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Old 26-03-2017, 07:17 PM
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RMS guiding errors and their consequences...

Hi all,

While waiting for clear skies, I started pondering, if it would be of any benefit to one day in a distant future get a bigger telescope.

My current setup allows for imaging at 1.61 app at f/4.5 and at 1.21 app at f/6. Taking seeing out of the equation, the most important aspect affecting actual level of detail in the data is guiding. From what I have seen so far, my mount has been guiding with RMS errors between 0.2-0.4 arcseconds. So during worse nights, and assuming standard distribution, about 68% of guiding will be within +-0.4 arcseconds, and the mount will be able to keep guiding to within +-0.8 arcseconds at all times (100% of samples). This suggests 1.6 arcseconds variation in total, about 1 pixel at f/4.5 and about 1.3 pixels at f/6. Worse guiding is most likely a result of worse seeing, so this would probably be masked by the seeing anyway.

However, for the better nights and with the mount performing the best it has so far, with RMS 0.2 arcseconds, meaning it kept on target to within +-0.2 arcseconds at 68% of the time, and at all times (100% of the samples) +-0.4 arcseconds = 0.8 arcseconds total variation, which is about half a pixel at f/4.5 and 0.7 pixels at f/6.

From those rough estimates, given I did not somewhere commit a mathematical crime, it looks like for exceptional nights it might be okay to put a slightly larger telescope in order to capture that extra level of detail with my current camera. Another option to justify buying a larger telescope could be upgrading the camera to one with larger pixels

Okay, enough of that, where are those clear skies?
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Old 26-03-2017, 08:59 PM
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Atmos (Colin)
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I guess what it really comes down to is image scale more so than focal length. Taking weight and money out of the question entirely, an ASA 14" F/3.6 Newt with a FLI-16803 will give an image scale of 1.45"/pixel. It is all about matching telescope and camera.

So far I am managed 5 minute subs with a FWHM of 1.7 at 1.158"/pixel, not entirely sure if that is seeing or telescope limited, suppose I'll never know unless I get some truly amazing seeing

So the simple answer to your question, yes, you can get a bigger telescope
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Old 26-03-2017, 09:18 PM
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Simple, bigger is almost always better - hehehe.
Besides I thought for the average site 1 arc/pixel is the target.

Greg.
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Old 26-03-2017, 09:24 PM
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That a nice FWHM Colin

Since you are using a premium mount my guess is that most of the time you are seeing limited and much less often image scale limited.

What I was curious about is how far we can push image scale until we become mount limited.

I remember that my AZEQ6 could occasionally guide at 0.4 arcseconds RMS, but usually was guiding at 0.6-0.7 RMS, so there would be not much benefit in imaging at 1 app with it. Since RMS error is for 68% only, guiding would be +-1.2 to +-1.4 arcseconds at all times = about 2.5 to 3 pixels error in total at 1 app (given there are no unexpected sudden large guiding errors). So light that should fall on one pixel would end up on up to 9 pixels on good nights.

Greg - I totally agree with you. 1 app is the target but few have mounts that will accurately guide to take advantage of such image scale.

My conclusion is that we can often aim for large scopes that will give us excellent image scale for those rare nights with exceptional seeing, but rarely we carefully take into account our mount's typical RMS guiding errors.
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Old 26-03-2017, 11:22 PM
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FWIW, I think that you can probably get a bit of a handle on the significance of guiding etc. using a simple approximation that assumes that the FWHM terms add in quadrature.

FWHM = SQRT(seeingFWHM^2 + scopeFWHM^2 + (2.35*guidingRMS)^2)

there are some complications : the scope and camera FWHM is not exactly the same as the scope resolution, since the camera will introduce more spreading via sampling point variations and crosstalk. In addition, the guiding RMS is definitely not independent of the seeing (as assumed in the above equation) and it will also vary with guide update rate and Dec drift rate (polar alignment accuracy). However, the above still gives some idea.

In general, all of our imaging will be dominated by seeing, often to the extent that guiding/scope FWHM contributions may not be all that significant, so there may not be too much point in aiming for perfection in mount and scope performance. In particular, on a night of good seeing, the mount guiding will be better than on a poor night, so the mount/guiding error tends to automatically keep itself within the seeing FWHM. Although of course, systematic errors (mainly backlash and residual PE) that result in non-round stars are generally not acceptable and will be more noticeable in good seeing. With appropriate sampling, a big scope will not really resolve much better in Australian seeing - compare your images with those from the 3.9m AAT, there will not be much in it. The main advantage of a big astrograph is in how quickly it can get to a desired SNR.

Last edited by Shiraz; 27-03-2017 at 12:39 AM.
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Old 27-03-2017, 07:00 PM
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Thank you Ray, Greg and Colin very much for sharing your thoughts on the topic
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Old 27-03-2017, 08:49 PM
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Sorry, not right on topic but related in terms of resolution. I don't understand this. This is from the Astrophysics site re the BARADV barlow used on the AP175. I always thought that theoretical resolution is determined by aperture and that this is rarely if ever achieved due to seeing. Yet here it seems that going from f7 to f14 there is an increase in resolution.

From the site:

"These two images compare the resolution with and without the Advanced Convertible Barlow using a QSI 683 (5.4 micron pixels) camera with the AP 175EDF at 1400 mm focal length. The left image is shown at 400% screen size and the right image with the Barlow is at 200% screen size, keeping the two images equal in perspective"


George (of AP) commented in a discussion on the TEC discussion group that he believed that pix size was a factor here where at f7 perhaps the pix are too large to permit enlargement to match image size at f14. If true then a longer focal length would have the potential to increase resolution. I don't know what to think! I do know that 4 years ago I took a pretty high resolution photo of M83 using this barlow on my TEC140. I'm curious what you all think about this.

Peter

M83: http://www.pbase.com/prejto/image/151603452
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Old 27-03-2017, 09:04 PM
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It actually looks very similar to what I notice between drizzle integration and just standard integration. There are details that under standard integration that don't look that great but really pop after drizzle integration.
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Old 27-03-2017, 10:58 PM
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Interesting question Peter.

While I agree with Colin that it looks similar to a result of drizzle integration, the main difference is that higher resolution data acquired with a Barlow is not a result of mathematical manipulations as it is the case with drizzle integration.

But if both methods produce similar results, then why bother with large telescopes? Unfortunately I don't have an answer to your question Peter, but sometimes I ponder whether the reason for us seeing more detail in some splendid amateur space images is not because of them being taken with larger telescopes, but more so because of premium mounts being used, better cameras and filters, and perhaps even more importantly, inspiring results are due to superior data processing skills of those few truly gifted and experienced amateur astrophotographers.
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Old 27-03-2017, 11:33 PM
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It's also a bit harder to compare. After all it's easier to get the smaller scopes - they're effectively everywhere, so therefore you'll have more results overall using them. Larger telescopes are more expensive and so there won't be as many around - lesser pool to work with...

I think for, um, "scientific purposes", I should be given a large telescope to get some data for comparison.
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Old 27-03-2017, 11:48 PM
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In the case of the Astrophysic's barlow on the AP175 I think we can assume that both images were taken with a state of the art mount and OTA!

Peter
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Old 28-03-2017, 09:20 AM
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You're most likely correct Peter, I was more referring Suavi's last paragraph and question. I think more "inspiring" results are just because of the pool of processors for the small telescopes far outweighs those for the large telescopes.
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Old 28-03-2017, 09:27 AM
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Maybe take the images with a grain of salt Peter - I doubt that even AP has managed to find a way around atmospheric seeing and sampling theory.

It is only claimed that the images "compare the resolution" - in the little info I have seen, it is not claimed that they are separate images taken under identical conditions, one with and the other without the Barlow. Do you know of any info on how they were taken?

Might be wrong, but my guess is that the image without the Barlow has been synthesised from the one "with" by heavily downsampling and then resampling back up. ie, it is intended only to illustrate in general terms that a Barlow can sometimes produce better results - which is what you want if you are selling them. The presence of strong processing artefacts in the "without" image may possibly be a clue as to what has been done. An alternative possibility is that the camera is a Bayer camera - if it is, the results would possibly be a little more understandable.

If the seeing is really good and a non-Barlow system is undersampling, then a Barlow can certainly help, but that comparo looks more like a guide than a true result.

Last edited by Shiraz; 28-03-2017 at 12:11 PM.
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Old 28-03-2017, 04:34 PM
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Who knows, Ray. I only know what I've read on the AP site. When I think of a company like AP I find it hard to believe they would fabricate an image to sell a product. But, as I said, who knows?

The next time I see Roland I will ask him directly about this!

Peter
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Old 28-03-2017, 06:05 PM
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I don't know either, and would also be very surprised if they fabricated an image - but synthesising an image from real data to illustrate what they have observed is something different. Or maybe they just used a Bayer camera.

Whatever, angular resolution is primarily determined by aperture and/or seeing. Adding a Barlow only changes the image scale and, provided the sampling is right, it does not change the resolution for DSO imaging (if it was possible where would you stop - maybe a stack of 5x Barlows could get Hubble-like results and there would be no point in big scopes or AO)

I guess it would be interesting for someone to try it out.

Last edited by Shiraz; 28-03-2017 at 10:17 PM.
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Old 28-03-2017, 06:26 PM
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The one without the barlow looks undersampled with a OSC camera so increasing image scale will make quite a significant improvement. It kinda looks like a single exposure looking at the star colours, they're not overly uniform.
I think what Ray is getting at is that (and I agree) increasing focal length doesn't improve seeing
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Old 28-03-2017, 07:11 PM
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One thing I don't quite follow on this thread. In my experience you notice weak guiding at any image scale. Sure 2x2 binning sometimes rounds out guiding errors to some degree but not fully.

Having more arc secs/ pixel will not hide guide errors. Of that I am quite certain. I have imaged at lots of different image scales. They all show guide errors.

In fact I find the CDK17 most forgiving of guiding and easiest to get round stars on and that has the lowest arc/sec/pixel of my setups.

So 1 arc sec/pixel it is for those with average seeing of around 3 arc secs.

Greg.
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Old 28-03-2017, 07:44 PM
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That is very interesting observation Greg that goes against ones intuition. I do not have any experience in guiding at long focal lengths (longer than 1.2m), but I can certainly second what you wrote about binning - it does not fully hide guiding errors. When guiding was bad with my previous mounts, even binning 3x3 resulted in eggy stars in some subs, in spite of my hopes that stars will be perfectly round in all subs.

It seems that accidentally I started a quite interesting and useful discussion :-)
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Old 28-03-2017, 08:28 PM
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I suspect we (or at least I) get better seeing than people assume is typical in Australia. I routinely get 1.8" FWHM L subs with my Esprit 120. Using Ray's handy spreadsheet I can reverse-engineer an approximation of the seeing at my site based off:

0.12m aperture
0.6" RMS guiding
1.8" FWHM stars in the resulting images
= ~ 1.28" seeing

Again using Ray's handy spreadsheet, upping my aperture to 0.2m would reduce the FWHM of stars in the resulting image to approx. 1.63"

I realise these are approximations, but don't know exactly how accurate Ray's formulas are. I do know the aperture of my scope, typical RMS guide errors and resulting star FWHM with confidence though.
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Old 28-03-2017, 09:37 PM
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For me to get a FWHM like that happens very rarely. I am usually closer to 2.5" on average. The best I've ever got was a night around 1.5" up at Heathcote for a while.
I've had 1.7" briefly in Ha one night in Melbourne. Well that's two instances of below 2"
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