squared on tube.

ASI224 is too small sensor to seriously notice coma. Diffraction limited field has radius of F^3/90 (in millimeters) - where F is F/ratio of the scope - that means that diffraction limited field (as far as coma is concerned) has radius of ~1.4mm.

ASI224 has diagonal of 6mm - so for central part of sensor - coma would not be noticed even for planetary performance. DSO imaging is impacted by seeing.

Here is an example with F/6 telescope and x0.5 reducer and sensor the size of ASI224:

image.png.340e8558a872aaa4e7399ce66915ad6b.png

At F/6, diffraction limited field is 2.4mm but with x0.5 reducer that will be 1.2mm on sensor - less than 1.4mm with your setup. No coma to be seen.

 

 

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On 20/09/2021 at 10:54, vlaiv said:

Focusers on such scopes are rather poor. I've seen people 3d print replacement focuser adapters because original are not quite squared on tube.

I have noticed that and have tried to fix the wobble as best I could. Changing the focuser on this scope is beyond my skill capability :)

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  • 3 weeks later...
Posted (edited)

Here is my latest image capture. The Cocoon nebula (IC5146) almost 4000 light years away from us and spanning 15 light years. The bright star near the centre is lighting up the nebular glow and also clearing out the cavities. Cant imagine that we are looking at a star nursery where a few million stars are being born.

This image is made up of around 200 subs of 30sec each, stacked and processed in Siril and polished in Gimp. 

ic5146.thumb.jpeg.1322d2bc1646c896061ad29b9ae4af5d.jpeg

Edited by AstroMuni
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Posted (edited)
On 20/09/2021 at 10:54, vlaiv said:

You should probably concentrate on collimating your scope first. Second thing to try to sort out would be focuser. Focusers on such scopes are rather poor.

Got scope collimated and PTFE strips added to focuser to stabilise it, with help from a friend @skybadger The image above is post these changes :)

Edited by AstroMuni
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48 minutes ago, AstroMuni said:

Got scope collimated and PTFE strips added to focuser to stabilise it, with help from a friend @skybadger The image above is post these changes :)

I would say stars do look better.

That is with ASI224?

Things do still look a bit over sampled. ASI224 is small sensor. Maybe try to do mosaic? 3x3 with each panel being binned?

 

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1 hour ago, vlaiv said:

Maybe try to do mosaic? 3x3 with each panel being binned?

I will need a bit more info on how to do this :) Still learning. I use Ekos to capture my images. I think ASI224 allows a max of 2x2 binning.

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1 hour ago, AstroMuni said:

I will need a bit more info on how to do this :) Still learning. I use Ekos to capture my images. I think ASI224 allows a max of 2x2 binning.

Yes, sorry about that. I find these things rather easy as I've been working with computers all my life and I often forget that there is learning curve.

Here is what it means in nutshell. Your image has quite large stars. It means that it is over sampled. Over sampled images loose SNR because light is spread too much over pixels. They look blurry as well - like when you magnify image too much.

What I'm proposing is to do following. Imagine that your image above was "more zoomed out" - like this:

ic5146.jpg.cb420d304fd1823ab9d5a7b92b190adf.jpg

but instead of being small - it was larger - like having that additional parts of the sky imaged next to basic image.

This is called mosaic and is made out of panels. You shoot individual images like you did of central portion - but also surrounding parts as well. That is first step. Some imaging software has mosaic planners - which simplify things, but you can do it via Stellarium or other planetarium software. Here is example:

You start by determining overlap points - like this (put one in each corner):

image.png.f69353bf8805b1e1f9c7b669af1bc738.png

(Use shift + left click to put marker and shift + right click to remove it)

Now move FOV so that it is positioned as particular panel of mosaic - and put marker in the center:

image.png.231e5eda613c2928b9baf03429462442.png

This is top right panel from 3x3.

You can now read off RA/DEC coordinates for that marker - so you know where to position the scope. If you can't do that precisely - then you'll have to look at the stars and aim for particular star close to marker to be in the center of the FOV.

image.png.136278f57e0c238bf212232cc3eec771.png

That way you can get positions for each of 9 panels.

You divide your session into 9 equal parts and take subs for each of those 9 panels (you only need one set of calibration frames).

Once you are done - stack them like you normally would - like you did for image above, but once you are done stacking - while data is still linear - you bin your data x3 (for 3x3 mosaic). This is done in processing software. If you don't have any software that supports this option - get ImageJ (it is open source scientific image manipulation software written in Java so it will run on any OS) - it has option to bin your data under:

Image / Transform / bin:

image.png.ec1717bd2ad9b547a78e5434b58210ef.png

Just select 3x3 in this case and average method:

image.png.8e891d83b8bb81f400b42b059406047a.png

Do that with all 9 images. Then make composite image out of them. You can do this with various ImageJ plugins that will create mosaic (it is called Stitching). Advantage of doing it like that is that you can do it while still linear.

Alternatively - process each of 9 panels and then use Microsoft ICE to stitch them into final image. There is also iMerge software that will stitch already stretched images together. Or you can do it in Gimp if you want - by using layers, translation and rotation.

For example - that is how I created following image (you can actually see panels because image is very stretched and I did not use flats):

image.png.0dba4ca636404a8c29dac3c65afde35f.png

Above image was also done with very small sensor / planetary type camera on similar focal length to yours - at 500mm.

I know this all sounds a bit intimidating - but why not give it a go?

 

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5 hours ago, vlaiv said:

Once you are done - stack them like you normally would - like you did for image above, but once you are done stacking - while data is still linear - you bin your data x3 (for 3x3 mosaic).

Ekos has a mosaic capture option. Will give it a shot. Need to check if SIRIL will stitch and align the 3 x 3 into one larger image. I use a Linux machine so limited options for stitching. 👍

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  • 5 weeks later...

I have acquired my first guide scope and managed to attach it to the screw thread provided for attaching camera to top of tube rings. Had a few discussions with FLO who were very helpful. Also my local astro society were equally helpful in choosing the right scope etc. Now to practice taking images with guiding. Fingers crossed.

205296929_guidescopeattached.thumb.jpeg.ebb7f73c0ee3ae82c210b9123fb6ba79.jpeg

Edited by AstroMuni
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