C9.25 with OAG and Cameras in place

[Tim]

Took this photo to show Todd8137 my SCT setup, but thought it might be of interest to others too.

Without the reducer in this configuration the scope is guiding at F10. Tricky, but I can get 30min+ exposures with round stars. With a 0.63 reducer in place it all becomes a bit easier

Pic shows:

C9.25>Celestron OAG (with DSI Pro guidecam)>spacers>Filter Wheel>Atik 428ex mono camera.

Cheers

Tim

[tollytastic1]

Hi Tim, very interesting photo so thanks for posting. 30 minuite subs at F10 is very impressive indeed, may even be tempted to give it a go!

Paul

[Tim]

They are still rolling off the camera now, OSC index of 0.48 and RMS 0.48 in PHD. I smooth out the guide commands further in eqmod.

You don't have a massive choice of guide stars, but you only need one

I'll post an example in a few minutes

[Tim]

Here's a crop of a single sub of an image I am currently doing, just to show the star shape achievable with 30 minute subs at f10. Naturally at these sub lengths I am using narrowband filters.

It's dark, I have only given it a slight stretch.

The mount is a bog standard, four year old EQ6, which has had no work or modifications whatsoever. Focal length of telescope in the above configuration is around 2400mm. Image is binned 2x2, resultant scale is around 0.8 arcsec/pixel, probably pushing the limits of the seeing.

Cheers

Tim

[Magnum]

Hi Tim, I have just been given a Vixen OAG to use with my meade 8" SCT on my EQ6 PRO.

Do you have any advice on using an OAG?

First problem I have with mine is that with the DSLR attached directly to the back, when its in focus I cant get my webcam to achieve focus as it needs to go further in, so looks like I will have to turn a bit off the top of the eyepiece holder.

Second when viewing with an eyepiece or webcam the guiders field of view is the shape of a rugby ball and I seem to be looking right at the edge of that view, The only adjustment I have is the eyepiece holder is moveable over the prism but even then I seem to be right on the edge of the field of view making it almost impossible to get a guide star on the screen.

Third when I do get a star on the screen it does not focus to a point, either side of focus is rugby ball shaped and in focus is a short straight line, ? is it possible to guide on stars this shape?

I guess what im asking is, are these problems inherent to every OAG or does it sound like there is something wrong with the one I have?

Cheers

Lee

[Tim]

Hi Lee,

I'm not familiar with the Vixen OAG.

You should be able, and you need to, get reasonably round stars. The focus doesnt have to be as pin point as for imaging, but you do need a clearly defined centroid, so star images that are stretched, doubled, squared won't give as good results.

You should be able to extend the backfocus to the camera with some spacers, just a few mm, rather than turning down the OAG. If you measure from the centre of the prism to the camera CCD, that is roughly the measurement you need from the centre of the prism up the stalk into the webcam chip

(I'd be surprised if you could guide with a webcam on an SCT, might be wrong)

If you can't figure out the OAG prism, send me some pics or something and i'll take a look, something doesnt sound right. Better by pm, I regularly forget to return to threads like this one :s

Cheers

Tim

[Magnum]

Tanks Tim

I already turned 10 mm of the eyepiece holder today so both cameras are now parfocal with each other with no spacers.

I just went onto a star but had the same strange star shape, blobby cross when in focus then becoming a sharp line either side of focus, and as i move focus in and out i can also see that the line flips 90 degrees either side of focus.

The only adjustment i have with the prism is that i can unlock it at rotate it.

I will try and get some photos for you.

Lee

[Tim]

Is the prism far enough into the light cone? It may or may not have a height adjustment?

has the actual prism itself gotten knocked off square or something? They sometimes have a little allen bolt holding them in place.

Hmmmm, thinking about it, if the OAG is picking up a star right on the edge of the field, maybe the scope itself has some astigmatism. The line shape stars flicking 90 degrees either side of focus is typical of that.

Try a quick star test with an edge of field star and see if you get a similar result without the OAG in place. It might just be that the OAG is picking up an existing problem.

[Tim]

Link for typical star shape with astigmatic optics

Starizona's Telescope Basics

I'm assuming you aren't using a diagonal in the train?

[Magnum]

I have just tested with my apo so I don't think its a problem with the scope.

I think I could get the prism another 1 mm into the light cone by making a washer tomorrow.

The prism is held into its mounting via 2 tiny grub screws, I though maybe the prism was pinched so I have just tried loosening them then doing them up just enough to hold it, but it made no difference.

Lee

[Magnum]

Ok Here are some photos of the guider.

Lee

[Magnum]

And some photos of the star shapes from my webcam.

I ave just realised that I have hijacked you thread, SORRY!! hope you don't mind.

Lee

[Tim]

Good pics Lee

That is some prism! You should be able to guide easily with that, mine is half the size!

I wouldn't bring the prism any more into the centre, it'll cast too much of a shadow.

Just checking, does the plate that holds the prism to the rest of the assembly fit both ways round? ie, the prism is actually facing the right way round isn't it?

[Tim]

Hard to say from the pics, but maybe a little more tilt on the prism to present a better image square on to the webcam will do it. Is the webcam chip dead central in the body?

I have a 1.25" webcam with focusing lens in that might help you get the light from the prism to be nicely presented if you get really stuck.

I'm off to bed now, pm tomorrow if you need to

Cheers

Tim

[Magnum]

Yes its the right way round, also I just tried a high power eyepiece and I get the same star shapes. Im really confused now.

I wonder could it be a bad prism?

talk tomorrow

Lee

[Tim]

I'd be surprised Lee.

Try tilting it overmuch towards the centre of the tube and see what happens.

[Magnum]

Ok will try that

cheers

Lee

[Tim]

Racking my brain really here, but have to check;

Is the prism in the right place, ie, it hasn't moved forward or backward a bit has it? Somehow it isn't pointing its image properly at the camera.

[hayju01]

Hi, Great info here but I have some questions so rather than hijack this thread i created another http://stargazerslounge.com/beginners-help-advice/179629-oag-autoguider-c9-25-a.html#post2197913

any comments most welcome.

Thanks,

Justin

[iwatkins]

To Lee.

Double check the prism plate is a clean fit. I had similar stars, very slightly cresent shaped, a few years ago due to a slightly cross threaded retention thumb screw on the plate causing a slight bowing of said plate. Prism was fine but I suspected it was the cause too. I just needed to disassemble, clean and reassemble and everything was fine.

Cheers

Ian

[ollypenrice]

Very impressive, Tim. I think you have a heck of an EQ6 there. Neither of mine would do that, though one of them might do it ocassionally. The guide values you are getting are in good agreement with what we get from the Mesu 200 and, like you, Yves and I find this means tight, round stars. The Mesu isn't tricky, it's plug and play, but it's a bit more expensive so fair's fair! I've also seen other EQ sixes (stripped down and TLC'd ones) which gave near-identical guide values to my Tak mount on the same night. A good EQ6 is a good mount, full stop. They are not all that consistent out of the box, though, but where would we be without them? Bankrupt, probably...

Olly

[Tim]

It's overdue a service Olly that's for sure, but when it still plays this nicely i'm loathe to alter it.

I suppose I could do a PE run one of these days and correct for PE, but to be honest I've never noticed a particular problem with PE.

[ollypenrice]

For sure, if it ain't broke...

Olly