BrendanC

Interestingly I do actually clay bar my car, and can verify this! Of course, whenever I do, I always get the song Gay Bar by Electric Six going around in my head.

Actually... I've changed my mind! Doh....! Main reason is, I had a nasty artefact in my shots which I was absolutely convinced was because of the mirror. I've since done some tests and nope, it's a dust bunny on the sensor. I can work around that with flats, but I'm also getting an air blower to see if I can shift it. If not I might invest in some swabs, cos I'd just be happier keeping the sensor clean. So, much as I'd love to have a beautiful clean mirror too, even just looking at my flats, stretched, tells me I don't need to. At least it was an interesting debate. Thanks to all who contributed.

Thank you. The plan is to immerse the mirror upside down in very shallow water so that just the face gets wet, nothing else.

Just figured out how to get the sensor exposed in the manual clean mode on the camera and yes, there's definitely a dust mote pretty much where that mark would correspond to. I'm going to get a blower to see if that can shift it. In the meantime, flats are the workaround. On the plus side, I don't need to clean the mirror it would seem!

I guess. That's what I used before and it pretty much solved the problem, I just kind of noticed it in the stretched image. This guy seems to have a good, safe way of cleaning the sensor: https://www.youtube.com/watch?v=b-7ciRxh6s8 I'll persist with flats in the meantime until/unless the dust bunnies really start to bug me... Thanks for your help.

Darn. I thought that was the case. So, what's the best way to go about fixing this? Sensor cleaning seems yet another circle of hell I need to pass through...

One more thing: I just took the camera out and took a few quick exposures. Here's one, stretched: This is without anything attached - no coma corrector, no lens, no mirrors. It looks clean to me. Is this a good test? Does it now imply it's the scope? And if so, why didn't 'the blob' move around when I moved the camera? Confused!

So the other day I processed an image and noticed an 'anomaly'. I checked the flats and sure enough, there it is: DSS had done a good job of applying the flat but I could just about see that there was something not quite right in the image. Et voila, there it is, in the flat (and all the other flats incidentally, and the master flat). On looking at the primary mirror, I could see a fairly large mark where it looks like an insect may have met its maker (this is the best photo I could get): So, my first thought was 'Obviously, it's the mirror - which probably needs cleaning anyway'. But then I decided to do an experiment. I took a few shots with the camera in the focuser, at a low ISO and exposure, almost like taking sky flats but in the room with the light on, rotating the camera between shots. This is what I got (stretched to show details): # If the problem was the mirror, I would have expected 'the blob' to have moved around the image, as I moved the camera around. But it hasn't. It's stayed put, except that its shape changes a bit. This 'shape shift' seems to correlate with the direction of the light - you can see that the dark side of the blob is always towards the light, almost like a crater on the moon. This implies it's the camera. Given that I've checked the coma corrector, the only remaining thing is the sensor, even though I have auto sensor cleaning enabled for every time I switch it off. So, before I either take my scope apart to clean the primary (scary) or take my camera apart to clean the sensor (very scary), what do you think? I'm kind of hoping it's the mirror. Thanks, Brendan

Here's the result from a quick run-through with StarTools (I binned it by 50% to speed up processing a bit). I'm sure this could be improved upon too. Main thing is, yes, there's more information there. Try downloading StarTools and playing around with it. The demo is fully functional except for saving images, which you can work around by taking screenshots. rosette stacked2.tif

Exactly, should have said - it's the collimation I want to avoid, as much as possible. I hate collimating. Thanks!

Hi all, My primary mirror definitely needs cleaning. I've only done this once before with my old 130p. I've got the distilled water, the medical grade cotton balls, watched loads of videos etc. One thing I'd really love NOT to have to do is remove it from its cell. I just don't want to have to remove the screws and brackets and then get the primary back in there with the screws too loose, too tight, or whatever else could go wrong. I've seen videos where people leave it in the cell, and read about people doing it too. So, is there any really really really good reason why I shouldn't leave it in its cell? Or is it OK?

I know! That's what I've been saying all along! That's what I need help with. So, having slept on it, I think I get it now. The vernal equinox happens at a certain date/time each year, generally 20/21st March. That's the precise date/time at which the sun crosses the celestial equator. Right Ascension for an object is taken (in practice, was taken in the year 2000, is taken according to JNow) as its distance from the sun at that precise point, expressed as hours/mins/secs. The direction it's in is immaterial, and this is where my 'hunch' at the start of all this thread was wrong. An object will only be due north/south as it crosses the meridian, which is nothing to do with the point at which it crosses the celestial equator. Makes sense, because having been to Avebury for the summer solstice for over 20 years, I know that the solstice can happen any time, whether a convenient PM or a less convenient AM (there's that pesky meridian popping up again...) Thanks! These things matter. It's like when I realised the moon's phases couldn't be described by a perfect sine wave, and someone pointed out that's because it's an elliptical orbit. It's just noticing small things that don't quite add up, and needing to know them because they're pretty fundamental.

I think this helps. Thank you. I'm going to sleep on it and see if it still makes sense tomorrow.

So, that's the same as this, right? 'The RA is the location of an object when the sun reaches the meridian on the day of the equinox.' I know I'm wrapping myself up in ribbons, but the full story is this: the other night I gave an online talk to a group of people, just giving them a tour of the night sky via Stellarium, sharing my photos, that sort of thing, with Q&A afterwards. It went really well, even fielding questions like "Is the universe expanding and if so, what's it expanding into?' (short pause, then wax lyrical about redshift, Hubble, dark energy) and they might want me to do another one. So, I was just thinking about 'stuff', as you do, mainly arcmin, degrees, RA, Dec, and so on. Then I got to thinking about how I'd explain RA to people. It went, in my head, something like this: "The RA is the position of an object relative to the sun at the exact time and date of the vernal equinox. So, if an object has zero RA, that means it's exactly... er... where?" I thought it made sense for it to be due North. But it isn't. And I didn't know why. It's one of those things that you sort of take for granted, using RA and Dec day in, day out, doing clever calculations and feeling very smug. Then you suddenly stop, and think about it, and realise you don't quite understand something very fundamental, which you really should. It's like realising your cat is really a dog. However, I think I do now. As you say, it's the sidereal time when it crosses the meridian. Which, I think, is what I'm saying too: it's the location of an object when the sun reaches the meridian on the day of the equinox. Does this work? Or should I just go back to banging the rocks together?

Thanks, but which bit? I've read just about every web page I can find. They all give exactly the same definition of RA which is what my understanding is (or rather, was). Here's my dilemma, in a nutshell... EXHIBIT A - Location of NGC 7822 at the exact time of the equinox last year. Right definition, wrong location, if - as I think it should be - it should be due north. I could be wrong. I know I'm wrong. I want someone to tell me why I'm wrong. EXHIBIT B - Location of NGC 7822 at the actual exact time of the equinox according to J2000. Absolutely right definition, wrong location again. EXHIBIT C - Location of NGC 7822 at midday on equinox last year. Wrong definition, right location. How can this be? I want someone to tell me! It seems to me that the only way to square this circle is to adopt the definition that the RA is the location of an object when the sun reaches the meridian on the day of the equinox. That's the only way I can see that the RA actually reflects where NGC 7822 is on the day of the equinox. That is, noon, not 9:37am or 7:35am. I've read pages and pages and pages and watched videos and videos and videos, but none really discuss the origins of how RA is derived from first principles. And certainly none of them help me understand where I'm going wrong here. I just know that the answer is probably really simple and if someone could just explain this to me, I'll suddenly see the light and slap myself on the forehead.

Correct. Believe me, Wikipedia was my first port of call when I realised I didn't quite get this. So, in that case, why does an object with 00:00:00 not appear directly north at that date/time? Given that the sun is at RA 00:00:00, and is overhead at midday on the equinox date, then this would appear to be the definition of RA. But it isn't. It should be the date and time of the equinox. There's clearly something missing from my understanding here. I just need someone to explain why I'm wrong, and to give me the correct answer! Why doesn't NGC 7822 appear due north at the exact date and time of the vernal equinox, as it should according to its RA and the definition of what RA is? I should add that I've been involved in astronomy and astrophotography for over two years now, have done an intro course at Oxford University, and have used RA and Dec coordinates quite a lot, for example when calculating the day that a given object will be directly overhead me at midnight based on its RA. It's just this oddity I've realised, this gap in my understanding.

Yes, this works, I just tried it on Stellarium and that's correct. So, there's clearly a misunderstanding on my part about what RA is regarding the vernal equinox. If the definition I thought was right, then it would be due North at the precise date/time of vernal equinox in the year 2000. But it isn't, it's at midday on the day of the vernal equinox ie when it crosses the meridian. I guess I'll have to do some more thinking...! It'll probably make sense at some point. Thanks for the attempts to explain it to my feeble brain...

Sorry, don't understand! NGC 7822's JNow and J2000 RA values are 00:01:08.59 and 00:02:12.65 respectively - not enough of a difference to account for how far out of North it appears on that screenshot. I'm just trying to determine whether I'm even right that, if I go back to the date of the vernal equinox in 2000 (ie the J2000 epoch), objects with an RA close to 00:00:00 would appear North. Am I right about that? If so, then why doesn't Stellarium show that? And if I'm wrong, which Stellarium is indicating, then where am I going wrong? An actual screenshot would really help show me!

Hi all, So, I was just thinking about right ascension. I thought I understood it, but now I'm not so sure. My understanding was that celestial objects have their right ascensions set according to where they were at the exact vernal equinox relative to the Greenwich meridian, currently using the year 2000 as the base year. In theory then, I thought that meant an object with an RA very close to 00:00:00 would appear directly north at that time and place. One of the objects in my target list is close enough to test this: NGC 7822, with an RA of 00:01:09. So, given that the vernal equinox in 2000 was at 7:35 on 20th March, I thought it would be a simple case of firing up Stellarium, setting my location as Greenwich, the time and date for the equinox, locating NGC 7822, and it being almost exactly North. But it isn't. Here's where it is according to Stellarium: I'm clearly misunderstanding something quite basic here! I mean, I don't actually really need to understand this because, well, we just use our fancy-pants software to find things for us nowadays, don't we? But I'd still like to know where my reasoning is falling down here. Just for peace of mind, you know, to be reassured that the heavens do actually behave like clockwork, as they should. Can someone help please? Thanks.

I do it the other way around! Laptop outside, tablet inside. The reason for this is that more often than not, I really do need to sort stuff out at the telescope end, even after focusing etc. At one point I considered getting a headless solution such as a mini PC, but then thought I'd probably need a keyboard too, then some sort of local display would be handy... which really means, a laptop! No substitute for being able to type, point, click etc with immediate results when you're trying to fix something. I used to connect via long USB cables but had reliability problems. Same with using wifi extenders. So now I use a powerline which connects to the laptop's ethernet cable and uses the electric cable as a comms cable. It offers a very solid connection (it's never broken, to this day). Suddenly, you can control everything with no additional wires, very reliably, using Google Remote Desktop, from your phone, tablet, PC, other laptop, whatever, even over 4G if needs be. After much chopping and changing, this is by far the best arrangement I've used in terms of reliability, safety and convenience.

If you run an app such as Speccy, you can keep tabs on your machine's temperatures. In - 5C, the core CPU on my laptop didn't drop below 50C, the main internal hard drive didn't go below 25, and the data drive, stored in a caddy where the CD used to be, didn't go below 15. So I think it's safe to say, as long as a laptop has power, and is actively running, and is protected in a confined space, it's going to be fine. I'd urge the earlier poster not to leave theirs out and unprotected having nearly killed mine by freezing it to death.

Yes. I have a small hole drilled into the back, through which all the cables feed. I put the laptop in the box, together with the extension cable drum, plug everything in, and when everything is set up (mainly focused and framed), I put the cover on. I also recently decided to put a 1KG bag of silica gel in there too after noticing some moisture, presumably from condensation, and that seems to be keeping everything nicely dry.

I should add that I run a power cable out to my setup in the garden. I'm not sure I would trust the laptop battery to stay going all night, especially given the effect cold would have on it, and that would be very important for it staying warm. Sorry, just realised you did say in the field, whereas mine's, well, in the garden!

Indeed they can - see https://stargazerslounge.com/topic/350101-frozen-laptop-literally/?tab=comments#comment-3813012 After that experience, I now put my laptop in a box, with the power option set to never sleep. That way it happily keeps itself warm all night (confirmed by using the Speccy utility). Just went through a night at -5C with no probs.

It worked! First time! Works perfectly off the EQMOD simulator, so that's communicating with TeleViz and Stellarium. Slew to an object, and I can watch it gracefully slide across the sky in Stellarium, while the scope graphic also slews. Absolutely what I was after and it should make a huge difference to the session. Thank you so much!