gnomus

Last night was pretty poor seeing-wise, but I was able to make some progress (I think). Firstly, the ED80 did find a guide star and did seem able to produce reasonable guiding - here is a screenshot from PHDLog Viewer: I think this is reasonably good guiding - certainly miles better than I was getting with my CGEM. There were long periods when the mount seemed to be tracking well with no need for 'correcting impulses' from PHD2. Is this level of accuracy about what I should be expecting from the MESU, or should it be better? I took two exposures - one a 10 minute Lum and the other a 20 minute Ha. I seemed to have cured the issue of star elongation in the centre of the image. This is a crop from the centre of the 20 min Ha exposure. I have re-sized this to 400% (how's that for pixel peeping?) and the stars here look bang on to my eye: In the corners I think things are improved. I do not see any significant issues in the 10 minute exposure. In the 20 minute Ha, there is still some elongation, but this is not all in the same direction like before (as evidenced by Mark's PixInsight script in an earlier posting). Here is a 100% crop (all of these are from the extreme corners): You might need the 200% sized crop to see what elongation there is - this is from top right: To me the elongation seen here appears to be radial - that is, the TR corner goes NW to SE whereas the BR corner goes NE to SW, and so on all the way round. I wondered if this could be field rotation. The fact that I don't see elongation in the 10 minute exposure suggests that this is not an optical issue. I did run the PHD Guiding Assistant and it told me that my Polar Alignment error was 1.9 arcminutes. Previously it had reported an error of 0.1 arcminutes, so I think I must have shunted something a little when taking the scopes on and off during yesterday morning's reconfiguration. (Of course I have no idea about the reliability or accuracy of the PHD2 Guiding Assistant.) PHD2 still seemed happy enough with the 1.9 arcminute error, that is it did not ask me to try to improve this. Would such a relatively small PA error give rise to field rotation? Am I correct in thinking that the field rotation would look worse when using a wider field of view (my ED80 was 600 mm f/l compared with the 450 mm for the Tak)? If that is correct would the reducer, by giving me a wider FOV, make things worse? My analysis, therefore, is that my earlier problems were due to flexure, either caused by my SkyWatcher guide scope bracket not being 'solid' enough, or because of the place where I had attached my cabling. Flexure appears not to have been an issue last night, as evidenced by my perfectly round central stars. The elongation I now have appears consistent with field rotation due to the PA not being as accurate as it could be - probably due to my blundering when redoing my set up yesterday. I'd be very interested to hear what folks think of these new findings and my analysis of them. If anyone is interested in looking at the FITS files, these can be found here: https://www.dropbox.com/s/t5u7921md1r4d8e/Test%20ED80%20Guiding_600sec_1x1_L_frame1.fit?dl=0 https://www.dropbox.com/s/3wy9ehcpnimjn5g/Test%20ED80%20Guiding_1200sec_1x1_HA_frame1.fit?dl=0

Clear Outside is suggesting that I might be able to get out tonight. If I do I will try out the new set up. If I still get eggs, I will rotate the camera 90 degrees (I think that this will help determine if I have tilt). Are there any other tests that I could usefully run if I still get eggs tonight?

If my set up was unbalanced in RA, could that explain the elongation? I have tried to be careful balancing the Mesu but because there is no way of 'declutching' I do find it a little difficult to tell if I am in balance. On both axes there is a wide-ish range of settings that will allow the scope to remain stationary when I let go of it. I have tried two techniques - one where I apply pressure upwards (or downward) on a point approximately equidistant to the centre - when doing this I try to get to a state where it appears that equal amounts of pressure are needed on each side to move the mount. The other technique I used was to move the weight or the scope to a point where it just started to move if I let go. Then I'd adjust it in the opposite direction until it did the same - then I would set the scope/weight at the midpoint of these two extremes. Are there any other (better) techniques for balancing the Mesu?

I am not entirely sure if I can answer this. I set up my camera using that trick you have mentioned on several occasions - that is, take an image whilst slewing the mount for 5 secs and then adjust camera angle to get it squared. In SGP's plate solve routine it states that my Angle is 0.83 degrees. At this angle, the full image looks like this (I've only posted portions of the image before) - this is very basic stack and levels stretch: Does that help at all? I've spent all morning getting my ED80 on top of my Tak using the Baader mini dovetail thing, then recabling and rebalancing - I hope this works.

OK chaps. I'll do some serious reconfiguring tomorrow and then wait for a clear night to test. I am not sure I am quite ready for off-axis guiding - do folks like the Atik OAG? (I have an EFW2.). I would have enough spacing, even if I did eventually get the Tak reducer. Thanks to everyone who has contributed to this thread - all very helpful.

That is interesting to see. Thanks for posting it. So, much more likley to be flexure than any optical issue then? Edit: Poor PA wouldn't look like this would it?

Thank you Sara. My wife has repeatedly told me that she'd like me to experiment more. Now I know what she has been on about.

I'll give it a go, but, before I do, can I see a photo of the hat?

I could try the ED80 but what about th f/5 vs f/7.5 (from memory) issue?

You enjoyed that last sentence didn't you Dave? I do still have the ED80 that I put a Moonlite focusser on, but that doesn't have the same focal ration as the ST80. I assume that trying to guide with my C8 (once I get dual mounting up and running) would be complete madness?

I do super- tighten the focus lock screw. I hadn't cottoned on to the three screws holding the focuser in place. I will check these later today.

Ah the perils of non-verbal communication. Please be assured Steve that I took no offence whatsoever from your comment - you are trying to help me after all. My 'Ouch' was meant humorously - perhaps "Touché" would have been a better response. I am however a little upset that you so lightly dismiss my credentials as a nincompoop. I am nincompoop-and-proud. (As this is the Internet, I should perhaps add a ). I used the ST80 when guiding the ED80, but it was not attached with the Skywatcher thingamajig. I had everything on the CGEM back then, and I was probably trying to keep the weight down. The Skywatcher does add quite a bit of height to the set up. I have this arrangement of holes on the top of my Tak clamshell: I could try bolting the ST80 rings direct to this (although those rings will only end up an inch and a bit apart). Alternatively I have a Baader mini dovetail that I could affix to the Tak clamshell. I will also try moving the point where I attach the cables to lower down - between the Tak and the Dovetail plate.

Thanks again Olly. I have spent some time going through various subs. At first I had convinced myself that the Ha filters all had the same issue - worse in the top left and top right corners - and that my Lums were worse in the top right and bottom right corners. But the more I looked at this, the less certain I was that I was seeing a consistent error. I am increasingly coming to the view that the issue may be one of a cable dragging. If that were the case then I might get different directions on my elongations depending on where the scope was pointing - isn't that right?. When setting up, I was a little unsure where I should attach the cable bundle. In the end I ran a piece of velcro around the join between the top of the Tak clamshell and the mounting plate for the guide scope bracket - I hope this image shows what I mean (ignore the velcro on the Tak lens hood: this was early on when I had temporarily attached a Telrad to the system to help set up): I did think about velcroing the bundle to the join between bottom of clamshell and the dovetail bar. I am planning to moving to a dual rig with Tak and C8 side by side over the next few days and I will try to find somewhere else to attach my cable bundle. I had intended to fix the guide scope in a set position with a Baader clamp until I read something on this site about how important it was that the guide scope and imaging scope be aligned! 'You say poh tah toh, and I say ......'

I have one of these SkyWatcher Guidescope Mounts http://www.firstlightoptics.com/guide-cameras/skywatcher-guidescope-mount.html and I have ensured that the guidescope and the Tak are pointing at exactly the same point of the sky. I do not have the reducer - was thinking of getting one but the stories relating to that adapter and its screws do not leave me brimming with optimism. Can I ask again about orthogonality of my unmounted 36mm filters. Could this be an issue or would I just be wasting my time checking this?

Thanks Sara. I quite understand the law of diminishing returns, it is the direction of travel (quite literally in this case) that concerns me.

Thanks for looking and confirmng my suspicions Mark. The elongation is all in the same direction - it is worse in some corners but it is present in the centre. I am dithering, so it is just possible that the 'blink' test is coincidental.I will recheck cabling and balance. With the Mesu, one cannot 'declutch' and It is not always clear where the precise point of balance is.

Ouch. However, I have not found this to be the case with astronomical or indeed any other type of gear, and I too have suffered from the 'hifi sickness'. What I am trying to do with my question is ascertain whether or not I do have a significant issue and, if I do, seek advice as to how to fix it (thereby getting the best out of my equipment). I may well be a nincompoop, but I was getting rounder stars with my ED80.

Thanks Steve You are probably right. I was quite pleased with the images last night as they downloaded from the camera. I don't mind the mini-spikes, I just was not expecting them. I understand that my image is not a disaster, but, at this price point, I was hoping for a significant step up. I think I got rounder stars with my ED80 (and on a lesser mount).

Thanks Steve (Is everyone on this site called Steve?) Links: https://www.dropbox.com/s/fbkvlc0rkdrykpe/Pelican_1200sec_1x1_HA_frame3.fit?dl=0 https://www.dropbox.com/s/6dl0o1d0mediw1e/Pelican_1200sec_1x1_HA_frame6.fit?dl=0 Frames 3 & 6 - the last ones taken before an autofocus run. I'm very grateful to you for this. Steve

I am using a fully threaded connection and all connectors are Tak parts. I do not have CCD Inspector (and am not sure I'd know what to do even if I did). I can post Dropbox links to a couple of subs if that would help.

I am quite certain I am pixel peeping! At 100% I am a little suspicious of those stars, but I really need to go to 200% (or 2:1 in PixInsight) to see the ovoids clearly. I will check polar alignment again if I get the chance (recent weather here has been beyond a joke). Could this be cause by a little bit of cable drag on an insufficiently tightened scope (see my previous post)? I can also rotate the camera - I presume that if I rotate it 90 degrees I would expect the direction of ovoid-osity to change by the same amount.

If you insist: You're sloppy. Thanks for your response. I went ahead and processed the 8 x 20 min subs I took last night. I still think I'm getting ovoids in the centre (highlighted in red). The other thing I have noticed on all the images I have taken with the Tak are small diffraction spikes (best one highlighted in yellow). I must say I was not expecting to get these with a refractor. Again, you'll probably need to click on the image to get the 100% crop. Am I expecting too much from the equipment? I thought I had reasonably good polar alignment. Certainly the PHD assistant tool reported excellent PA (but I don't know how reliable this is). Following your comments, I went out and checked the tightness of everything. I was able to tighten up the Tak rings and connections by a very small amount (a 'wee thochty' as they say back home). I am not sure if this could have been responsible. Something interesting did happen when I was doing this though - the whole scope rotated slightly in the clamshell. It is possible that I hadn't tightened this up sufficiently (trying not to crush the tube). Could this make a difference I wonder? The final thing I have been thinking about are the 36mm unmounted filters that I have in my EFW2. Fitting these filters into the wheel was a fiddly job to say the least. Is it possible that one or more of these are not exactly square? Would that make a difference? If so, how do you get these things in square????? I had hoped I'd be imaging by now - not still troubleshooting, but there you are.

Thanks Steve. If it is sensor tilt, what am I supposed to do about that?

I am someone who is still trying to figure out when I should ‘soldier on’ rather than admit defeat. Last night may have been a case in point. The moon was pretty huge - mid-90% size. Nevertheless, (and despite advice from people who know what they are doing) I decided that I would press on and try to get some Ha of the Pelican Nebula. I went out at around 8pm, I rolled the roof off, and I fired everything up. Then I took a peek over the observatory wall. In the half hour or so since going out, it had completely fogged up - dense fog. So, I shut everything down and rolled the roof shut again. About an hour later, things seemed to have improved considerably, though ‘Clear Outside was still showing ‘Red’ for dew point and relative humidity. I went back out and tried again. Most of the time I got really good guiding – flat graphs with <0.2 pixel RMS error. Every now and again the graph would throw a slight wobbly with Dec moving between +1 and -1 arcseconds for a few seconds, but overall it didn’t seem too bad – My Tak 85 and Atik 383L give me a resolution of 2.47” x 2.47”. I was taking 20 minute subs and, on my warm room screen, SG Pro seemed to be showing me reasonably good images. However …… Once I got indoors and looked at my subs on a decent screen I saw that I had some star trailing. I attach some 100% crops taken from the middle and the four corners of my image - you may need to click on the image to make it the 100% size.: It seems to me that all of these crops show stars with a top-left to bottom-right slope. When I finished for the night (some 3 hours later) it was quite one of the wettest evenings I had encountered. The equipment was all soaked and there was water streaming down the observatory walls. The dehumidifier seemed to have dried everything up by morning. Is this then simply a problem of 'seeing' or have the conditions pointed up a more fundamental problem with my set up? I use an un-reduced Tak 85 (at least for the moment).

Thank goodness. Does that mean that you won't now have to take the scope back to the UK with you, and that you are now "up and running"?