Differential flexure or field rotation

[Psychobilly]

Looking through the latest log files the biggest excursions are around +/- 0.6 pixel...

apart from when i change guide star and it takes a few seconds to "center"...

Peter...

Here's the worst from the recent sessions....

Vertical axis - Error in Pixels Horizontal time in Secs

[ATTACH]23230[/ATTACH]

[ATTACH]23229[/ATTACH]

[themos]

eq-mod.sourceforge.net/pecprep/index.html

pecprep also analyzes PHD logs

[Psychobilly]

Thanks Themos...

Just plugged the figures for the 500mm f8 and DDI IIc into CCD calc and its coming out at 3.48 arcsec per pixel ...

Here's the corner and center stars from that run - crops from full res image...

Top Left

[ATTACH]23248[/ATTACH]

Top Right

[ATTACH]23249[/ATTACH]

Bottom Right

[ATTACH]23250[/ATTACH]

Bottom Left

[ATTACH]23251[/ATTACH]

Center

[ATTACH]23252[/ATTACH]

Should I leave well alone?

DSS was giving FWHM between 3.4 and 3.6 for the subs for that stack

Peter..

[Ibbo!]

Hi Peter

I would leave alone

Also the problem could be overcorrecting so the guider is constantly going back and forward, try lowering the aggressiveness or guide rate if you can it might help

A bit counter intuitive I know

Steve

[Psychobilly]

Thanks Steve,

Most of my imaging is done with the 1000D on the Megrez 72 FF III which is coming out at 3.4 arcsec per pixel as well...

Peter...

[Starflyer]

It's my understanding that an osc index of 0.5 represents a perfect level of aggressiveness, <0.5 and PHD is always trying to catch up, >0.5 and the adjustments are over correcting.

That said, I have the same problem and can't tie it down, but I guess it's flexure somewhere in the system. I've been imaging mostly with the 10" f/6.3 newt and a cheapo Bresser guide scope. The Bresser focuser is slightly sloppy even with the adjustment tightened all the way up, time will tell if the WO66 (partially) solves the problem.

Here's some of the graphs from my latest M51 image, and a crop of the trailing problem I have, the graphs look almost perfect and the best I've ever achieved. Any advice muchly appreciated

[MartinB]

Starflyer, it could be the focuser or flex around the newt. The guiding looks ok. Peter, were these logs still using Vega? The erratic behaviour that you get chasing the seeing or using an oversaturated guide star. When you see this pattern, first check the ADU value of the guide star and if it is too high then find a dimmer star. If it doesn't improve then lengthen the guide star exposure making sure the star doesn't become too bright. In poor seeing it is best to go for a minimum of 3 second exposures to average out seeing fluctuations. This chasing of the seeing won't stop you from having round stars and take those fwhm estimates with a pinch of salt, if you run them through CCDinspector you are likely to get a different result altogether!

[Psychobilly]

Thanks Martin...

The later logs were from one of the "filler" panes of the cygnus mosaic the same one that the 5 crops came from.

I normally use the last alignment star to focus up the scopes and also do the initial calibration in PHD Guide.

Next time out i'll try longer exposures on a dimmer star and see what happens..

Thanks again.

Peter...

[Aza]

Another possible solution could be to use one of these:

SCA T-Adapter, Astrophotography, SCA laser collimator, laser collimator, astronomy product, telescope accessory, self centering adapter, accuracy alignment, collimation

(available from FLO)

I have seen this on another forum, where someone has stopped getting potato stars, by using this to eliminate the use of thumbscrews.

I am going to try this myself, once I figure out how to use it in conjunction with my QHY5 and WO72! As I currently use the diagonal to achieve focus.

[Psychobilly]

Try and avoid diagonals at all cost when imaging... and where ever possible use Threaded connections...

My guidings improved since I have moved to a dedicated guide setup wich is a 500mm f8 telphoto lens (t-mount) with a 50mm t extension fitted to a DSI IIC... everything is nice and square and solid... I can Achieve infinity foicus but just not at the infinity mark on the lens with this setup and its working great... and cheap as well...

It's freed up both the "proper" scopes for simultaneous imaging as well

Peter...