OAG at very long FLs

pmrid (Peter) · #1 23-06-2011, 05:58 PM

Wow. this is not easy. I am trying to eliminate elongation in stars from my imaging and have decided to go down the road of OAG'ing. But I'm using an EdgeHD14" SCT which means that I'm stuck with running at F11. This means a FL of 3900mm or so.
An OAG picks up stars off the edge of the light cone - hopefully a corrected edge but the way the thing has to be set up, the mirror in the OAG is actually only about 50mm or so from the back flange. The fixed distance from that flange to imaging plane according to Celestron is 146mm so the stars the OAG is picking up are not corrected. And at that FL, the imaging chip in a guide camera (mine is a QHY5) struggles to get me any decent kind of focus. I'm therefiore working with blobs rather than stars for guiding.
And at that FL, the graph looks like a lie detector on a politician.
Can anyone offer me any pearls of wisdom in the art/science of guiding at this FL?

Peter

Bassnut (Fred) · #2 23-06-2011, 06:19 PM

You definitly need OAG at that FL for a start. Out of focus or slightly distorted stars is not neccessarily a huge problem, out of focus can be an advantage. Why cant you shift the guide cam height to get closer focus?.

gregbradley · #3 23-06-2011, 06:35 PM

When an OAG is setup correctly the distance from the corrector to the plane of the autoguider chip and the distance from the corrector to the imaging chip will be the same.

So that is not the issue.

The issue here is you can often adjust the prism to go down lower into the light stream so it is not right out at the edge.

I use a Planewave CDK17 at 2959mm focal length and a MMOAG with an ST402 guide cam. I find there is a bit of flex with the eyepiece holder arrangement of the guide camera and MMOAG. I sometimes would get very eggy stars. I found the camera would rock slightly in the eyepiece holder and pulling it back (it was a bit stiff) would round out the stars.

Also I got a parfocal ring from Scope Stuff. That locks onto the nosepiece of the ST402 so even if I remove it it is set to the right focus like a stop when I put it into the eyepiece holder of the MMOAG.

Unfortunately though it really needs another screw (it has 2) so it can lock on with a slight amount of tilt. Perhaps drilling a tapping a small hole would improve it. I was considering a screw adapter of the correct length to rid it of any flex. Not sure if it is worth it - perhaps.

Anyway - try lowering the prism lower into the light stream. The MMOAG came with some spacers to do just that.

As Fred says the guide star does not need to be round to still get excellent guiding. The software calculates the centroid of the star to a subpixel accuracy anyway.

A lot of stars are double stars as well. Try another star if you have bad guide errors with one. I often find the simple selecting of another stars can suddenly improve guiding a lot.

Greg.

pmrid (Peter) · #4 23-06-2011, 06:55 PM

Thanks Greg, Fred.
Got all that. I am able to shift the guide cam in it's holder. That's not the issue. But getting a nice sharp focus doesn't seem to be within the means of the QHY5 at this FL. Still, with some Darks and goosing around I have been able to get guiding. I'm using a Lumicon giant OAG at this stage pending a decision to go upmarket. This is toe-in-the--water time.
I have been running a series on 6188 - seems a popular choice lately - just some Ha at the moment. The subs are showing nice round stars. I'm delighted with that. So clearly, this is what I have to do. There can't be any question of going back to any other form of guiding.
I'll throw someting up on the Deep Space thread later tonight after I have stacked a few of these.
Peter

ericwbenson (Eric) · #5 23-06-2011, 06:58 PM

The imaging chip and the guide chip are exactly the same distance from the back flange, if they are both to be in focus...

At 25mm off axis the C14 RMS spot size is 60-65 um, depending on the back flange to focal plane distance (essentially what the focus knob is set too since that changes the mirror spacing). At 15mm off-axis it's about 35 um, so choose the smallest OAG you can get away with. An Edge C14 should have smaller spots than this, but it is new enough that I don't believe anybody has posted quantative results yet.

BTW a "fat" guide star does not mean you can't autoguide, it just means the faintest star you can use is a bit brighter than in a fully corrected OTA.

Noisy tracking graph? Find out what the peak to peak variation is in arc seconds, you may be looking at magnified noise from the huge image scale, or bad news, your mount is too woobly.

EB

bert (Brett) · #6 23-06-2011, 09:30 PM

Peter,

I'd be concerned at the pixel size of your guide camera. The qhy from memory has around 5.4 micron pixels, and it does not support binning. Im guessing that is why you bouncing around so much. The image scale of your guide camera is off the charts.

From my experience I found that I run my guide camera at bin2 (bin 1 is 7.4 micron) under a meter of fl and bin 3 for over that. I once made the mistake of trying to guide at bin1 at 2300mm. What a mess that was, just by changing the binning to bin3 settled it right down.

Brett

Tandum (Robin) · #7 24-06-2011, 01:01 AM

Maxim can X2 bin a QHY5 but the phd wiki says :-
binning is going to hurt your guiding accuracy by cutting the spatial resolution in half

gbeal · #8 24-06-2011, 05:01 AM

Give it a shot Robin, can't hurt. I bin the Lodestar, and the ExView bins automatically. Worth a crack.
Gary

Tandum (Robin) · #9 24-06-2011, 09:40 AM

I don't see any difference at the FL's I use Gary but I normally guide at about 1/8 of Peters FL

g__day (Matthew) · #10 24-06-2011, 04:45 PM

I use a C9.25 and a Losmandy OAG and a Meade DSI II Pro mono for guiding via PHD. A few suggestions (thought my FL is only 2.3 metres compared to yours).

1. Ensure everything is rock solid firm - if even one screw is a bit loose - holding your cameras - you'll get differential flexure and that will spoilt your shots.

2. I had to add a extension tube to bring the DSI II into focus - but it sat a bit loose in the OAG guide tube. I wrapped the DSI II end in sticky tab - a few revolutions, until it was quite snug, and guiding improved dramatically.

3. I focused my main camera using a Bhatinov mask on Antares, then focused the guide camera using Juptier on with the Bhatinov mask. upiter at 5 second exposures was just bright enough to give differaction spikes on PHD so I could focus correctly.

Lastly thought - I am sure you have checked for any gear slipagge - triple check - from every angle. I though my gear was really tight and firm - until CCD Inspector showed the main camera was 7 degrees! tilted - it had slipped and been re-tighten at a slipped position! Differential flexure is probably your main enemy at long focal lenght guiding - so check from every angle in the daylight that your gear on the imaging train is rock solid!

Cheers, Matt