cfinn

My first DSO image was of (surprise surprise) the Orion Nebula, but it was taken with a rather special instrument. A 20 inch Grubb Parsons Cassegrain. I was an Undergraduate student in Astrophysics at the University of Edinburgh at the time, and I took this image at the end of a research project using the telescope to estimate the distances and ages of star clusters in Auriga using multi-band photometry and Hertzsprung-Russell diagrams. Myself and my fellow project team members at the time couldn’t resist the chance to take a pretty picture after all the real science was done, so here it is, along with a picture of the telescope in situ from the link above. Members of Mid Kent Astronomical Society will doubtless be interested to see this as they are now the lucky benefactors of this scope following its decommissioning only a year or two after I graduated! The scope is currently being refurbished by members of the society and the hunt is on to find a new permanent home - details here.

I think it would be great if there was some kind of online tool where you could post your images and it could automatically diagnose what kind of aberrations are present in your image and suggestions as to whether it can be fixed easily (e.g. collimation, coma corrector, field flattener spacing etc.) vs something needing repair or professional adjustment. Could be very tricky to implement, but I could be tempted to give it a go if it does not exist already. Just as a proof of concept. It would need a large sample of images labelled with the issue(s) present + examples with no issues, several hundred at least to be able to train a convolutional neural network which could do the job adequately enough. In general I think advice would be welcomed by most, but via a PM rather than on a forum for everyone to see (unless specifically asked for). It's always worth going through all the less sinister possibilities first as well, before jumping to the conclusion that the telescope needs to be replaced or sent back for repair.

I think opinions will differ on this, but I’ve seen many reports of people guiding very successfully (to ~0.7” RMS) on this mount with payloads very close to the stated maximum of 20kg for astrophotography. I might be wrong but I think the often quoted 2/3 rule originates from when manufacturers would quote the max visual payload, where tolerances are not so tight as they are for astrophotography, so you would need to reduce that number by at least 2/3 if you were interested in the latter. However, it is stated very clearly that 20kg is the max payload on the EQ6R for astrophotography in particular, and many people do indeed seem to have success close to that limit. I think the bigger problem is not weight but the length and weight distribution of the OTA you are using. Longer OTAs have a higher moment arm, which makes good balancing critical. Weight distribution is important, because if the centre of mass of your rig is not in the right place, i.e. dead centre, then balancing on all three axes can be even trickier. This will affect guiding more than weight in my opinion. My setup is 16-17kg on an EQ6R and from experience my biggest problem is definitely down to the length of the OTA and balancing.

Here is my last minute entry. Perhaps controversially, I have done most of the heavy lifting in post by utilising machine learning courtesy of Luminar AI and Topaz DeNoiseAI. Surprisingly effective... Channels first combined in AstroPixelProcessor with only around 30% of the Ha blended in. Continuing with APP, I performed background calibration, star colour calibration with the adaptive blackbody + extinction model and a fairly heavy crop to remove edge artefacts and frame the galaxy centrally. Then into Luminar AI where I used the Enhance and Structure AI sliders just a little bit to bring out the spiral arms. I followed this up with a small adjustment using the small and medium details sliders to bring out the inherent grainy texture and detail in the dust. I then dialled down the highlights to fix the core. Lastly I ran the image through Topaz DeNoiseAI using the "AI Clear" algorithm with noise reduction and sharpening both set low. Following this I dialled up the sliders to recover original detail and reduce colour noise, which cleaned up artefacts quite well. This whole procedure is certainly lazy, but I'm happy with the results!

I would be happy to give a talk at some point on my PhD thesis, though would need a few weeks notice to prepare! My research was on the intergalactic medium (IGM) and it's relationship to galaxies. I used high resolution spectroscopy of quasars from the Hubble Space Telescope and ground based observatories to map the location and dynamics of metal enriched gas in the IGM in and around galaxies at medium redshifts (z ~ 1). These observations were compared with state of the art simulations of the Universe produced with Durham University's supercomputer - the Cosmology Machine. This led to some new insights into the nature and consequence of feedback from active galactic nuclei (AGN) and supernovae, which shapes the evolution of galaxies over cosmic time. I'm also happy to talk a bit about what life is like as a working astronomer and share some of my experiences visiting and working at world class facilities in the Atacama desert in Chile.

Does anyone know what metal/alloy the Esprit lens cell is constructed of? Steel is supposed to be the best because it has very similar thermal properties as the glass, which minimises the risk of this happening.

Ok, thank you for the clarification. Your suggestion is a reasonable one, but of course should not really be necessary. Sky-Watcher have been good enough to recognise that in offering me a replacement.

Yes, you’re absolutely right, I’m quite sure the replacement will be fine. I should emphasise that I’ve not set out in any way to rubbish the reputation of these telescopes. It was just helpful to me to verify that replacing the scope was the right course of action.

What did you do about yours in the end? Did you get a replacement or go for something else?

Thank you, this is encouraging to hear. I’ll report back once the new one arrives (end of this month supposedly) and I’ve had chance to test it. It will of course be warmer so I may not know for sure if the problem is still there until next winter...!

Just thought I would share that Sky-Watcher UK have agreed to send a replacement. Fingers crossed this one turns out better and I was just unlucky.

Yep, superb. This is what sold me in the first place. You have to pay 2-3 times the price for this performance elsewhere. Hopefully Sky-Watcher can learn from these examples and improve their QC and/or lens cell design.

Thank you. That is good to know, but of course it shouldn’t have to come to that. It’s a shame, because when I first got the scope it gave an excellent star test (in warmer temperatures). The optics are first class and this is essentially just an assembly/design issue.

Yeah blimey, you’re not wrong! I’ve emailed Widescreen Centre to report the issue and explore what my options are. Let’s see what they say.

That’s interesting. You could well be right.

I ordered mine in August and it arrived in November.

Thank you, this is helpful and I’m sorry to hear about your experience. It certainly looks like your issues are much worse than mine, and it is a shame they could not be resolved. Your story does not fill me with confidence that an easy fix will be possible, so maybe a replacement is a safer option, though it’s a real pain with availability of telescopes being so poor at the moment. I bought mine from Widescreen Centre, so I’m hoping that if I approach them about this they will be as understanding as doubtless FLO would be based on my experience with them so far and from what I see on this forum. I’m also keen to hear from other owners to see how common this issue is, because this will influence what decision I eventually make.

Hi all, I'm keen to understand how common it is to see pinched optics on the Sky-Watcher Esprit APOs in cold weather during the winter. I get hexagonal stars with my Esprit 120 when the temperature dips close to 0C outside, even with a dew heater strap. The effect becomes less noticeable as the ambient temperature increases, so that by around 5C I have nice round stars again. You can see the effect clearly on the image of M106 below (particularly the brighter stars towards the top left hand corner of the image). The temperature outside for this image was getting on for -2C. I know that this is caused by the six collimation screws holding the objective in place, which pinch the lens cell a bit too much when the temperatures get cold. Do most Esprit owners see this effect to some degree in cold temperatures or is it only in some isolated cases like mine? If the latter, then this suggests it should be possible to fix this by loosening these screws ever so slightly so there is no pinching in cold weather, but secure enough to maintain collimation of the scope. Certainly not a procedure I would want to do myself, but I presume someone like Es Reid would be able to fix this easily enough. I'm generally very happy with the scope I have, but I would be happier if I could have clean round stars in my images all year round. Would welcome some thoughts/opinions on this. Charles

Many thanks. I’m using a QHY Polemaster for polar alignment and am reasonably satisfied I’m getting it as accurate as I can manage. As Michael mentioned, I’m not seeing many dec corrections, so assuming my PA is good enough. I’m also using multi star guiding already and 2x2 binning on the guide cam is not sensible for me as I’m using a 240mm FL guidescope. I think your comments around guide speed and direction are interesting. I’ll give that a go. Charles

Thanks Michael, I appreciate you taking a look at these for me. To be honest, I don’t know how to change the calibration guide speed in Ekos, or what is normal here. There is a setting for the number of iterations and the pulse length, so maybe it’s the latter? Next time I think I’ll use PHD2 and do a proper run of testing. I was trying to image here, rather than specifically testing the guiding parameters. Sounds like I need to take a look at Declination backlash and stiction so I will do that. Charles

Another guide log from last night. Min move increased to 0.4". Guiding exposures of 2 or 3 seconds just made things worse, so I had to stick with 1 second. Made great efforts to balance the mount properly on all 3 axes, before making RA a little east heavy. It's a real struggle to get guiding RMS < 1". At one point RA axis goes crazy and oscillates +/- 10"! Re-calibrated a couple of times and tried upping the number of iterations in Ekos. Dec looks strange to me - could be backlash? Opinions again would be very welcome. Performance is some way below what I was expecting from this mount, which is disappointing. guide_log-2021-03-07T19-27-58.txt

Thanks Michael. It might well be worth it to me to revisit some of these things. The calibration in particular I have not paid much attention to and I just let Ekos do its thing, but I will have a play and/or switch to PHD. I was vaguely aware that calibrating near/on the celestial equator was a good thing to do and that you should re-use this calibration elsewhere. By default Ekos will re-calibrate every time you switch to a new area of sky, but I do believe there is an option to override that.

Yes, interesting. I had the problem described in this video where I could not achieve balance along a third axis and interestingly I found that adding the finder (which I don't use for imaging) fixed it. This confuses the hell out of me, because without it there doesn't look like there is anything off-centre that could cause an issue. I will probably revisit this and think about it some more.

Agree in general I should go for longer exposure and higher min move for exactly the reasons you suggest, though I do believe since I am using the new multi-star guiding I should theoretically be a little more robust to seeing. In any case, unfortunately this did not solve my problem and the same settings give me very good results nearer the pole. I have run the guiding assistant in PhD. Attaching results here.

I should also say, I have to use the counterweight extension to get balance in RA with two 5kg counterweights. I am aware that this is less than ideal and am thinking about getting an extra counterweight or two so I can balance with the weight further up the bar. Not sure if this is a possible cause or not.