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Edge HD 8in for DSO

astrophotography cassegrain ccd Celestron CMOS imaging equipment SCT dso
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#26 Astrojedi

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Posted 26 November 2019 - 04:46 PM

It is true that one needs an sct e.g. for small galaxies or planetary nebulas.

 

But still, many dso's can be photographed around 1400mm. And for dso's one wants a speedy system.

 

Many dso's are simply too dark for an f10 scope, although that changes with every cmos generation that is more and more sensitive.

 

I admit that I still have to try using the edge more often natively and with a higher gain number or a more sensitive camera than the asi1600. Maybe f10 is then more usable than at unity gain?

 

 

The problem is not so much that the native edge 800 has a long focal length.

 

It is that it has f10 and not f7 or f5.

 

One now has things like the rasa. But that is a dedicated wide field concept.

 

Ideally, One would like something at 400mm for really wide fields. Such systems exist en masse (Celestron has the rasa now, and one can use affordable flat field refractors)

 

Then I would like something at 1200-1400mm for middle sized dso's at f5-8 (for which celestron only has problematic focal reducers with coma problems)

 

and one would like something with 2000-3000mm for really small planetary nebulas and small galaxies. At best this should also have f<10 but that would become untransportable. For that, Celestron has the edge hd line.

 

I personally use the edge 800 for small things, but for really small planetary nebulas, one needs probably an edge 11 or something... 

 

The problem is that we do not really have something unproblematic at around 1400mm focal length. 

 

The refractors in that area are extremely expensive.

If I use the 190mak newt from skywatcher, I get collimation problems. I would get spikes in the image and I would have to rebuild the scope basically.

An RC will give me the same collimation hell and create even stranger spikes.

 

So I think there would really be a market for a telescope of around 1400mm and f8-f5.6 which does not produce spikes and delivers the performance and problem free handling of the native edge 800.

 

At their native focal length, the models from the edge series are absolutely fine telescopes.

 

I am not totally sure what point you are trying to make. If you want to image at 1400-2000mm FL with a 8-10" scope then by definition it will be a slow scope. If you want a fast focal ratio long focal length scope it will start to get big / unwieldy very fast (not to mention expensive).

 

I think there are some misconceptions around F ratios and optimal sampling which are hangovers from CCD imaging and continue to persist. With low read noise cameras F ratio is less relevant and the impact on SNR is much less.

 

The EdgeHD 8 paired with a small pixel highly sensitive camera is sufficient for most small objects. I use the EdgeHD 8 at F7 and F7.5 with my ASI183MC & MM all the time for imaging smaller objects e.g. galaxies, planetaries etc. It produces superb SNR and detail. I love imaging galaxies and have never felt the need for a larger scope. For larger objects I pair with either the Redcat51 or my AT60ED. Delivers lovely high resolution wide fov images.



#27 WadeH237

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Posted 26 November 2019 - 05:08 PM

So I think there would really be a market for a telescope of around 1400mm and f8-f5.6 which does not produce spikes and delivers the performance and problem free handling of the native edge 800.

This exists, and it is...

 

The EdgeHD with the F/7 reducer.  I have been using that combination for a few years now and it works great (again, assuming a sufficient mount, some experience to deal with the realities of imaging at a longer focal length).


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#28 Benni123456

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Posted 26 November 2019 - 11:20 PM

 

The EdgeHD with the F/7 reducer.  I have been using that combination for a few years now and it works great (again, assuming a sufficient mount, some experience to deal with the realities of imaging at a longer focal length).

I think i have shown here countless examples of my edge 800 with 0.7 reducer. And all fail to be coma free and flat field.

 

Please show me one, just one single picture with an edge, an apsc sensor and the 0.7 reducer which shows a coma free flat field that has no visible color aberrations and a resolution of 0.5". I have yet to see one.

 

The edge natively delivers that. And it maybe mine does so too with reducer at 138mm after careful collimation but i have yet to find that out....


Edited by Benni123456, 26 November 2019 - 11:23 PM.


#29 Benni123456

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Posted 26 November 2019 - 11:22 PM

 

The EdgeHD 8 paired with a small pixel highly sensitive camera is sufficient for most small objects. I use the EdgeHD 8 at F7 and F7.5 with my ASI183MC & MM all the time for imaging smaller objects e.g. galaxies, planetaries etc. It produces superb SNR and detail. I love imaging galaxies and have never felt the need for a larger scope. For larger objects I pair with either the Redcat51 or my AT60ED. Delivers lovely high resolution wide fov images.

I think for planetary nebulas the edge 800 is a bit small. Some are really tiny. But then one has to transport it. Perhaps an edge 9 or 11 would be a compromise that is still transportable for these things.



#30 WadeH237

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Posted 27 November 2019 - 12:10 PM

Please show me one, just one single picture with an edge, an apsc sensor and the 0.7 reducer which shows a coma free flat field that has no visible color aberrations and a resolution of 0.5". I have yet to see one.

I don't have an APS-C size sensor, but here is a sample from my 4/3 sensor (an ASI1600MM-cool).

 

This is a mostly unprocessed image.  It was taken with my EdgeHD8 at F/7 with an image scale of 0.5 arc seconds per pixel.  The subs have been calibrated and integrated and I ran MureDenoise on the channel masters.  Other than that, I did DBE, PCC and a stretch.  The image is uncropped and otherwise unprocessed.

 

The spacing between the reducer and the sensor is 105mm.  I suspect that my focus offset for the blue filter was not correct for this run, as the blue subs are slightly out of focus as compared to the red and green.  Also, I suspect that sensor in my camera is slightly tilted.

 

There is some slight elongation in the corners, especially in the blue channel (as I mentioned, due to a slight defocus).  As you say, it is not completely coma free and flat, but it's not all that bad - and I am almost certain that some of it is tilt inside the camera.

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#31 Benni123456

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Posted 27 November 2019 - 05:33 PM

 

The spacing between the reducer and the sensor is 105mm.  I suspect that my focus offset for the blue filter was not correct for this run, as the blue subs are slightly out of focus as compared to the red and green.

Actually the problem can not be reduced with a different focus for each filter (I tried that).

 

The problem what you see is that the edge corrector has a color error (i.e. the spots show a different shape for each wavelength) and the reducer makes it worse.

 

And this color error is minimized for the pure edge at around 134mm. 

 

Can you, just for fun, try to make a photo at 134mm and then compare?

 

Because In mine, the stars are much smaller at 136mm with the reducer and the color error also slows down.

 

The fwhm at 136mm with the reducer in mine is close to the native edge. 

 

Maybe with yours it is different. I would really like to know..

 

Best regards...



#32 WadeH237

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Posted 28 November 2019 - 01:18 PM

Can you, just for fun, try to make a photo at 134mm and then compare?

My system is disassembled and in the house right now.  I have a new camera on order to replace the ASI1600.

 

When the new camera arrives, I am going to spend some time tweaking the spacing to get it dialed in as closely as possible.  The one thing is that the new camera has a Sony ICX 814 sensor, which is smaller than the 4/3 that I am using right now, so it's not a great test of the corners.



#33 Benni123456

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Posted 28 November 2019 - 03:06 PM

Thank you for trying.

 

I guess even with a small sensor, the stars should be smaller at the right distance.

 

With 105mm, they were huge blobs in my case. Now  they are so small at around 136mm with the reducer, that they look like hotpixels when I view the entire image (this is what the edge, as a 200mm scope, should deliver in fact.

 

At 105mm the image was worse than my 65mm refractor. I was really annoyed by this because then the edge, or at least the reducer would have been a total waste of money.

 

Now by using it at 136 at f7 I am much much more optimistic that I can find the correct spacing and use it at f7. And even then, the small deviation from round is not that problematic, I think. It was more the lack of sharpness over the entire image that was the problem for me at 105mm.

 

At f10 it is an absolutely fine scope. But it takes long to image at that f number.

 

Unfortunately, in winter, there are few clear nights in germany. And it is therefore difficult to try out several spacings



#34 Astrojedi

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Posted 02 December 2019 - 07:25 PM

I think for planetary nebulas the edge 800 is a bit small. Some are really tiny. But then one has to transport it. Perhaps an edge 9 or 11 would be a compromise that is still transportable for these things.

 

Here is the Cat's Eye nebula with my 8". This is just a live stack of 50x4s with no post processing - lucky imaging, longer exposure and deconvolution could bring out a lot more detail. Of course you could do better with a larger scope but there is no limit to that. By some random yardstick as you are using you could easily argue a 14" is too small.

 

cats-eye-2.jpg


Edited by Astrojedi, 02 December 2019 - 07:27 PM.



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