Avoiding focuser sag - tips?

[lukebl]

I've worked out that slightly elongated stars I'm getting in my imaging are caused by focuser sag. I worked this out by various experiemnets last night with the camera in different positions. I found that the margin for error was very small, and that poor star shape occurs with even the slightest misalignment. Although I've tightened up the focuser, there is still a bit of play which I think makes all the difference. I am using a 250mm f/4.7 Newt and a SXVF-H9 CCD with filter wheel and coma corrector.

Just wondered if anyone has any tips on how the make the focuser as rigid and aligned as possible. It seems to me that the weak point is the two screws in the 2" eyepiece holder which grip the Coma corrector. The coma corrector seems to pivot on these, resulting in a tiny bit of play or misalignment. Is there any way of replacing these with something which will grip the coma corrector firmly and evenly all round?

[Psychobilly]

Could you shim it to make it a tighter fit?

Peter..

[ollypenrice]

Some kind of packing or shim would help. Silver paper? Even just a coat of Hammerite?

Could you drill and tap a third screw into the focuser?

Alternatively a quality focuser. (More expense!). This is a real problem with Newts where the CC extends the leverage exerted by the weight of the gear on the outer end. It may not apply in your case but flex in the OTA can also cause loss of orthogonality.

Olly

Edit, Sorry Peter, we crossed!

[kookoo_gr]

If your problem is to have the camera centered at the focuser drawtube i would recomend o hotech sca adator First Light Optics - Search Results i bought one and i have no problem with my camera being centered at the drawtube. If your problem is with the focuser drawtube itself then i would suggest a new focuser

[Starlight 1]

Could you shim it to make it a tighter fit?

Peter..

This is the best way that Peter put in cut open a coke can.

[lukebl]

Thanks for the advice chaps. I'll look at ways of tightening the fit with something. Not sure about the focuser. It's the standard dual-speed one which came with the scope, and I can't stretch to a new one right now.

If your problem is to have the camera centered at the focuser drawtube i would recomend o hotech sca adator

Surely, though, I would have the same problem as I would still need to fit the hotech adaptor to the tube, which might wobble in exactly the same way that the coma corrector does? In any case, I can't see where the coma corrector would go in the optical train.

I know that a big CCD hanging off the side of a Newt isn't ideal, but whenever I see Yfronto's magnificient images, I know that it's perfectly possible!

[Merlin66]

You could always think about adding a "support" bracket to your OTA to hold the camera etc square to the optical axis......

[lukebl]

Thanks for the advice, gents. Not wanting to tempt fate, but I think I've cracked it. I inserted a strip of laminated paper around the focus tube so it sat snug in the focuser. Last night I managed a whole series of perfect 10 minute exposures with little sign of slippage, flexure, coma or guide errors. Possibly the first time since I started on this imaging lark three years ago. Hope it's not just a flash in the pan!

Here's last nights effort, anyway. NGC7741 in Pegasus. A really nice little barred galaxy, with loads of distant galaxies in the background, which I don't think has cropped up here before. I think it would also benefit from an hour or two with an H-a filter.

c 1h 20 mins each of R, G, B in 10 min subs. SXVF-H9. 250mm f/4.7 Newt. PHD/ST80/QHY5 guiding.

[Psychobilly]

Congrats Luke.. lets hope you have seen the last of those "problems"...

Peter...

[Glider]

If you search the forum for similar posts on using thin single sided adhesive PTFE tape for packing focusers and you think it would be useful I've got a mile of the stuff and will post you some, a bit more robust than paper packing perhaps.

[Ant]

I realize that this is quite an old psot.... but...

I am coming to the conclusion that I have a similar problem. Did your solution help (long term) or was it just a "flash in the pan"?

How do you keep the laminated paper in the correct position and still maintain the ability to focus?

Cheers

Ant

[lukebl]

I realize that this is quite an old psot.... but...

I am coming to the conclusion that I have a similar problem. Did your solution help (long term) or was it just a "flash in the pan"?

How do you keep the laminated paper in the correct position and still maintain the ability to focus?

Cheers

Ant

Hi Ant. Can't help really. Problems are still ongoing, combined with general guiding issues, and I've been reading your parallel thread with interest! It sometimes works, sometimes (more often) doesn't.

The laminated paper sort of wedges the focus tube so that it's stiff without being immovable. But I still seem to spend every (all too rare) clear night trying to get focus, guiding and camera alignment correct and don't actually get round to a good imaging session.

There seem to be many factors involved here, such as possible bending of the actual OTA with the weight of the focuser and camera, plus general sag in the focuser. Imaging with a 10" Newt and a small-format CCD is a lot of hard work. But if MikeD can do it, then so can I!

[jgs001]

Not sure how much it'll help on this particular issue, but it worked wonders for my poor stock R&P's... teflon tape... sticky backed. I've got a fair amount left... so if either of you want to try it, let me know...

[Ant]

John that would be grand mate.

I'll PM you my address, let me know what I need to send you to cover tape and postage and I'll send it over.

I haven't got a clue how to tell you how much I need though...

Cheers

Ant

[jgs001]

Ant, how long is the drawtube ??

[Ant]

I'll have a check tomorrow John. From memory I have about 8cm of "travel". I'll have a measure tomorrow evening. The correct focus point is 46mm...

Do you put three strips at 120 degrees apart along the draw tube? Never occurred to me to do it that way

Cheers

Ant

[lukebl]

...Do you put three strips at 120 degrees apart along the draw tube? Never occurred to me to do it that way

Cheers

Ant

Yes, it's occurred to me that putting the tape around the whole tube is a bit wasteful. John, I think the measurements I gave you were a bit generous!

[jgs001]

It'll depend on the focuser itself really. You need to put them along the drawtube at the point the felt liners in the focuser are. My konus has 4, the 80ed has three, but I only stuck the tape on the lower two as I could adjust the top one. I just need the approx length of the drawtube.

[lukebl]

An update.

Well, I attemped putting the teflon strips in position, which John kindly supplied me with, but that just seemed to jam the focuser, so that it would rack in but not out despite all efforts to adjust the focuser. So I removed the Teflon, and put it all back together and carefully adjusted the hex screws till the focuser was nice and tight but still racked in and out smoothly with the camera in position. All looked very nice, and the focuser was nice and tight with no play or wobbling. HOWEVER, on taking an image, I find that I'm back to square one, with oval-shaped stars. Well and truly stumped now. Here's an enlargement of part of a frame showing the problem (I know that the elongation is nothing to do with guiding, as it happens on very short exposures too).

Does ANYONE with a 250 PDS focuser have this problem, which I'm certain is caused by focuser sag? I know there are a few of you out there (MikeD?) who are imaging with a 250mm Newt and 314L/SXVF-H9/HX916 cam. Which focusers do you have, and should I replace mine, I wonder?

[Merlin66]

Luke,

have you checked out the direction of the movement v's the position of the camera in the focuser and the direction of Mr Newton's gravity?

Doing this would obviously confirm one way or another that it is the focuser and not something else.....

(Could you rig up at least a temporary "splint" from the OTA to the camera to give it independent support - see if there's any improvement?)

[lukebl]

Luke,

have you checked out the direction of the movement v's the position of the camera in the focuser and the direction of Mr Newton's gravity?

Doing this would obviously confirm one way or another that it is the focuser and not something else.....

(Could you rig up at least a temporary "splint" from the OTA to the camera to give it independent support - see if there's any improvement?)

Good thought. I'll give that a try.

[Zodiacal_Light]

I've got an Orion XT10i and also seem to be getting flexure from something. The guide scope is an Orion mini 50mm and has no problem keeping the guide star centered. However the main image show oval stars.

It could be the focuser, but couldn't it also perhaps be the primary mirror shifting? I've tried pushing on the bottom of the primary mirror and it does indeed lift up a bit. Not the cell but the actual mirror itself is loose.

[lukebl]

I've just run some of my images through CCDInspector and, boy, I seem to have some problems! It seems to indicate that collimation is way out, even though it looks OK with the Cheshire, but presumably a sagging focusser would look like bad collimation too, wouldn't it?

[MikeD]

Sorry I have only just found this thread:o

I use a standard skywatcher single speed Crawford focuser That was given to me when someone upgraded to the new skywatcher low profile focuser (I used to use the old rack and pinion focuser before that).

I also had problems with egg shaped stars but from differential flexure and only on exposures over 5 minutes.

After many years fiddling I came to the conclusion that the differential flexure I had may have been centred around the newts focuser, although I don't think sag was the problem more the focuser flexing the OTA itself but I was never sure.

My fix was to switch to an off axis guider and I have never looked back.

Sorry if this doesn't help you with your issues.

Below is a ccd inspector analysis from a recent image with my comma corrector, TS off axis guider, DMK21( for guiding)filter wheel and HX916 all hanging off the focuser.

Mike.

[lukebl]

Thanks for the info, Mike. Something puzzles me a bit, though. You say that you finally came to the conclusion that the problem was caused by the focuser flexing the OTA, and that the problem went away when you got an OAG. Surely, though, it would still sag, particularly with the additional weight of the OAG straining the focuser?