Still getting some rotation - What could it be?

[steppenwolf]

Thanks for posting this image. The silver knurled knob is the tension bolt for the focus knob shaft and I am guessing that the two Allen bolts you have outlined perhaps act as a pivot and anchor but to be honest, I haven't seen one to this design close up so can't be sure.

What I would say though is that constant adjustment of these two should not be necessary although I wouldn't be surprised if you had to adjust the silver knob when swapping between camera and eyepiece focussing.

Running out of ideas here!

[johninderby]

Hi

While there are a number of things that could be out of adjustment here is one other thing you could try.

The four outer screws one (in each corner) allow you to adjust the closeness of the focusing mechanism to the drawtube. If this isn't set right it could cause the problems that you are describing. You might try tightening them or loosening an 1/8 turn and see if that improves things.

John

[swag72]

Thanks Steve - Here is a couple more images taken tonight. I have done as Peter suggested. 2 images the same, the second taken at 90 degrees camera rotation to the first. The elongation appears to be on the right hand side of the image this time. Can you look at those and see what you think. If it is the focuser, would you expect the sag in the same place regardless of camera rotation? I guess that you would.

Thanks John for identifying what the screws do - All checked and tightened today, perhaps that's not the thing to do. I guess to get that right I'll have to adjust it at night once the camera is on the tube and just ascertain what gets the best image?

[jgs001]

Sara, can you take a picture of how the focuser attaches to the body of the scope, one on top, one underneath... not sure it's necessarily gonna prove much but...

Do you always fit the camera in the same orientation to the draw tube ? How does the FR attach to the rear of the draw tube ? The screw threads on my 80ED are under a ring on the rear of the tube, and there's a lock ring above that... can you fit everything together and get a couple of shots assembled too ?

[swag72]

Hi John, In this thread I have attached 3 pics I took tonight of the scoring marks on the tube when it is fully extended.

Camera not always fitted on same orientation, depends on where the scope is pointing as to where I have the camera. I hope that the pics in the other thread will answer your questions. Thanks for looking John. I appreciate the time people are taking with me over trying to get this sorted.

[jgs001]

I was wondering, and it looks like it's the standard three screw arrangement that fixes the focuser to the scope, if perhaps, it had got knocked out of alignment a little... if the odd stars always appear in the same part of the image (allowing for camera orientation changes, could be tough to work out) then perhaps that's the issue. The laser would show that up instantly of course...

It does look like it's been operating under quite some pressure with that scoring...

[SteveP]

Sarah

I know from my own experience that adjusting the focuser to keep the draw tube orthogonal can be a right PITA. It's surprising how very small adjustments to the tensioning screws, say 1/8th of a turn, can have an impact. In the end, I decided to minimise the draw tube extension (and thus increase its stability inside the focuser housing) by using slightly longer adapters in the imaging train. I now only extend the focuser by 3-4mm to reach focus and the problem has been resolved... and that is with 4kg of filter wheels, OAG, camera etc hanging off the end of the focus tube

Also, looking at the pictures you posted elsewhere, the fact you have scoring on the draw tube suggests to me that the screws are already too tight

HTH

Steve

[martin_h]

Sarah

I know from my own experience that adjusting the focuser to keep the draw tube orthogonal can be a right PITA. It's surprising how very small adjustments to the tensioning screws, say 1/8th of a turn, can have an impact. In the end, I decided to minimise the draw tube extension (and thus increase its stability inside the focuser housing) by using slightly longer adapters in the imaging train. I now only extend the focuser by 3-4mm to reach focus and the problem has been resolved... and that is with 4kg of filter wheels, OAG, camera etc hanging off the end of the focus tube

Also, looking at the pictures you posted elsewhere, the fact you have scoring on the draw tube suggests to me that the screws are already too tight

HTH

Steve

That wont work in Sarah's setup because she uses a focal reducer, so has to maintain the 55mm spacing.

[swag72]

Thanks for all of that info. Next time I am out I'll get the guiding all going and just take 30 sec subs with an allan key handy and adjust those tensioning screws (The 4 on the corners of the plate, right?) and just keep snapping until I get some nice round stars!!

Sounds time consuming, but easy, yes?!!!! Watch this space, there will be more stupid incoming questions I am sure!!

[Psychobilly]

Sara I spent an hour or so searching the forum and web lastnight for a couple of threads that I have read in the past.. I found one but not the "whole set" so I have pm'd soemone to see if they are able to help...

Peter...

[SteveP]

That wont work in Sarah's setup because she uses a focal reducer, so has to maintain the 55mm spacing.

Unless I'm missing something, I beg to differ since I use a similar arrangement when I need to use a field flattener on another scope where the BFD is equally critical.

To simplify the arrangement in Sarah's case, the setup would be ...

Focuser tube > Adapter1 > Reducer > Adapter2 > Camera

The length of Adapter1 is whatever is needed to bring the DSLR to focus with the reducer in situ and with 2-3 mm of draw tube extension.

Steve

[swag72]

What type of adapters are you thinking about Steve? Also, forgive me for being naive, but what will the adapters actually achieve? Do they increase the distance from the camera sensor to the focal point and so you don't need the tubing out as far? Sorry, I'm sure that's all the wrong wording!

[Paxo]

What Steve is suggesting is using an extension tube immediately between the end of the focusser drawtube and the flattener, this will not effect the use of the flattener and maintaining flattener to chip distance. The reason for suggesting this is that when the image is brought to focus, significantly more of the drawtube (the same length as the extension piece used) remains insite the OTA significantly reducing the chance of flexure or sagging.

Steve

[roundycat]

If these screws keep working loose have you tried threadlock?

Dennis

[SteveP]

Sarah

I'm suggesting an adapter which screws onto the rear of the 'tubing' (aka focuser tube) and onto which your field flattener then screws. Then attach your camera to the flattener as you do currently. All that's happening is that you're swapping the extension of the focuser tube with an adapter. And consequently retain much of the length of the focuser tube inside the focuser itself. As you say, the aim is to reduce the amount which the focuser tube extends and thus minimise the sag created by the focuser itself

Does that make sense?

Steve

[johninderby]

Of course there is one solution to your focuser problem

John

"Feather Touch Rack & Pinnion focuser"

[Merlin66]

John, you well may be correct...but a laser collimator and a 3mm Allen Key is much cheaper.

[johninderby]

John, you well may be correct...but a laser collimator and a 3mm Allen Key is much cheaper.

Yes it's best to be sitting down when you see the price of the FT R&P. Having said that though I've never regretted getting one for my FLT98.

John

[swag72]

That looks like a nice piece of kit!!! I really will try to sort out my current focuser. I've got a 50mm extension tube, but I don't think It will all fit together as the SW thread is different to normal thread size I believe.

So do you think that a laser collimator may be really useful and save me lots of time and faffing about? Anyone make a recommendation for one - It's not something I thought I'd need so have never looked at them.

[Psychobilly]

The advantage of CCD inspector over the laser collimator is that you are making changes with the imaging device inplace... so the focusser is under the "true" load...

Peter....

[swag72]

So Peter, if I use CCD Inspector, I would be doing the following.

1) Take picture

2) Put image through CCD Inspector

3) Analyse result (could be a problem!!)

4) Adjust screws and retake picture

5) Keep following 1-4 until CCD Inspector telle me it's sorted.

Yes?

So what would I be wanting to achieve in CCD Inspector? I found the diagrams hard to decipher to be honest.

[Psychobilly]

Thats how I would probably go about it...