Spaced Out

OK, so I am preparing to pour a budget concrete and brake disk pier. I have one last question before I get cracking....... It might sound dumb but...... For the top I am using a brake disk and I have created an azimuth peg on the top for my HEQ5. The disk (and some threaded bars) will be sunk into the concrete so just wondering how people align these north correctly ? I was just going to use a normal compass, would this be adequate ? Obviously there’s no going back once I’ve poured it so I’d like to get the azimuth peg lined up the very best as I can to save having to faff about later re-drilling the disk ! Thanks in advance for any advice.

Thanks for this, yep the 90cm to mount base I mentioned is only a few cm higher than the lowest height for the tripod (HEQ5). I did think about going lower but thought I'd be better off using the height I'm used to. Trouble with this is there isn't a perfect height for every use and I don't have a crystal ball regarding the future ! I think I'll just set it for how I use it now (which is quite low) and then worry about any changes if they happen, as you say, it is easier to add something to a low pier than chop down a tall pier !

Hi All I’m about to build a budget concrete and brake disk pier in the garden (no obs). My main focus is imaging but very occasionally I may want to observe stuff or show friends stuff. Deciding on pier height is giving me a little headache. I suppose the priorities are that I need to try and keep it low so that our garden fence block as much light from neighbouring properties as possible, I guess low is better for stability too and I’m using Newtonians at the moment so again lower is probably better ? However, I do wonder about the future, maybe changes in telescopes and also the occasional bit of observing. Anyway, I’ve measured the current tripod/mount height that I use, if I adopt these measurements for the pier (which seems logical) it will be 90cm to the top of the mount plate/base of the mount. Does this sound normal ? I see a lot of piers are quite a bit taller, am I too low ? Am I missing something important ? Thanks in advance for any advice you can offer.

Thanks, I'm feeling frustrated that my little problem persists but I've certainly learnt quite a bit so far going through this whole process. The things I used to be really scared to touch (like collimation, moving the secondary about etc..) don't seem scary any more. That's got to be a step in the right direction !

Thanks, I'll give this a go. Yes I rotated the spider vanes around but no change, it was only when I turned the focuser upside down that I saw the problem reverse itself. Also looking directly through the centre of the focuser at the spiders the vertical vanes definitely look a touch thicker than the horizontal ones, then when I move my head slightly to the left they narrow up and look fine, I'm guessing this all points to an off kilter focuser of sorts ?

Thanks for your reply. Yes I measured and marked the point directly opposite the focuser and used a cheshire to centre it. Difficult as 2 of the collimation screws are hard/impossible to reach with the focuser tube in ! I used a collimation cap with a small central hole to centre the secondary because this was the advice I have read online, I'll try using the cheshire instead tho.

Well, time for an update. I've tried a number of things to resolve this issue without success, I’ve rotated all of the spider vanes round by one to see if it was a single vane issue, I removed the coma corrector, rotated the camera around lots, removed and centred the focuser the best I could, turned the focuser upside down, hacksawed the focuser tube down to reduce intrusion and in the end totally stripped and rebuilt the scope 3 times. The result...... my scope is now flocked and matt blacked, but I still have the problem ! On the plus side I think I have narrowed down the issue to some sort of focuser tilt ? Turning the focuser upside down reversed the issue in the image so from that I assume it must be a focuser problem ? I rebuilt the scope 3 times and centred the focuser as carefully as possible each time, but the problem remains. When collimation is as good as I can get it and I look through the collimation cap I can see the cap reflected but also the edge of the focuser tube at the top right side, suggesting the tube isn’t central ? When I look through without the collimation cap at the spider vanes, the vertical vanes are clearly a touch wider than the horizontal vanes which are a very fine line, if I move my head slightly to the left the vertical vanes narrow up nicely the same as the horizontal ones. I’m a newby and still feeling my way around here so please correct me if I am wrong, but from this I am assuming that the focuser or focuser tube is tilted somehow ? Every time I have rebuilt the scope I have carefully centred the focuser the best I can using the 3 collimation screws. I am wondering if there are further tilt adjustments available for the focuser/focuser tube to line it up perfectly with the secondary ? There are few more allen screws on the bottom of the focuser but not sure what they all do tbh !

Hey don’t worry and thanks for the help ! My gut feeling is that it is probably focuser tube intrusion, it does stick quite a way in ! However I’m going to follow the advice here, I will centre the focuser and re-collimate first. If that doesn’t fix it then I’ll chop the focuser tube down. I’m sure the flocking won’t resolve this issue but I was thinking of flocking/darkening the tube and mirror cleaning anyway so I figured why not do it all while I’m fiddling about with everything ! Good advice, thanks !

Right I’ve fumbled about with the primary and moved it down the tube to the point where it fell off one of the screws ! I managed to get this screw back in and tighten it up a little, so the mirror is now as far forward as I can safely get it, still I have the split spikes on the left side of the image ! At this point I’ve decided to take the nuclear option and strip the scope down. I will lop off a section of the focuser tube (i’ve marked on the tube where the focus point is for the DSLR & MPCC) and centre the focuser. While I am at it I shall clean the mirrors, flock the scope and get the blackboard paint out too. Once rebuilt and collimated if I still have this issue I may just chuck it in the bin ! Any advice on this little project ? I’ve just watched the astronomyshed YT vid on mods which has explained the dismantle/re-assemble process quite well, it’s spurred me on a bit. With regard to chopping the focusser tube, I’m only going to use this scope for imaging so I am happy to try it, how much should I be lopping off ? Is it still OK to have a bit protruding into the scope at the point of focus ?

Ok.... so I’ve pushed the primary forward by (I’m guessing) 8mm or so on the adjusters, a quick test on a bright star and the problem is still there. The lock screws still have a bit of thread visible now (about 5mm), not sure if I should push it any further or if it would make much difference if I did ? I’ve tried using an eyepiece on a bright star to see how that looks but I’m finding it really difficult to make the spikes out clearly. Probably my tired old eyes ! Is there an easy way to assess if the focuser is tilted somehow ?

Before I have a fiddle I am just wondering how far forward the primary will move on the rear adjusters ? I've just moved it forward a few turns now (maybe a few mm) and the lock screws still have a bit of thread visible (about 5mm). If I go too far will the mirror disengage ?!

Thank you for taking the time to write this response, it makes sense. I think I'll try bringing the primary forward first and see how we go with that idea. I'm a bit scared of moving it too much in case if falls off or something ! I'll take a look at it this afternoon and see how easy it looks. I've only got the two standard thumb screws on my focuser but the MPCC and camera seems to lock nice and flush/tight with those so not convinced that it is moving/tilting, but I'll take another look. If the moving the primary doesn't help then I'll look at the focuser tilt. If that doesn't work then I'll probably get the hacksaw out. I'm now thinking that if I have to do that I might just strip the whole thing down, flock the tube and clean the mirrors while I'm at it ! Could help me learn more about how it all works, or, could be a nightmare getting it back together again !

Looking at my issue with the split stars, would this focuser intrusion account for the split stars appearing on one side of the images only then ? At this point I am willing to try anything but it would be nice to be fairly confident before I get the hacksaw out and butcher it !

Thank you for this. That looks quite scary initially, but it's actually quite straightforward I guess. Did this mod solve a similar issue ? Do the remaining longer parts of the tube cause any problems ?

Wow ! Thanks for this explanation. Would this account for the split stars appearing on one side of the images only ?

Hi, thanks for this. Not quite sure I understand what you mean ? Sorry I'm a beginner and a bit of a duffer when it comes to this, probably why I am struggling ! The focuser was at the bottom in the test shots. Re- stars moving top to bottom, if you look very carefully at the original picture I put up at the beginning all the bright stars from a little bit left of centre exhibit these split spikes (from top to bottom) and all the others right of centre don't. This little issue is really messing with my head !

Thanks I did not refocus between exposures. For this quick test I just chucked my mount outside and wanged a scope on it, so no laptop or ultra fine tuning of the focus, just got it as good as I could using the back of camera LCD. Hmmmmm.......

Well I’m feeling that this may have beaten me. Yesterday I spent all afternoon adjusting the spider vanes and collimating the scope the best I could. I was meticulous with getting the spider vanes centred and as straight as possible, each vane is now equal in length from the tube to the centre of the centre screw down to around 1/2mm. I spent hours on this and then checked it using the card disc with a centre hole I’ve made previously for collimation, I then made another paper disc just to double check that too ! After fiddling about with the vanes I had moved the secondary about so I then spent ages collimating the scope again. Once that was complete I did a quick star test which showed the scope was pretty well collimated. I then did some quick tests on a bright star and the problem persists. Below are 4 images of the same star moved across the frame. When on the left side the star diffraction spikes split but in the centre and on the right they look OK ? The images are awful quality (1 second exposure sat on a mount with no power at 20,000 ISO) but they show that the problem is not resolved. I’m left scratching my head now. I don’t know what else to try and I’m wondering if I might just be better finding a telescope shop somewhere to identify the problem and sort it out for me ? Not really a route I wanted to take. Any further thoughts or ideas would be most appreciated at this stage !

I'll try and collimate it tomorrow and re-test this on the next night with any stars available.

Yeah done that, it does seem central. I'll recheck the secondary tomorrow when I try to collimate it again. I thought the vanes look a bit bendy in places too ! Is that normal ? Bought the scope 2nd hand and never touched the spider vanes before the last few days so no idea what's happened to them previously. Still doesn't affect the diffraction spikes in the centre of images tho which look fine.

Just did a rapid photo test outside. It was a race against the cloud but with a star centred it looks perfect, with a star on the left of the frame it is splitting. I didn’t have time to do one with the star on the right because the thick cloud rolled in.

These split spikes only seem to appear on the left side of my images, I’m wondering if that means anything ?

Thanks. I’ve made the adjustments and now the spider vanes in question are a bit more horizontal (see the photo). I’ve got the spider vanes as good as I can get them by eye in terms of being centred and at right angles. I’ve just done a quick test outside on a bright star and the problem persists ! Now I’ve moved the spider vanes the scope is a little out of collimation too (I did a star test to check), so that’s another little job for tomorrow, I've only done collimation once before and it took me 2 days to sort out ! I guess for observing this is just a minor problem but I am trying to get into imaging so I want this scope to be set up as well as I can get it, the split spike thing is really starting to bug me now !

Thanks so much for taking the time to respond to this, I’ve been twisting and turning the spider vanes about the place for 2 days now without any success, these split diffraction spikes stubbornly remain the same ! Looking at this again I think you could be right with your diagnoses. Looking at the vanes face on the two that are left and right on my pics seem slightly raised at the secondary holder and curved upwards a little bit, while the others are straighter. So, how can I remedy this ? Sorry I’m a novice and this is all new to me. I’m guessing I loosen the top screw and tighten the bottom to pull the secondary down slightly ?

Hmmmm...... I’m having to revisit this thread because I am still having trouble with this. I’ve identified the vane that seems to be the issue and I’ve spent over an hour this evening trying to sort it out without any success. I’ve tried everything including being fairly brutal in twisting the vanes in different directions to see what happens, this made zero difference and the split diffraction spike just persists whichever way I turn them. I’m just wondering if this is definitely a spider vane issue or could it be something else ? What a headache !