F4 Astrograph

[Dave Moulton]

Hi all,

Just playing around with the new toy a GSO 8" f4 Newt.

The picture is 30 mins in 5 min subs with a SXV-H9. Looking at the coma do you guys think the issue is collimation, the spacing of my MPCC or maybe a bit of both.

I have a cheap Laser collimator but due to slop in the steeltrack focuser and the laser I don't rate the method that highly.

Any tips or tricks guys

Dave

[barkis]

The GSO 8" f4 is a great imaging scope Dave, as can be verified by TJ. He has done some great work with his, and I'm sure he will be along to give you some help with your problem. That's if he is not too tied up at work. You could drop him a PM, he will be only too happy to help.

Ron.

[CELESCOPE]

Hmmm i dont know Dave , it dont look like coma to me , but then could be my old eyes lol , more like the camera not seated right, the stars to me seen the same all over , dont think its collimation.whateverit is it is very minimal.

Rog

[steppenwolf]

This doesn't look like coma to me, more like the camera has a tilt? The spacing of the MPCC is critical or it will not function correctly. Provided the laser collimator is itself well collimated it should do a reasonable job but a sloppy Steelrack is a bit disappointing.

[Dangerous-Dave]

That has come out nicely too, great work. Nice seeing a few images of M101, an oft-neglected target.

[Tim]

Hi Dave. That pic makes me want a mono camera

As Steve suggests, I'd guess at the camera being ever so slightly tilted. I find it tricky tricky to get the setup absolutely square. Try to find the cause of the slop and get rid of it as best as you can. As you will have seen, the difference a slightest of turn of the focusser makes at F4 is quite a lot, and if one corner of your chip is even 0.5mm further from the secondary than its diagonally opposed corner, then you'll get that effect. I get it quite a lot tbh.

Take a look at the spikes on the large star to the left, and you will see that focus at that point is slightly off. If the stars on the other side of the image are sharp and the spikes crisp and single, then I'd def go for the focusser holding the camera train at a slight angle. If you can rule that out, the next thing to check would be the tilt of the secondary i guess, again, tiniest error will result in a dramatic effect on the stars.

To check your collimation, find a bright star, and defocus it to a donut shape, and centre it in the frame. The circle should be......circular. If you move the star to each corner of the frame, it should distort a little, stretching in the direction of the corner, but it should do this by the same amount for each corner, you may find it is more distorted towards one corner atm.

Probably being absolutely good as you can be on each aspect is the best, as any slight misgiving will show up. Slower tubes seem a bit more forgiving!

Hope that helps a bit. Are you coming to SGL4?

[Dave Moulton]

The GSO 8" f4 is a great imaging scope Dave, as can be verified by TJ. He has done some great work with his, and I'm sure he will be along to give you some help with your problem. That's if he is not too tied up at work. You could drop him a PM, he will be only too happy to help.

Ron.

Cheers Ron. Better to come from this setup hopefully

Dave

[Dave Moulton]

Hmmm i dont know Dave , it dont look like coma to me , but then could be my old eyes lol , more like the camera not seated right, the stars to me seen the same all over , dont think its collimation.whateverit is it is very minimal.

Rog

Rog, Interesting you should say that and you may well be right as at the moment I have to back the nose piece out of the focuser about 10mm to get to focus.

Dave

[Dave Moulton]

This doesn't look like coma to me, more like the camera has a tilt? The spacing of the MPCC is critical or it will not function correctly. Provided the laser collimator is itself well collimated it should do a reasonable job but a sloppy Steelrack is a bit disappointing.

Steve Regarding the steeltrack focuser. There is slop but not in the focuser itself it occurs when you tighten a nose piece into it , a small amount of sag, very noticeable when you put the laser in.

Dave

[Dave Moulton]

That has come out nicely too, great work. Nice seeing a few images of M101, an oft-neglected target.

Cheers Dave, need to get the stars tighter now and we are sorted

Dave

[Dave Moulton]

Hi Dave. That pic makes me want a mono camera

As Steve suggests, I'd guess at the camera being ever so slightly tilted. I find it tricky tricky to get the setup absolutely square. Try to find the cause of the slop and get rid of it as best as you can. As you will have seen, the difference a slightest of turn of the focusser makes at F4 is quite a lot, and if one corner of your chip is even 0.5mm further from the secondary than its diagonally opposed corner, then you'll get that effect. I get it quite a lot tbh.

Take a look at the spikes on the large star to the left, and you will see that focus at that point is slightly off. If the stars on the other side of the image are sharp and the spikes crisp and single, then I'd def go for the focusser holding the camera train at a slight angle. If you can rule that out, the next thing to check would be the tilt of the secondary i guess, again, tiniest error will result in a dramatic effect on the stars.

To check your collimation, find a bright star, and defocus it to a donut shape, and centre it in the frame. The circle should be......circular. If you move the star to each corner of the frame, it should distort a little, stretching in the direction of the corner, but it should do this by the same amount for each corner, you may find it is more distorted towards one corner atm.

Probably being absolutely good as you can be on each aspect is the best, as any slight misgiving will show up. Slower tubes seem a bit more forgiving!

Hope that helps a bit. Are you coming to SGL4?

Tj, thanks for the comments. Yes the mono cameras are nice. Out of square ccd may well be having an effect but I very much doubt it is 0.5mm. Half that is a distinct probability though. So work to do there.The laser sags when you tighten the clamp on the focuser so How to solve that without buying a hotech or something I can't imagine. If ti happens with the laser then it does with all the camera s as well I guess.

As to the collimation I do have one head scratcher. When looking down the focuser with a collimating cap the reflections of the focuser tube while square is offset when you look at the secondary towards the top of the secondary. It is as if the secondary sits too low in the telescope tube. People tell me this is normal on a fast Newt, not sure.

Dave

[Tim]

No, that's right Dave, there should be a bit of offset. I have some diagrams somewhere that help in understanding why this has to be so.

There is quite a lot of adjustment in the depth of the secondary though, so if you find your stars are odd shaped in one direction you could play with that perhaps.

Is your laser itself collimated? I'm inclined to use mine only as a rough guide now, and do the rest by eye.

I had some odd reflection issues with mine, but I think I have pinned it down to the way I had my filters. The reflections also indicated that the secondary was slightly tilted off square.

[Dave Moulton]

No, that's right Dave, there should be a bit of offset. I have some diagrams somewhere that help in understanding why this has to be so.

There is quite a lot of adjustment in the depth of the secondary though, so if you find your stars are odd shaped in one direction you could play with that perhaps.

Is your laser itself collimated? I'm inclined to use mine only as a rough guide now, and do the rest by eye.

I had some odd reflection issues with mine, but I think I have pinned it down to the way I had my filters. The reflections also indicated that the secondary was slightly tilted off square.

TJ, Cheers for that. Clear tonight and my first day off so I will have another bash and star collimate this time should be interesting to see if i can improve things any

I'l keep you posted

Regards,

Dave