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Russe

Imaging with the 130pds

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2 hours ago, moise212 said:

replace this weekend the secondary mirror with a larger one

Ah, really? AFAICT it's the only sw Newtonian that doesn't lose light by missing the secondary. Admittedly, our test is crude. Hold paper at the open end of the tube with the telescope pointed at the sun. On the 130, nada. On all other sws we've tried, you get a nice focused point of light, enough to burn the paper where the light from the primary is not intercepted by the secondary. There are some details here.

Oh, and tremendous globular. Not much wrong with the secondary there!

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2 minutes ago, Gerry Casa Christiana said:

So 138mm divided by diameter of telescope in mm :) 

That's Rayleigh's. For an 130mm mirror, Rayleigh's limit is ~1.06" while Dawe's is ~0.89".

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3 minutes ago, alacant said:

Ah, really? AFAICT it's the only sw Newtonian that doesn't lose light by missing the secondary. Admittedly, our test is crude. Hold paper at the open end of the tube with the telescope pointed at the sun. On the 130, nada. On all other sws we've tried, you get a nice focused point of light, enough to burn the paper where the light from the primary is not intercepted by the secondary. There are some details here.

Oh, and tremendous globular. Not much wrong with the secondary there!

Thanks! Actually, the image is cropped a bit and, however, I struggled a lot to remove the gradients and towards the corners they have waves patterns. What I heard is that there are 2 secondary mirror types. Ones for visual (which should be my case) and ones for imaging. I measured mine to be ~36mm wide, while the imaging one should be 47mm. It also appears that I have some astigmatism, but I find the weird gradients far more disturbing. I need to flatten the images a lot. If I were to go for IFN, I'd better do something else.

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1 hour ago, Gerry Casa Christiana said:

So 138 divided by diameter of telescope in mm :) 

I only had to check it becuase its not very often you come across a camera with such small pixels (2.4  microns) - 99% of the time DSO imagers dont have to think about it becuase they usually never get close to the resolving limits of their telescope.

Though I've not really seen any negative consequences of breaking that limit, there must be some penalty - but there arent that many discussions on the subject that I have found.

I did find a sensor with 1.6micron pixels, but I cant see any practical application for it... you'd need a large aperture and a short short focal legth to satisfy that - and we all know that would be insanely expensive!

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1 hour ago, Uranium235 said:

Though I've not really seen any negative consequences of breaking that limit, there must be some penalty

Hi,

Surely there aren't actually any *actively negative* consequences? I mean, the telescope's resolution will continue the same regardless of how many camera pixels its image falls onto? So in theory, the only downsides are secondary, things like hotter camera (more pixels, probably less efficient cooling p/p) and also far larger file size per mm^2 of sensor?

John

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3 hours ago, Uranium235 said:

Though I've not really seen any negative consequences of breaking that limit, there must be some penalty - but there arent that many discussions on the subject that I have found.

Presumably if your scope oversamples, then you may as well bin to get a better signal:noise, if it undersamples (130P-DS with a DSLR, say)  just live with it or fit a barlow :-)

My recent moon shots were with a 150mm scope, theoretically about 0.75" resolution, but 0.25" per pixel - in theory about 0.35" would be ideal (nyquist theorem - sampling should be at twice the frequency of the highest frequency you want to record - applies to spatial frequency of an image as well as sound*).

0.75" equates to a resolution of 1.5km on the moon but in practice I seem to have got features down to under 1km after stacking.

Even though it's technically oversampled, the shapes of craters and mountains etc. are smooth down to the pixel level. Although oversampling may not give any extra image information, it does improve the appearance of the image, just as 'oversampling DACs' give a more pleasant sound.

 

*To explain this in more detail - to record a 20KHz signal you need a 40KHz sample rate to record the high and low parts of the signal. Equally if Raleigh says you can resolve two stars 1" apart, you need three pixels spaced at 0.5" to record the darker zone between them, or four pixels spread over 2" to be able to always record the stars as two points.

 

Edited by Stub Mandrel
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I'm familiar with the nyquist theory in both imaging and sound engineering (though its more obvious in sound) - but I suppose if you go past the rayleigh limit and into oversampling its capability then there really isnt much of a win or lose, as youre always going to be limited by the aperture (a bit like having a limiter on your car). Especially when you take into account that for long exposures, the sky in the UK is never good enough to support that kind of resolution.

Perhaps futher tweaks could be made, the 130 has been through a house move and hasnt had its collimation checked for about 6 months - so tweaking that (or at least checking it) is probably worth it. But for now the biggest issue remains the dark frame calibration of the ASI178, as it seems to prefer darks taken in the same session (though not too difficult due to the short exposures required).

However, those tiny pixels seem to have turned the 130 into a very unlikely galaxy smasher :)

Group2_L_80_web.thumb.jpg.b53f4e95bb50c6e657114f0eab445736.jpg

(only 20x2min)

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One more processed. I just installed yesterday PixInsight, but for now I seem to get better results with my old processing style.

This is 7h of Lum and 48min each RGB. Light pollution - not max, but still red.

One more M92 to be processed.

Alex

M51-F588-2018-04-22-LRGB_p01_half.jpg

 

And one 100% resolution crop.

M51-F588-2018-04-22-LRGB_p01_crop.thumb.png.2675a8712b69e409f3d00a81fe4cec56.png

Edited by moise212
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2 minutes ago, Uranium235 said:

I'm familiar with the nyquist theory in both imaging and sound engineering (though its more obvious in sound) - but I suppose if you go past the rayleigh limit and into oversampling its capability then there really isnt much of a win or lose, as youre always going to be limited by the aperture (a bit like having a limiter on your car). Especially when you take into account that for long exposures, the sky in the UK is never good enough to support that kind of resolution.

Perhaps futher tweaks could be made, the 130 has been through a house move and hasnt had its collimation checked for about 6 months - so tweaking that (or at least checking it) is probably worth it. But for now the biggest issue remains the dark frame calibration of the ASI178, as it seems to prefer darks taken in the same session (though not too difficult due to the short exposures required).

However, those tiny pixels seem to have turned the 130 into a very unlikely galaxy smasher :)

Group2_L_80_web.thumb.jpg.b53f4e95bb50c6e657114f0eab445736.jpg

Very nice details! You just beat me with your post.

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1 minute ago, moise212 said:

Very nice details! You just beat me with your post.

:D

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4 minutes ago, Uranium235 said:

I'm familiar with the nyquist theory in both imaging and sound engineering (though its more obvious in sound) - but I suppose if you go past the rayleigh limit and into oversampling its capability then there really isnt much of a win or lose, as youre always going to be limited by the aperture (a bit like having a limiter on your car). Especially when you take into account that for long exposures, the sky in the UK is never good enough to support that kind of resolution.

Craig Stark's famous pdf goes into more detail - he suggests you work out what is the limit on your resolution (seeing or aperture) and what that limit is, then select a pixel size of roughly half that. I'd guess teh AS178 is very well matched to the 130PD-s under our wonderful British skies :icon_biggrin:

Out of interest - where did you take that image of Bodes?

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4 minutes ago, Stub Mandrel said:

Out of interest - where did you take that image of Bodes?

In me back garden :) I've recently moved to the edge of town and in a northerly direction there is nothing for miles so that part of the sky is quite good. West is awful though, South is a bit "meh" and East is good(ish). Plus the bodes were near the zenith - so not too hard to get a good image.

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Hi everyone

just thought I would share an experience over the last week that may help others.
I stripped the 130PDS last year to do a few modifications. whilst doing so I had to re-collimate the focuser.
Now everything was put back together and measurements all got taken using a set of digital calipers.

All this winter I have been in a constant battle trying to get rid of coma, I thought it could have been an issue with the camera when turning it full spectrum but nothing seemed to solve it.
Last week after taking it off the mount for the summer season I decided to give it a good clean and move the primary up slightly (instead of chopping the focuser tube).
Whilst checking everything I re-checked the collimation of the focuser, this time with an older well made set of calipers I was given earlier this year.
The collimation was off by a few mm in both directions. I went and got the digital calipers out and everything measured correctly.
After checking both sets of calipers against a steel rule it turns out that the digital calipers are indeed out of wack.

I have since re-collimated the focuser using the manual calipers, thrown the digital calipers in the bin and moved the primary up by a few mm.
I will not be putting the scope back on the mount for a few weeks yet but I think I may have found my issue.

I guess the moto of the story and what I am trying to get across is check your calipers.

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Creepy. I find collimation a frightening and nerve-wracking experience, despite the fact that ive read plenty of guides on how to do it. Last time I collimated (after taking the scope apart for putting black felt inside), I ended up going round in circles with aligning the secondary dead-centre and then being forced to tilt toe primary so much it was almost falling out of the tube! :D In the end, I got the right angle  with the secondary and it seemed to work.

I did cheat just a little tiny bit though :D--I slightly adjusted the secondary vanes to bring the secondary into the centre. Only by about 1 mm, I decided to get a compromise between the mirror being centred and the secondary being true to the focuser. ;) 

John

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The answer to collimation problems comes from adding good collimation screws to the secondary and to use such a wonderfull collimation tool named Hotech laser.

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11 hours ago, Stub Mandrel said:

Craig Stark's famous pdf goes into more detail - he suggests you work out what is the limit on your resolution (seeing or aperture) and what that limit is, then select a pixel size of roughly half that. I'd guess teh AS178 is very well matched to the 130PD-s under our wonderful British skies :icon_biggrin:

Out of interest - where did you take that image of Bodes?

Last Friday I tried to use my brand new ASI385MC with my other scope, the SW200PDS, the idea was to use the combination as a globular cluster, planetary nebula and galaxies smasher. The 385MC has a 3.75 micro pixel but with doubled sensitivity with regard to the 185MC.

But maybe I will give a try to the 130PDS as well after Uranium tests.

Take a look here below:

 

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Even the best digital calipers are vulnerable to low batteries and especially dirt on the scale which need not be particularly noticeable to cause significant misbehavour.

If find that a wipeover with methylated spirit is usually the most effective way of restoring order.

Potentially even cheap ones are accurate, but pay more to get better finish/operation, better battery life and greater temperature stability. Best brand is Mitutoyo Absolute followed by Moore and Wright IMHO, I have both but still use cheapies for various duties.

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Another little run, this time with the Baader MkIII corrector. Oversampled, but not bad for just two hours:

130pds + ASI178MM (UV/IR filter only)

Uncalibrated at the moment, needs new flats :)

Group1_noflats75.thumb.jpg.7bce2356ca82639eee1fb84a00f8c607.jpg

 

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Which settings did you use Uranium?, I mean gain, offset, times....

For my attempt of NGC4565 with the SW200PDS and the ASI385MC I did use a gain of 420, time 15s, I do not remember the offset, but the histogram was around the 75%.

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2 hours ago, Susaron said:

Which settings did you use Uranium?, I mean gain, offset, times....

For my attempt of NGC4565 with the SW200PDS and the ASI385MC I did use a gain of 420, time 15s, I do not remember the offset, but the histogram was around the 75%.

I used the max dynamic range preset in the ascom driver control panel.

Exposure was 60x120s, it could have been 3min subs but I didn't want to take new darks. To be honest, 15s is a bit too short.

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Now calibrated. Still a bit noisy, but perhaps double subs would sort that out. Quite a lot of background galaxies lurking in-shot:

27004392377_1a8cd52b5a_k.jpg

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Today I am going to try the couple M53-NGC5053, fingers crossed.

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lol... that image just got into Flickr explore :D not bad for a bargain basement camera and telescope.

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