If a 127mm MAK is the autoguiding OTA - what CCD would you recommend?

[g__day]

Interested in folks opinion here. I wanted to try guiding my C9.25 with a longer focal length tube, so I switched from a piggy backed 80mm refractor (focal length 480mm) to a Meade DSI to 127 mm / 1500 mm focal length MAK into a Meade colour DSI.

One problem is now stars are huge! Rather than be near pin point for example Canopus last night via PHD on a 3 second frame grab cycle was about the size of a pea! Say 3-5 mm across. I am not convinced PHD can find the centroid of this star accurately - especially as atomspheric effects kick in.

I am really trying to nail autoguiding better - so any suggestion would be much appreciated. The mount, Losmandy bars etc - have the capability and rigidity, its a permamnent set-up - so my next target is a CCD better suited to the 5" MAK.

All thoughts and suggestions greatly appreciated!

http://ccdware.infopop.cc/eve/forums/a/ga/ul/5321099852/inlineimg/Y/Triplet_rear_all.jpg

[Terry B]

Why are the stars so big?
Have you tried the guider in the main scope? Are the stars still just as big?
Is is the focus?
To reduce the size if focus is perfect just reduce the exposure time &/or gain.

[g__day]

No idea!

Exposure time - I want over 1.5 secs to avoid atomspheric effects, gain - do'h will try that next to see if it helps!

Focus I have played with - its about as small as I can get the stars - maybe too much blooming - very faint stars are very sharp.

Matthew

[gbeal]

Perhaps try a dimmer star, it will go hand in hand with your longer exposure time requirement, plus it may be better looking??

[g__day]

Agreed - if one is in the frame - still the questions I'd like to know is for that OTA - in Sydney surburb skies (i.e. average viewing at best) what would be the ideal CCD (pixel size, signal to noise, economics).

Is a DSI good enough or should I either marginally or alot better to balance CCD to OTA combination well?

Cheers!

[citivolus]

I wonder how much cooling of the Mak would come into play for star size with guiding?

[g__day]

Not sure - the MAK is in a astro lab and temperature is pretty constant during the day. Although focus seems very sharp when you have a quality eye piece in the MAK - focusing on the DSI is laborious. Even with the gain turned right down I might describe what I see as blooming - a white, bright central dot - surrounded by a haze. ANt this is the same from an hour after sunset to 4 hours later - so I expect heat isn't the key factor - maybe its collimination?

Of course cloud cover doesn't help any - and to date with all the cloud cover I am focused on minute improvements to alignment and tracking - I'm a one problem at a time kinda guy if I can prioritise things.

Life should be eaiser when imaging something faint - funny that its the bright stars causing me grief!

[allan gould]

I routinely use a 90mm MAK to guide with. I keep the exposure times low and have not noticed bloated stars. I use an XY shifter to pick a nice star on the screen and guide on that. I find that for a 2500mm scope the mak guides very nicely but I must state that I use an integrating video camera (Watec 120N+) with a TVGuider.
I think that from what you are describing you are getting star bloat and that you either should use a dimmer star or drop the guiding rate to 0.3 or 0.5X siderial which may help. Maybe guidemaster would be a better choice of program to use with a DSI.
Regards, Allan

[g__day]

Just spoke to Don at BinTel who advises - pixel size is too small for that focal lenght - get a $49 focal reducer for the DSI itself!

Nice one Don!