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NigeB

NGC 2683: A 'frac user's experience with an EdgeHD 14

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Evening All

A few weeks ago I decided to take my TOA150 off the mount for a while to give the Celestron Edge 14 a proper try-out. The refractor is beautifully straightforward to use - it "just works", has a wide field which makes plate solving easy, and a reasonably trouble-free focuser. I anticipated some problems moving to a long focal length mirror-shift SCT.

I've fitted the Edge with a Lakeside motor focuser - rather than going the Crayford route I've put it on the normal primary mirror focus knob. I expected this to throw up challenges due to mirror shift, but after some experimentation with SGP settings I found the right combination of focus direction and backlash compensation which generates good V-curves and little net image shift between the beginning and the end of the focus run. I can definitely recommend the solution which Peter at Lakeside has come up with for this fitting.

I'm using the OTA at native F/11 (3910 mm focal length), with an Atik 460ex CCD and Lodestar guider via an OAG on a Mesu-200 mount. Plate solving was an issue to begin with - the 460ex imaging area is quite small, and the system is slow. I've found that adopting 4 x 4 binning and 15 sec exposures for plate solving captures, usually works, and I can now centre a target in the middle of the frame from any part of the sky in 2 or max 3 plate solve cycles.

By far the biggest challenge with the entire thing so far has been finding and keeping guidestars - in the wide field of the refractor it's hardly ever a problem, but with the 14" it's a real challenge for some targets because the area of sky captured by the Lodestar is so small. Very often there aren't any usefully bright ones available in the 4-8 second guide exposure times I'm using in PHD2, and now and again I've hit problems in which the guide star is lost because it has moved out of the guide square in PHD2 after a focus run - although I seem to have managed to tune most of that out now (I really wish there was a setting in SGP to perform a re-centre after auto-focus - that would solve lots of problems). 

Guiding at f/11 has posed no problem at all - really flat PHD2 curves. The "Target" view in PHD2 has shown clustering of points within the 0.5" circle most of the time this week. Collimation is probably not brilliant, and I've found having to think about that business to be a bit of a pain compared to the trouble-free experience of the refractor - not that it's difficult, just that it's something you have to do.

At the beginning I spent a night jumping around from object to object, marvelling at how targets that are tiny in the refractor field of view suddenly fill the frame. But eventually I settled on NGC 2683 as the target for a proper first attempt at imaging with this combination. 

Result shown below. Exposures: 26 x L, 10 x R, 9 x G, 9 x B subs - all 600 sec with 2 x 2 binning.

It's clearly far from perfect - I've tried not to introduce too much noise in processing but there's definitely noise there with mottled background being a real issue - though that's my processing not the instrument. I also have a problem in L with a couple of dust bunnies that aren't flat fielding out properly - either the filter wheel is not returning to exactly the same position, or I've had a small shift in the orientation of the camera. Need to check collimation as well.

I think it's promising although there's more work to be done on the image and on the setup. However, by coincidence I'm posting on the same evening as @wimvb who has posted an image of the same target taken with an MN190 and 460ex, and the level of detail in Wim's image compares very well indeed with mine, despite the difference in the aperture and plate scale of the combination used - make of that what you will. I have my own views on why, and ultimately it's the refractor which will be mounted for most of the time - but the 14" certainly offers a lot of different options for imaging and I think it's worth working to tame the challenges which the design introduces.

 

Nigel

 

NGC 2683

 

Edited by NigeB
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Very nice image, Nigel. You definitely got a lot more detail in the main galaxy and its small neighbour than I have. That's the advantage of a much larger fl and aperture. But ultimately, I think it's the European skies that work against us.

When I discussed possible candidates for my scope update, Göran first suggested the EdgeHD 8 and MN190, while I was contemplating an astrograph like TS 10" ONTC or Lacertas 10" Newton reflector. But at about the same time, I grew tired of star spikes, and finally I chose the MN190.

Regarding the background in your image: the small scale noise looks like it's from deconvolution. You can avoid it by either using a strong mask, or by increasing the regularisation parameters slightly (especially the value for layer 1). I always spend quite some time tweaking the settings.

The colour mottle can be killed by MultiscaleMedianTransformation on chrominance, with a 50% mask (applied inverted)

MMT.jpg.542c049c8c81042ad8ccfa57a1ff5b06.jpg chromamask.thumb.jpg.e22bda7d017f9cec395d18a83b12833b.jpg

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Very nice result there, Nigel, with plenty of detail.

It almost looks to me as if there is another galaxy superimposed on it, just above the centre at a slightly more horizontal inclination - an optical illusion, I guess due to looking through the central bulge.

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8 hours ago, wimvb said:

Very nice image, Nigel. You definitely got a lot more detail in the main galaxy and its small neighbour than I have. That's the advantage of a much larger fl and aperture. But ultimately, I think it's the European skies that work against us.

When I discussed possible candidates for my scope update, Göran first suggested the EdgeHD 8 and MN190, while I was contemplating an astrograph like TS 10" ONTC or Lacertas 10" Newton reflector. But at about the same time, I grew tired of star spikes, and finally I chose the MN190.

Regarding the background in your image: the small scale noise looks like it's from deconvolution. You can avoid it by either using a strong mask, or by increasing the regularisation parameters slightly (especially the value for layer 1). I always spend quite some time tweaking the settings.

The colour mottle can be killed by MultiscaleMedianTransformation on chrominance, with a 50% mask (applied inverted)

 

Hi Wim,

Many thanks! Yes, I think seeing is the key limiting factor. The 14" was an itch I had to scratch, and it doesn't disappoint, but previously I've noticed how similar deep sky images from the 'frac and my C11 are. Planetary imaging is a different matter and that may be a good reason for keeping the 14" on the mount, but I've not tried that yet (unlikely to either, unless the trees in the wood opposite suddenly fall down so I can see below about 30° altitude!)

I think you probably made the right decision with the MN190 vs the Edge 8... The mechanical compromises with the Celestron do take the "edge" off slightly.

Thank you very much for that processing advice - this is really helpful, and I'm going to try re-processing the image adding this technique - I'll re-post when done.

Best Regards

Nigel

 

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14 minutes ago, NigeB said:

unless the trees in the wood opposite suddenly fall down so I can see below about 30° altitude!

I can relate to that. I've never been able to image the Orion neb properly, due to either houses or trees being in the line of sight. The trees are nowadays my own. I may remedy that problem some day. With a chain saw. ?

19 minutes ago, NigeB said:

I'm going to try re-processing the image adding this technique - I'll re-post when done

Good luck.

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Great image Nigel and an entertaining and  interesting read. I have an EdgeHD11 essentially unpacked since I got it in the US three years ago at a very good price that became too tempting. But it has been sitting in a closet since then as my fracs have kept me more than happy. But it is nice to know it is there and that I can take it out and put it on the mount one day.

As you say your image is not that much different from Wim's, but it does show a bit more details. One thing that Wim's scope appears to do better is to keep the star sizes down. It seems to be a well known fact that SCTs give big stars. Does anyone know why? Both scopes are reflectors but maybe it is the central hole in the primary mirror that somehow makes stars bigger? Not a major problem and if someone cares it is easily fixed with some star shrinking during processing.

Edited by gorann
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Well done on the technical side! Nice galaxy, too.

I think the acid test is to compare the same object imaged in both instruments.

Olly

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1 hour ago, gorann said:

It seems to be a well known fact that SCTs give big stars. Does anyone know why? Both scopes are reflectors but maybe it is the central hole in the primary mirror that somehow makes stars bigger?

I think that that is part of the reason. The larger central obstruction (secondary and hole in the primary) pushes more light out to the edge of the diffraction pattern. And if bright stars are overexposed, the brightness will be further out. This seems to be the major reason here, since there is detail in the galaxy (not overexposed), but the stars are fat (pverexposed). Another reason is that the fl gives a much smaller field of view with any given camera (here the Atik 460), and if that field is stretched across a computer screen, then stars will look bigger. Further, the longer fl demands more of guiding. If you can guide 0.5" RMS  when you image at 1.2" (1 m with 5.86 um pixels, my camera and scope combo), you're doing fine. But if you get the same guiding RMS when you image at quadruple the fl (3.9 m combined with 4.54 um pixels, that's roughly 0.23"), then you will see fatter stars.

1 hour ago, gorann said:

it is easily fixed with some star shrinking during processing.

I would actually use the same technique as proposed in other threads by @ollypenrice: stretch one layer (image) for detail in the galaxy and another layer (image) for stars. Then combine the images with a mask. In PixInsight one would use masked stretch for the second image, and pixelmath to combine.

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I think that's a very nice result.  Sure, it's a shame that there is a fair bit of noise in the background but you have nice round stars and no chromatic aberration (which I seem to suffer sometimes with my Edge 11...)

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4 hours ago, gorann said:

Great image Nigel and an entertaining and  interesting read. I have an EdgeHD11 essentially unpacked since I got it in the US three years ago at a very good price that became too tempting. But it has been sitting in a closet since then as my fracs have kept me more than happy. But it is nice to know it is there and that I can take it out and put it on the mount one day.

As you say your image is not that much different from Wim's, but it does show a bit more details. One thing that Wim's scope appears to do better is to keep the star sizes down. It seems to be a well known fact that SCTs give big stars. Does anyone know why? Both scopes are reflectors but maybe it is the central hole in the primary mirror that somehow makes stars bigger? Not a major problem and if someone cares it is easily fixed with some star shrinking during processing.

Thanks Gorann! I think my EdgeHD14 is a similar story - good price so an opportunistic buy, but ultimately I find it hard to convince myself to remove the frac from the mount. But I think it's definitely worth a try.

Regarding the soft stars - I think Wim has captured the situation very well in his reply to you. Certainly my experience with side-by-side comparisons between the TOA150 and C11 co-aligned is that the performance of these two designs is much closer than often portrayed in some forums - once you take into account the very high plate scales of the SCT and try to match that with the refractor, things look a lot more equal between a good frac and a well collimated SCT. Not perfectly so - that central obstruction in the SCT does spread the power out around the central maximum more than in the case of a refractor - but as Wim has identified, it's only part of the story. Much is down to simple image scale.

I don't regret for one moment the money I spent on the 'frac, but it shows just how good modern SCT's are; in terms of "bang for buck" they're really amazing - even if a little more maintenance intensive to get good results from.

Regards

Nigel

 

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Nigel, your post got me closer to put my big(ish) SCT on the mount, but first I have to replace my EQ8 with the Mesu 200 that has been on my living room table for over a month now (no major problem since we usually eat in the kitchen). I just need some clear nights in a row to get the energy to take on all the hassle of going from a handset to running it all from a computer and losing a night or two of imaging.

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1 minute ago, gorann said:

Nigel, your post got me closer to put my big(ish) SCT on the mount, but first I have to replace my EQ8 with the Mesu 200 that has been on my living room table for over a month now (no major problem since we usually eat in the kitchen). I just need some clear nights in a row to get the energy to take on all the hassle of going from a handset to running it all from a computer and losing a night or two of imaging.

 

Now that you have multiple mounts, you can always put the sct on the eq8 next to your obsy. Assuming you have the eq8 tripod as well. ?

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22 minutes ago, gorann said:

Nigel, your post got me closer to put my big(ish) SCT on the mount, but first I have to replace my EQ8 with the Mesu 200 that has been on my living room table for over a month now (no major problem since we usually eat in the kitchen). I just need some clear nights in a row to get the energy to take on all the hassle of going from a handset to running it all from a computer and losing a night or two of imaging.

 

Gorann - definitely worth doing, but I feel the pain! Note that I've tried mounting my TOA150 + C11 together on the Mesu, and it's fine; I found some of the currently available side-by-side mounting plates introduced a bit of flexibility and lengthened damping times noticeably, and there were some issues with cameras clashing with my pillar - the dual mounting plate pushes the OTAs out over the side, so you need to watch out. In the end I found a Casady tandem bar used which is a phenomenal lump makes for a stable setup. So in principle you don't need to have either/or, you can have both your Esprit 150 and C11 (and probably the ED80 as well) up and running together. 

Good luck!

 

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Right now I have my Esprits 100 and 150 side by side, but for the galaxy season your suggestion is one to consider (and it had struck me) . Maybe I find my AP becoming too much routine one day and will start looking for trouble..... By the way, what happens when the Mesu runs your cameras into the pillar?? Nice to know!

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1 hour ago, gorann said:

By the way, what happens when the Mesu runs your cameras into the pillar?? Nice to know!

Well, it's definitely to be avoided of course, but accidents do happen, and there are two features which offer some protection. This is a friction drive mount and there are no clutches. Even when it's tracking, it's possible to move the telescope by hand without releasing anything if you really want to. So conversely, if there's a clash, the system will slip rather than crumple your camera, and that should save major issues. There are encoders on both the motor and the axis in RA and Dec; if the Sitech detects a mismatch between the motor vs axis encoder (I think it's around 10° difference), it stops tracking (there's a message which appears on the on-screen Sitech panel). I've had that happen a few times when I was pushing my luck with meridian flip adjustments, and absolutely no harm done to mount or camera. If the clash is during a slew, then of course, there's a fair amount of momentum in the telescope assembly, and even if the mount stops, the motion can't be halted instantly without something giving - but again the friction drive seems to me to be the one most likely to keep your hardware intact because of the ability to slip if required.

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Thanks a lot, Sounds reasuring! It often happens that I have a nap before a flip, especially if it is at 3 am, and sometimes I do not hear the alarm......

Edited by gorann

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Update - I managed to get some clear skies and image the same target with the TOA-150 and a x1.5 extender (so it's running at f/11, matching the Edge's focal ratio).  Exposure details below.

NGC 2683 (The UFO Galaxy)

I've spent more time processing than I did with the Edge image ( @wimvb - many thanks for your tip regarding the mottled background control - it worked really well.) I've also found the reason for the really bad flatfield results in the previous attempt - a loose filter wheel adapter ring.

I find that in side-by-side comparison, when enlarging the refractor image to match the view given by the 14", the star sizes are similar - so seeing seems to be the limiting factor.

Exposure details:

Takahashi TOA-150 Refractor 
Takahashi 1.5 Extender
Atik 460ex CCD +EFW2 Filter Wheel & Baader LRGB Filters

L: 20 x 1200 sec + 2 x 600 sec
R: 10 x 1200 sec
G: 10 x 1200 sec
B: 7 x 1200 sec

(All 1x1 binning). A side-by-side comparison of the Edge 14 and TOA 150 versions, with the TOA image zoomed to match approximately the Edge 14 (full frame) version is also shown below.

The point being... nothing in particular, but it reinforces my view that the "softness" often claimed for SCTs is due more to plate scale than intrinsic optics - the central obstruction will have an effect, but this is not what people are seeing when they remark on softness. 

N.

 

Screen Shot 2019-03-04 at 22.40.53.png

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If we are seeing limited, as we are, I can't see any point in trying to image at scales below the limit of the seeing. Your TOA150 at native FL is working at 0.85 arcseconds per pixel. That's plenty! The truth is that you will rarely or never be able to achieve that resolution with UK seeing. You will be able to make a bigger object image by using an extender or the 14 inch but that bigger image will contain no details which are absent from the images by the refractor at native FL. This is what people call 'empty resolution.' You could just as well upsample the rafractor image to the size of the reflector image and have the same empty resolution.

In your instructive side by side comparison above I think the refractor image is a tad more highly resolved than the 14 inch and the stars are smaller and have less saturated cores.  I'm convinced that your TOA150 at native resolution will match or out perform the 14 inch for deep sky long exposure imaging. I would say, don't be fooled by the size of the output image from the 14 inch. Set the refractor image to the same screen size and look for the smallest resolved details in both.

What the TOA will not do is match the 14 inch visually on small targets or for fast frame imaging of planets or, possibly, deep sky imaging if using the lucky imaging approach with a CMOS camera. People are getting remarkable results this way on small bright DSOs because the approach beats the seeing and allows the big scope to show what it can do. If it's seeing dependent I don't believe it can.

Olly

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So in short, use fast frame rate CMOS camera for galaxies in the UK. Good to know. 

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2 hours ago, Lockie said:

So in short, use fast frame rate CMOS camera for galaxies in the UK. Good to know. 

Sort of. It works on bright dso's, but don't expect to catch tidal streams or ifn. With all dso imaging you can't beat the integration time. In general the result of your efforts depend on total time spent on a target. This means that if you go for 1 s subs, expect to fill your hard drive at 3600 subs/hour. You then need to sift through these in order to select the ones with best detail. These approved images determine the total integration time. After all, it's called Lucky imaging, so you need to select the lucky ones.

Dutchman Emil Kraaijkamp developed his stacking program Autostakkert! to help with this job.

http://www.astrokraai.nl

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33 minutes ago, wimvb said:

Sort of. It works on bright dso's, but don't expect to catch tidal streams or ifn. With all dso imaging you can't beat the integration time. In general the result of your efforts depend on total time spent on a target. This means that if you go for 1 s subs, expect to fill your hard drive at 3600 subs/hour. You then need to sift through these in order to select the ones with best detail. These approved images determine the total integration time. After all, it's called Lucky imaging, so you need to select the lucky ones.

Dutchman Emil Kraaijkamp developed his stacking program Autostakkert! to help with this job.

http://www.astrokraai.nl

I was going to say, the software picks the best frames for you, but good point about the hard drive space.

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Interesting, as I will be in a similar, though less extreme position soon.

I already have a 130mm f/7 triplet apo (TS, not TAK unfortunately), and a 12" f/6.8 ODK incoming. Putting the SX694 on the triplet, and a yet-to-be-bought Atik 16200 on the ODK gives very similar FoV, though the 'frac images at 1.04" PP, and the ODK at 0.61" PP. The Atik, however, has sufficient pixels to allow useful binning or cropping.

Will see how things work out.

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Great report NigeB.

Very interesting.

 

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