Guiding advise

[Coco]

Can anyone shed light on whats going wrong here.. I can manage 5-6 mins but any longer I'm getting eggy stars..

Since imaging at 1,000mm Ive noticed this... below 600mm It hadnt been a problem.

The SW80 is bolted to the top of the MN190 which has been converted to Losmandy mount.. Im sure its not flexure..

Should I be using shorter times on the QHY5 ... ? Help

Help would be appreciated..

[shaunster]

is it just in one corner? can we see a full frame?

[Jannis]

is that at the center also, or just on the sides?

[Coco]

Thanks...this is typically across the whole image.

I thought it was my guiding.. my scopes super duper balanced.. a 10 penny piece will tilt the scope what ever part you place it on..

Its doing my head in!

[robbieince]

Hi guy

Try longer guide exposures sat 3 secs. It helps to damp out chasing the seeing.

Also if you are convinced flexure is out - how about field rotation? Is your polar alignment dead on? Is your guide star off axis from your image field?

Try some really long exposures to see if the problem is proportional to exposure time.

These can be a pig to pin down. Shout if you want another pair of eyes.

Regards

Rob

[ollypenrice]

Balance; perfect is usually not what works best. The east should be a tad heavy to keep the mesh on the same side. When perfectly balanced you get oscillation across the play in the gears.

Polar alignment is a candidate, as above.

Maybe the corrections are a little aggressive. Can you slow them a little? (In my system, AstroArt, you do this by telling the software that the scope moves faster than it really does so the corrections being sent are reduced.)

Flexure: you can never be sure you have eliminated it!! Check the focuser where it screws into the main tube, check everything.

Looking carefully at your stars I think it is backlash arising from perfect balance. You have, essentially, two perfect stellar images slightly juxtaposed. You don't have one smeared stellar image but two good ones offset. Classic backlash. If you could remember which way the camera was orientated with regard to RA you could add to the evidence. If I'm right the elongation should be along the line of RA.

The joys of longer FL imaging. This is why I am always banging on about learning the ropes on a shorter one!

Olly

[gfletcher]

I had exactly the same problem last night.

I was imaging the rosette nebula using 8 min subs.

Imaging through my 250px with ST80 guidescope.

I was using 1 sec intervals on PHD.

Only 3 of the 10 subs i took were usable, the other 7 had tracking problems.

Maybe 8 minutes is too optimistic and i should reduce to 5 min subs ?

Graham

[roundycat]

I'm with Olly, perfect balance in the form of being able to tilt the whole thing with the weight of ten pence is useless. You need a sizeable out of balance in order to beat backlash in both RA and Dec. If you have any backlash settings in your software, set it to zero.

Dennis

[kookoo_gr]

it could be comma form bad orientation of the camera. check the orientation of your cameras. i place my imaging camera so that i have vertical-horizontal movements at my image when i move my mount. also check the collimation of your scope with ccdinspector

at what focal length are you guiding? if u use a finderscope as i guider i would suggest a longer FL as a guidescope. also i would suggest looking if the fan of the imaging camera gives you vibrations. place a small stick at the blades of the fan to stop it and then try again. if you have no egg shaped stars then remove the stick and try a few more shots without it. i had a similar problem with my atik 16 ic even at 1 sec of imaging time and i placed an on off switch at the fan. the fan is for removing the excess heat form the electronics. the sensor's heatsink is the camera's outer casing so if it is the fan causing that problem then you can use a switch.

from my experience perfect balance is not the main issue of trailing if you have serviced your mount correctly and especially if you have removed the backlash from the gears of the motors. i balance my scope perfectly at my mount and i have achieved 15 mins of exposure with my newton telescope (now i use an RC scope)

lock the draw tube of the focuser of both telescopes in order to remove flexure what type of focusers do you have and are they placed correctly?

another main cause of trail are the mounting rings of the imaging scope and more specifically if it is a newton scope (since its length is prone to vibrations) and you have a vixen type dovetail since they are more susceptible to flexure. in order to face this you will need a wider plate (lomsandy preferably) for placing rings with a wider base ( http://www.dodgenoptical.com/images/parallaxrings.jpg ). also bear in mind that the mounting rings (if you use a vixen dovetail) are not perfectly level. at mine i used a fine file and filed the base of the rings in order to increase the surface that touches at the dovetail.

after polar alignment make sure that all the knobs (altitude an azimuth along with the screw that screws the mount head to the tripod) are tight

best of luck at solving your problem

[gfletcher]

These are the resultant images after DSS and processing in CS4

The first one is the result of 3off 8 min subs and the second one is using 7 off 8 min subs of which 4 of them showed the egg shaped stars.

[gfletcher]

The image using the 7 subs has admittedly less noise but shows the effects of star trailing

Graham

[perfrej]

I second the notion of going for a slight unbalance in the rig. Backlash and oscillation around equilibrium are my prime suspects in a drama like this.

On my NEQ6 with a heavy Skywatcher Explorer 250, Orion guide tube and a DSLR I have about 3 kg of unbalance on both axis and I track dead on with 1s, 2s, 4s and whatever-s exposure in PHD guiding. I find 2s to be a good average though.

You should go at it again and just move the dovetail down a few inches and push the counter-balance weights up the shaft a bit.

Let us know how that turns out.

Clear skies tonite...

[opticalpath]

I agree with the advice to slightly unbalance in Dec and to the east in RA, to reduce effects of backlash. However, if the diagonal trailing has got both RA and Dec components, it could still be differential flexure which can be a b****r to track down.

Here is something to try, to eliminate (or prove) flexure as a possible cause:

Set up your autoguiding and once it has settled take a short exposure (say 0.5 sec) of a fairly bright object star. Let the autoguiding run for an hour then take another short exposure. The individual exposures are so short that the star image in each frame should show no trailing. BUT if the two images show the object star has moved in the frame to a significantly different pixel position .... and the autoguider still shows the guide star centered, this would prove that there has been some shift between the relative pointing directions of guide scope and imaging scope.

In fact, if you took a seies of 0.5 sec exposures, say one every 15 minutes, for a couple of hours, flexure would show up as a steady (usually diagonal) drift of the star position in the frame. Stacking the images with no alignment will show this up.

A backlash problem alone would tend to smear the star image back and forth over a small range giving an elongated shape, but the star would stay centered around the same approx. pixel position in each frame. Flexure would show up as a bigger displacement of the star position, if you wait long enough. Examining a series of short exposures taken periodically over a long period - without aligning them - will reveal this.

Good luck tracking down the problem.

Adrian

[Sparrow]

Adrian - I'm quite interesrted in the symptoms and solution you describe for flexure. I get those very symptoms - only very mildly but I've done everything I can to illiminate or reduce backlash so I'm wondering whether pigybacking like my setup can lead to this whereas a side by side set up wouldn't - just a thought.

Back on topic for the original question, looking at the graph I'd agree with previous comments that balance and backlash are prime suspects. I solved this by making the scope nose heavy when pointed to the East where I always work, and reducing the backlash settings in the mount handset. By default they're at 99 and I reduced them down to about 50.

[opticalpath]

I think that backlash-induced errors would would be like oscillations and would not accumulate over time: the guider would just keep giving correction impulses until it 'catches up' with the lost motion, so would return the object stars to the correct start position eventually .... until the next drive reversal and delayed correction. The star would elongate, but would keep returning to the start position. The amplitude of the movement back and forth stays within limits and elongation of stars would not get much worse with longer exposures.

Flexure, on the other hand looks like a slow but steady drift in one general direction: you see the stars marching slowly in one direction from one frame to the next. Individual long exposures show stars slightly elongated in the same direction, but crucially, there will be a cumulative shift of position from one frame to the next. When we align and stack the images, we still see the elongated stars in the summed image due to trailing during each exposure, but the flexure drift from one exposure to the next is 'aligned out'. That's why you really have to examine each frame, or an UNaligned sum, to notice the steady drift caused by flexure.

Elongation of stars caused by flexure DOES get worse with longer exposure.

Adrian