A Question of Balance and Guiding?

[Thalestris24]

Hi all

I've been trying to image M31 recently but my guiding has been poor and erratic . However, immediately after trying M31, I've also been gathering data for M33 which is about 15 deg south of M31. In that case the guiding is fine . I'm wondering whether this is likely symptomatic of a balance problem? I have my setup slightly east heavy but when pointing at M31, which is between about 37 and 45 deg when I'm imaging it, the scope balance is obviously biased towards the mirror end. So, should I make the scope even more east heavy to try and compensate?

Also, while I think about it,  I've been doing 10min subs - are they likely to be long enough to pick up the galaxies' nebulosity? (Lp is bad here...) So far I've acquired 10 x 10mins for M33, fewer useable subs for M31. Not tried stacking any yet.

Thanks for any advice / suggestions.

Louise

[lensman57]

Hi all

I've been trying to image M31 recently but my guiding has been poor and erratic . However, immediately after trying M31, I've also been gathering data for M33 which is about 15 deg south of M31. In that case the guiding is fine . I'm wondering whether this is likely symptomatic of a balance problem? I have my setup slightly east heavy but when pointing at M31, which is between about 37 and 45 deg when I'm imaging it, the scope balance is obviously biased towards the mirror end. So, should I make the scope even more east heavy to try and compensate?

Also, while I think about it,  I've been doing 10min subs - are they likely to be long enough to pick up the galaxies' nebulosity? (Lp is bad here...) So far I've acquired 10 x 10mins for M33, fewer useable subs for M31. Not tried stacking any yet.

Thanks for any advice / suggestions.

Louise

Your scope should be balanced at any orientation. You have an added problem with a Newtonian scope as the imaging gear and the guider hang over the side of the scope and cause complications as the orientation of the scope changes. The practice of having the scope east heavy is more to do with the less than perfect mechanics of the hobby mounts rather than guiding itself, I believe it is supposed to keep the worm gear meshed up. As for M31 you need some short subs for the core and subs of about 600~900s for the rest as the outer parts of M31 are quite dim compared to the core. M33 is a very difficult target at the best of times and requires very long subs and quite a lot of them in particular as you like myself suffer fom very  heavy light pollution and the dirty city air does not help either.

Regards,

A.G

[Thalestris24]

Hiya

So what might the solution to the M31 erratic guiding be? I balance the scope in all 3 axes (as per Dion's video) but then make it slightly east heavy. I only have the finder-guider but the qhy8l is quite heavy and sticks out a lot but is pointing down and slightly to the left for both targets. The orientation doesn't change much between M31 and M33, just the altitude angle really.

I can get 600s subs (with M33) but 900s might be a challenge with either! I may get a few more sessions in but I'll probably have to wait until next year for them to come around again...

Thanks

Louise

[AstroAndy]

What I do, in a pinch, is turn the tube so that the imaging gear and guide cams are kind of on top of the scope (keeping it slightly East heavy). To measure the balance, I point the scope straight up to the Zenith, and leaving it loose (levers not tightened), check whether it comes down on either side, if it doesn't, voila.

[Thalestris24]

What I do, in a pinch, is turn the tube so that the imaging gear and guide cams are kind of on top of the scope (keeping it slightly East heavy). To measure the balance, I point the scope straight up to the Zenith, and leaving it loose (levers not tightened), check whether it comes down on either side, if it doesn't, voila.

Hi

Yeah, I balance in the vertical also but seem to have a problem guiding when pointed at higher altitudes e.g. 40-45 deg

(Edit: when pointing east - all my imaging is done pointing east) I'm just not sure why that is.

Louise

[ollypenrice]

The serious Newt imagers orientate their cameras like this. http://www.astronomie.be/pieter.vandevelde/equipment/2014_provence.jpg

Camera optical axis parallel with counterweight axis and on the same side. This is Pieter Vandevelde's self-built instrument at my place. This way you use the least counterweight and have the best dynamic balance - ie it's easier this way to make the balance consistent in all orientations. Jonas Grinde's big robotic Newt is also set up this way. I haven't really used a Newt for imaging but these guys know their stuff.

Olly

[RikM]

I have mine oriented along the counterweight shaft as well. Actually, mine is a few degrees offset to account for the finderguider. I regularly shoot 20min subs in Ha, and have done 30 and even 60min for OIII.

[Thalestris24]

The serious Newt imagers orientate their cameras like this. http://www.astronomie.be/pieter.vandevelde/equipment/2014_provence.jpg

Camera optical axis parallel with counterweight axis and on the same side. This is Pieter Vandevelde's self-built instrument at my place. This way you use the least counterweight and have the best dynamic balance - ie it's easier this way to make the balance consistent in all orientations. Jonas Grinde's big robotic Newt is also set up this way. I haven't really used a Newt for imaging but these guys know their stuff.

Olly

Hi Olly

Maybe I'm not quite so 'serious'! But, anyway, mine is similar...

I obviously have the finderguider sticking out and the ccd to one side so they balance reasonably in the vertical. As I say, I'm not really changing orientations, just altitude (semantics?). And yes, that's my imaging window with the blinds, behind! You can see why the sticking out finderguider relative to the window size is a limiting factor...

I'm beginning to think the guiding problem is maybe caused by something other than balance... Not sure if it will be clear enough to try some more tonight - here's hoping.

Thanks

Louise

[Thalestris24]

I have mine oriented along the counterweight shaft as well. Actually, mine is a few degrees offset to account for the finderguider. I regularly shoot 20min subs in Ha, and have done 30 and even 60min for OIII.

Looks like yours is mine's big brother, Rik!

[ollypenrice]

Good. I wasn't clear how your rig was set up, Louise, but I think you have it about optimal.

Olly

[Christopher Davenport]

Hmmm not sure if this help, might not be a balance thing, but rather a PHD thing, close to polaris, PHD nudges are small, far from polaris the nudges need to be bigger.

Might be a aggressive setting or move to a different position after the guide setup routine.

Bad polar alignment become more apparent as you move away from polaris.

Just some ideas on trouble shooting.

[Thalestris24]

Good. I wasn't clear how your rig was set up, Louise, but I think you have it about optimal.

Olly

Ok, thanks Olly. Um, complete with 2.5kg York Fitness counterweight from Tesco!

Maybe I have some flexure which is only showing up when the scope points fairly high up.  I'll have to investigate!

Louise

[Thalestris24]

Hmmm not sure if this help, might not be a balance thing, but rather a PHD thing, close to polaris, PHD nudges are small, far from polaris the nudges need to be bigger.

Might be a aggressive setting or move to a different position after the guide setup routine.

Bad polar alignment become more apparent as you move away from polaris.

Just some ideas on trouble shooting.

Hi

It could be something to do with PHD2 or it could be mechanical/flex. I calibrated close to M31 but that didn't help. Somehow flex seems more likely especially as it isn't always guiding badly. It's been behaving erratically above 40 deg.

Cheers

Louise

[lensman57]

Hi Louise,

I take my hat off to you for your perseverance.  Imaging through a small window takes some dedication. I will not complain about the restricted view of my back garden again.

Regards,

A.G

[Christopher Davenport]

Only reason I don't think it is flex cause it is a finder guider, not much can move as your mount changes orientation.

My f guider is rock solid, not sure if anybody else knows of flex in a f guider.

Have you tried a drift align in the south and east/west?

[ollypenrice]

The movement could easily be in the primary...

Olly

[Thalestris24]

Hi Louise,

I take my hat off to you for your perseverance.  Imaging through a small window takes some dedication. I will not complain about the restricted view of my back garden again.

Regards,

A.G

Hiya

I don't have any choice unfortunately! Though it was my choice to take up AP in the first place so only have myself to blame! Maybe one day I'll win the lottery so I could buy a car and take myself off to a dark site or maybe move to Norfolk! A girl can dream...

Cheers

Louise

[Thalestris24]

Only reason I don't think it is flex cause it is a finder guider, not much can move as your mount changes orientation.

My f guider is rock solid, not sure if anybody else knows of flex in a f guider.

Have you tried a drift align in the south and east/west?

Hi Chris

My finder seems pretty sturdy though the objective isn't totally tight as I had to remove the locking ring in order to focus with the qhy5l-ii. I have some tape around it. It's something I can check. Hope the clouds stay away tonight. I can't drift align as I can only point to the east.

Cheers

Louise

[Thalestris24]

The movement could easily be in the primary...

Olly

Hi Olly

How could I tell/check that?

Thanks

Louise

[Thalestris24]

Hi

Just to add - on the plus side, from 10 x 600s of M33 I have some definite nebulosity showing along with quite good stars I'll try to get some 900s or maybe even 1200s subs though the latter might be hard to attain...

Edit:

I can get a good 60s unguided with nice round stars but... As soon as I start guiding I always have a feeling that the guiding itself is moving the mount around more than it needs to. I would have thought that in theory PHD2 would only need to make a correction once every 30s, say, rather than every ~2s, for each exposure. Am I being stupid here? Or is there a particular setting I can apply in PHD2 to improve things?

Nights are getting longer - yay!

Louise

[IanL]

You've probably already read it, but working through this may give you some ideas:

http://stargazerslounge.com/topic/188777-phd-guiding-basic-use-and-troubleshooting/

PHD2 has a few more options than PHD, but it is basically the same.  I've found that the changes to guide graphs in PHD2 tend to make things look a lot worse that they do in PHD, even though the actual corrections are pretty much the same.

Things to consider:

1. Cable drag - I can't see how you have the cables tied in from your photo of the gear, but having a long loop of loose cable from either the camera or guider can be enough to cause flexure. The best setup I have found is to run the cables from the cameras to one of the tube rings and attach them there with no slack (but not so tight as they pull on the cameras either). You can then have a loop to the mount head with enough slack for a full rotation of the Dec axis, and a second loop to the tripod head with enough slack for a full rotation of the RA axis, then back from there to the computer.  It might require longer cables, but actually if you don't have enough cable for that arrangement you're running a risk of the cable pulling the laptop anyway.

2. Is the finder attachment the type with one set of alignment screws and a rubber 'o' ring? If so that has definite potential to cause flex. Just because you can't see it by eye or feel it by hand doesn't mean it isn't happening. 1 arcsecond of flex is less than 0.0003 of a degree and you only need a few arcseconds of flex over the course of an exposure to cause visible problems on a typical setup. I think you've posted another thread about options for another guidescope where guidescope rings were also discussed - forget them as most of them are just not fit for purpose. Aim to get your guiding rig absolutely rigid to the main scope (or look in to using an OAG- but that brings its own problems).

3. In the same vein, you're imaging indoors - how solid is the floor?  My scope is set up on a solid concrete slab patio that is a couple of feet thick, even at 400mm focal length (guidescope) walking around near the scope is enough to induce noticeably more/erratic guiding corrections - unless you live in some kind of cold-war nuclear bunker it could well be part of the problem.  I tend to run my imaging rig from indoors once set up, and you might have to try something similar (most of the capture applications have the ability to put a delay in at the start of a set of exposures to allow the operator to clear the area).

Also, imaging through a window and over the roof is bound to induce a lot of thermal instability and affect the quality of your guide-star.  The only thing I can think of is to go for the longest guide exposures you reasonably can to try to minimise the effect so that you aren't chasing the seeing.  Bear in mind that you can't go too long or your guiding will not keep up with tracking errors.  30 seconds is almost certainly too long, but if you're doing 1 or 2 second guide exposures, try going to 4 or maybe 10 seconds to see if it helps.

If you think PHD is issuing corrections too frequently, tweak the min-motion parameter so that it will only issue a correction once the specified amount of error has built up.

4. Try to break down your bad guiding in to RA and Dec.  Are you getting bad guiding on both axes when you change altitude or just one axis?  RA guiding issues are usually simpler to figure out as it's usually just a matter of getting reasonable polar alignment, sorting out the balance and getting decent calibration.  You're already doing the right things in terms of trying to keep an east-side heavy setup and avoiding changes of balance by keeping the cameras and other kit close to the RA axis.

Dec can be more troublesome as you have to factor in the added complications of trying to avoid changes of direction in Dec guiding (as you then have to cope with backlash in the gears each time you change direction), plus figuring out which way to imbalance the scope depends on where you are pointing and also which direction you have drift.  ideally you want a small polar misalignment so there is a constant drift in one direction, and thus all the guiding corrections will be one way = no backlash.  You then need to move the OTA forwards or back in the rings to keep the balance pulling slightly in the opposite direction to the Dec corrections.

5.  Bear in mind that near the meridian and the zenith you're going to have problems with erratic guiding on this type of mount.  I don't know how far west or how high your rig will let you point through the window, but at the zenith the best option is to just wait for the target to track through a bit.  Lower down on the meridian I've had mixed results with my NEQ6. Sometimes I've been able to track a fair way through the meridian with no guiding problems (and also had good results with forced flips so the mount is counterweights high just before the target tracks through the meridian).  Other times it has been a bit of a train wreck.

[Thalestris24]

Hi Ian

Thanks for your long and comprehensive response - much appreciated.

I only seem to have guiding problems when trying to image M31. Because of the window I'm very limited as to when and how long I can attempt it but that's something I have to live with. Currently, for M31, that's between about 68/36 deg (Az/Alt) and 78/45 deg. Then I move down to M33 between about 79/31 and 89/37. 45 is my upper Alt limit, 89 my Az limit. The M31 guiding is erratic but M33 is fine. I can potentially change the az limits somewhat by moving the setup but the alt limit is set by the top of the window... Anyway, so something appears to be going wrong at higher altitude and/or more eastern azimuth.

My cables are cable-tied to the top of the pedestal which is only 55cm high. I can't see that cable drag is likely but I'll keep an eye on them next time.

Yes, the finder has an O-ring and two adjustment screws (standard SW 50mm).The erratic guiding is there straight away when I try to guide at M31.

I'm stuck with the floor I have. It has a laminated surface, presumably over floor boards. The setup sits on a foam mat and on top of that I have a wooden board between the two north-facing legs of the pedestal. My solution is to sit on the other side of the room and avoid movement!

I usually guide using 2s (mostly) to 3.5s exposures. I'll maybe try longer ones if I can find a suitable guide star. I'll also try increasing the min movement parameter.

I can only point east so a long way from the meridian and the zenith. It's only since I got the pedestal that I can reach an alt of 45 deg. Before, with the SW tripod, I could only get to about 32 deg...

Maybe the guide problems are a combination of things. Thinking about it, pointing at higher altitudes and more east will certainly have an affect on the centre of gravity of the whole setup and this might affect it's stability on the floor.

I'll just have to try a few things (as above)!

Thanks for your input

Cheers

Louise

[Thalestris24]

Hi

Oh well, I managed to get 4 x 900s of M31 last night. The guiding wasn't great but maybe better than before. Unfortunately I've picked up a nasty gradient stemming from the bottom left. I've no doubt it's due to the street light outside . Any ideas how I might get rid of it in processing without ruining the target? At least I have some nebulosity so quite pleased about that The stars are a bit eggy in the top left - field rotation??

I also managed 4 x 900s of M33. Guiding wasn't as good as the other night. I'm wondering whether the guiding troubles are simply/mainly down to the atmosphere though I can see on the graphs that I have some periodic error. Anyway, added lasts night's to 10 x 600s obtained previously. Once again, have the horrid gradient.

M31:

M33:

I'll be grateful for any feedback!

Thanks

Louise

[ollypenrice]

The demon background flattener and gradient remover is Dynamic Background Exctraction in Pixinsight. So far as I know it has no rival. Gradient Xterminator, an affordable Ps plug-in by Russ Croman, is second best (again so far as I know.) Now that I have PI I only use DBE.

How to see if your mirror is moving? Tough question, without trying an OAG to see if it solves the problem.

Olly

[Thalestris24]

The demon background flattener and gradient remover is Dynamic Background Exctraction in Pixinsight. So far as I know it has no rival. Gradient Xterminator, an affordable Ps plug-in by Russ Croman, is second best (again so far as I know.) Now that I have PI I only use DBE.

How to see if your mirror is moving? Tough question, without trying an OAG to see if it solves the problem.

Olly

Thanks Olly

I just watched a DBE video tutorial  It looks like it does the job! Would need to save up for PI... I would normally just use the wipe function in StarTools. That removes gradients but it also appears to remove the nebulosity at the same time! I'll have to check if there's a way round that. I think I also need to think about whether it might be possible to prevent the problem. Prevention is always better than cure! A longer hood might help. This is the darned culprit:

I wish it wasn't there!

Cheers

Louise