problems tracking / collimation?

[Gerhard]

Hi all!

Yesterday I was out taking a few pics of M56 and M71 (no nebulas or galaxies, because the moon is still quite full).

This was the first time I tried my new coma corrector, which I think works fine. One RAW sub:

A few problems, though. As you can see from the above RAW example, there is no coma anymore, but all stars are kind of triangular in shape.

Is this due to collimation error, ie warping the primary by tightening too much?

I tracked (unguided) using also PEC...

Second problem. On RAW sub that wasn't so good...:

These subs are frequent, one in every 4, on average... I'm guessing my HEQ5 is old and not too precise anymore... Or could it be something else still?

Here is the stack of the good subs (no post production), to show the beneficial effect of the coma corrector again:

Ideas?

Gerhard.

[Owmuchonomy]

Just taking the second light frame you posted this is representative of backlash in one of your axes. Before a session I ascertain the orientation of my chip relative to the mount axes by taking a single sub and moving the mount in each of RA and DEC. Then I can easily diagnose any issues relative to the mount axes. I would put a pack of Jaffa cakes on your mount showing backlash in RA. There are lots of reasons for your finding. How good is your polar alignment?

[Gerhard]

Just taking the second light frame you posted this is representative of backlash in one of your axes. Before a session I ascertain the orientation of my chip relative to the mount axes by taking a single sub and moving the mount in each of RA and DEC. Then I can easily diagnose any issues relative to the mount axes. I would put a pack of Jaffa cakes on your mount showing backlash in RA. There are lots of reasons for your finding. How good is your polar alignment?

hmmm... I saw this option in the Synscan menu. I'll have to look into it... So you're saying that while you're taking the sub you move the mount in RA and then in DEC?

And then how does one see if there's backlash? I'll study about this.... @work now, so cannot get distracted too much...

I think (hope) my polar alignment is quite good: I always perform drift alignment as well after aligning with the polar scope... I don't wait half an hour to see if the star drifts, but at least 5 minutes for both RA and DEC... And yesterday they seemed quite stable....

[StargeezerTim]

Regarding the triangular stars...I had a similar problem and it turned out to be pinched optics. I cleaned the mirror and I think I tightened the clamps too much when I reassembled it. It was easy to rectify, just loosen the mirror clamps a little.  Tim. 

[Gerhard]

Regarding the triangular stars...I had a similar problem and it turned out to be pinched optics. I cleaned the mirror and I think I tightened the clamps too much when I reassembled it. It was easy to rectify, just loosen the mirror clamps a little.  Tim. 

will do! :-)

[Gerhard]

hmmm... I saw this option in the Synscan menu. I'll have to look into it... So you're saying that while you're taking the sub you move the mount in RA and then in DEC?

And then how does one see if there's backlash? I'll study about this.... @work now, so cannot get distracted too much...

I think (hope) my polar alignment is quite good: I always perform drift alignment as well after aligning with the polar scope... I don't wait half an hour to see if the star drifts, but at least 5 minutes for both RA and DEC... And yesterday they seemed quite stable....

I read somewhere (couldn't resist googling backlash problems... :-) ) that backlash is seen when the slew movement is not responding well to the controls on the keypad.

This I have seen A LOT with my mount. So next time I'm out, I will look at this definitely, and adjust my backlash settings accordingly...

Next time will be a while, though... clearoutside shows bad weather all the way up to next tuesday....

[glowingturnip]

the backlash compensation on your mount is good for observing, but not so good for imaging.

Basically when you're observing, and change direction on the direction pad, there's a dead zone when you press it and nothing happens, until eventually it catches (and invariably overshoots).  While it's not moving, that's the backlash being taken up.  You can program the mount with a backlash compensation setting that you have to find by trial and error, so that when you change direction, the first few presses act faster than normal, so it takes up the backlash quicker.

The PHD guys recommend that you don't use backlash compensation for guiding though as that can result in unpredictable behaviour.  Try the following:

- if the guiding errors are occurring in the RA direction, moves the weights slightly so that the set-up is 'east heavy'.  ie if the scope is on the west of the mount, move the weights further out, but if the scope is on the east of the mount, move the weights further in.  That will mean that the RA tracking is always pushing the mount against the slight imbalance, and it doesn't wobble in RA backlash.

- if the guiding errors are in Dec (and they often are), it's trickier.  Try the 'resist switch' algorithm in PHD which tries to avoid changing direction and entering the backlash unless it has to.  Also if you find you have a clear drift one way or the other in Dec, then you can try setting the dec guiding to 'north only' or 'south only' instead of 'auto' - try both and find which one is trying to adjust the drift.  That will mean the mount will never voluntarily enter the Dec backlash.  You can also try making the scope slightly tail-heavy to counter Dec backlash to keep it on one side of the gears, but that has limited effect (eg will have no effect if pointing straight up)

[Gerhard]

the problem is, I don't guide... maybe it was not clear, but the problems I have are with tracking with the mount only... So I have no PHD on which to rely...

I wish... :-)

in fact, I think if I started guiding, I would get better subs... I'd sincerely hope so, since that is the whole point of guiding... :-D

[Gerhard]

I will be redoing the PEC training as well, as precise as possible...

[Tinker1947]

I used the handset to do a 2 star align, the use the polar align option for the PA, run through it twice on different stars and while i didn't extend the sub time to find the limit a 3 minute sub had no trailing just points of light, Balance and PA are crucial...

[Owmuchonomy]

I will be redoing the PEC training as well, as precise as possible...

That is unlikely to reduce your backlash. Unguided, the best I ever reached was 90 second subs unguided at 715mm focal length. The best thing I did to my mount's hardware was this:

[glowingturnip]

if it's unguided, it's unlikely to be backlash isn't it ?  PHD won't be pushing the mount around, so will just be the RA drive constantly pushing, and the Dec doing nothing.  More likely periodic error unfortunately

[Gerhard]

if it's unguided, it's unlikely to be backlash isn't it ?  PHD won't be pushing the mount around, so will just be the RA drive constantly pushing, and the Dec doing nothing.  More likely periodic error unfortunately

I'm just now coming to understand what backlash is, so if you say so! ;-)

if it's periodic error, which I also suspected, could a good PEC training maybe have some positive effect? eliminate at least those errors?

This periodic error would disappear if one starting guiding, no? But maybe THEN I would have backlash problems...

it's a mess.... :-)

[glowingturnip]

yes, PEC training will help, but you'll need to be very good !  Make sure that the correction speeds are low, something like 0.125x tracking speed, and you'll need s steady hand and eye and a reticule eyepiece to keep it centered.

Guiding will make an awful lot of difference, and doesn't cost too much to get started - I use a Celestron T70 travelscope got off of ebay for £40, and started with a simple webcam (a meade LPI also off ebay)

[Gerhard]

yes, PEC training will help, but you'll need to be very good !  Make sure that the correction speeds are low, something like 0.125x tracking speed, and you'll need s steady hand and eye and a reticule eyepiece to keep it centered.

Guiding will make an awful lot of difference, and doesn't cost too much to get started - I use a Celestron T70 travelscope got off of ebay for £40, and started with a simple webcam (a meade LPI also off ebay)

for PEC training I will be in fact VERY still, not moving the position of my eye relative to the eyepiece, and with slow speed. I have a reticule eyepiece, 12,5 mm, with 2x barlow to do this job as precise as possible...

for guiding the biggest part will be getting a laptop... I only have a desktop at home (just bought it new a year ago, when I didn't have my scope yet... of course now I would have taken a laptop...), and my work laptop I cannot use... The guide scope is already ready for me: the president of my club, who sold me the newt, also has a guide scope especially for my OTA. 

So, laptop and CCD... plus I don't know what I'll have to pay for the guide scope...

maybe some extras, like an adapter to put the laptop on my battery... I don't see all this happening before new year's....