Drawtube and Field Flattener wiggle in tube.

[AlistairW]

Hello,

I am after some ideas for an issue I am seeing with my setup. My focuser drawtube is a standard 2" and I simply push the Flattener\Reducer in to the tube and tighten the compression ring with the aid of 3 radial thumb screws.

My issue is that I cannot get the FF/FR square in on the focuser tube. As soon as I tighten one screw the FF/FR tilts. I do this very carefully by tightening one screw at a time but is still very hit and miss. With the maximum pressure I can physically exert on the thumbscrews, the assembly still wiggles. So I replaced the thumbscrews with Allen bolts, which meant I could get a lot more torque and tighten the FF/FR in the tube, but I expect there is still some more tilt due to the nature of tightening one screw at a time - (plus I am now worried I could pinch the optics, - though the casing is a very solid metal on the FF/FR).

I had thought about the Hotech self centring FF, but I don't want to just throw money at the problem.

I was wondering if it would make sense to make a thicker compression ring, so that the amount of play between ring and screw thread is less and therefore cannot add any more wiggle.

I am beginning to think that compression rings and thumbscrew are a bit of a pain for imagining, where you need to be very accurate.

Thanks

Alistair

[Oddsocks]

Hi Alistair.

This is a perennial problem as you have discovered.

Your idea to use a thicker compression ring won't work because it is too narrow and acts as a pivot, it won't matter how tight the ring is the flattener will still pivot about that point once a heavy camera etc is hanging off the back. Using Allen bolts instead of the thumb screws will distort the barrel of the flattener and risks shattering the glass besides pinching the optics.

I also have a GT81 and the Hotech SCA flattener. The Hotech system does provide a very stiff and axially aligned mounting. Optically it is rated for f5 to f8 systems and it's best performance is mid range at f6.5. With the GT81 at f5.9 it works quite well though the resulting images are not quite as good as the Televue TRF2008 flattener which I also have and prefer. With the Hotech stars seem a little "fuzzy" compared to the same object when using the Televue though the overall resolution seems the same, the star colour through the TRF2008 seems closer to an image without the flattener, star colour with the Hotech SCA flattener appears warmer, but it is really splitting hairs and at the GT81 focal length there is not a lot in it.

When using the Hotech the normal locking thumbscrews in the focuser must not be used else you might cut or damage the rubber expansion rings in the body of the SCA or deform the sliding metal rings that separate the expanding rubber sections. This means the safety undercut function of a normal flattener, eyepiece or accessory is not used and you totally rely on the Hotech SCA system to secure the camera and anything else in the image chain attached to the focuser. This is a little risky and I use a safety lanyard tied loosely around one of the tube rings, around the back of the camera and back up to the tube rings, just in case the SCA should slip the safety lanyard will prevent the camera falling on the ground!

To test and see if the stiffer mounting and good axial alignment provided by the Hotech system will help you, use a piece of thin polycarbonate sheeting cut from some supermarket packaging, the type used for for the tray part of prepackaged sliced meat or cheese is just barely a mm thick, cut enough to wrap around the body of your existing flattener and feed it into the draw tube of the focuser at the same time you push the flattener in, this will take up the slack in the draw tube and help axially align the flattener. You need to experiment with different thicknesses of polycarbonate until you find the correct thickness that allows one full wrap around the body of the flattener while taking up all the slack in the draw tube. Tighten the thumb screws securely and use a lanyard as fail safe protection in case the flattener slips out the draw tube despite tightening the thumb screws, remember with the polycarbonate sheet wrapped around the flattener the compression ring will not engage with the safety undercut in the body of the flattener. Take some images and assess the results, if you have solved the tilt then most likely the Hotech SCA Flattener system will work for you, if the tilt is still there then something else is wrong such as lens cell collimation or focuser draw tube bearing alignment.

When using my Televue TRF2008 with the GT81 I use the wrap-around polycarbonate sheet and a lanyard for security and it works well, one day....someday...I will glue the polycarbonate sheet in two sections to the body of the TRF2008, one piece wrapped around the flattener body above the undercut, the other below, this will support both ends of the flattener in the draw tube and still provide the open access to the safety undercut for the brass compression ring to engage with. As long as a fairly benign glue such as Evostick impact is used it should be possible to peel off the polycarbonate strips in the future without leaving marks if moving to a focuser with a tighter tolerance of draw tube opening.

I have been using the same bit of polycarbonate sheet in this way with the GT81 and TRF2008 for around five years and though it only gets an outing half a dozen times year the polycarbonate shows little signs of wear and keeps the flattener perfectly aligned in the draw tube.

Ultimately, ditching the draw tube / compression ring method of assembly and using screw connections throughout is preferable and this is what I do on my bigger scopes but unless you have access to lathe to turn your own adaptors it can become rather expensive...

[Davey-T]

Hi Alistair I had the same problem with the WO F/R 1V on WO110FLT and resorted to buying the WO twist lock focuser thingy, expensive but it works.

Dave

[AlistairW]

Thanks for the replies.

I am wondering about the backfocus for the Hotech - is it 55mm for the WOGT81 in your experience Oddsocks ?

[AlistairW]

Whilst I have not tested this - what I have tried is to place a 2" diameter piston ring clip at the position of the first undercut of the WO FR/FR. Originally I was finding the 3 thumb screws were tightening on the undercut ridge, and introducing a lot of play. The piston ring, not only acts as a spacer, but now allows the FF/FR to butt up tight to the drawtube. But, I still have to lock it down hard with my Allen bolts. It certainly does not wiggle anymore - I do worry about the pinched optics from the FF/FR. Not tried this yet but guess I will be looking for tilt and triangular stars. Never easy this hobby !

[AlistairW]

WO twist lock I don't think will work for me as I have switched to a Moonlite.

[AlistairW]

One other thing I might try is Aluminium tape arond the FF/FR barrel. Then build the tape up to take up the slack. Screwfix sell it, and its 40 microns thick.

[Oddsocks]

1 hour ago, AlistairW said:

I am wondering about the backfocus for the Hotech - is it 55mm for the WOGT81 in your experience Oddsocks ?

For my system it is best at 53mm with the GT81 using a Nikon APS-C sensor, 23.8mm x15.8mm, but is ok at 55mm with the smaller KAF8300 sensor in my QSI camera, 17.96mm x 13.52mm.

 

57 minutes ago, AlistairW said:

One other thing I might try is Aluminium tape arond the FF/FR barrel. Then build the tape up to take up the slack. Screwfix sell it, and its 40 microns thick.

That should work well too, I tried the extra thick "Bacofoil" kitchen baking foil before I moved on to polycarbonate, I found the aluminium foil tore rather easily but if it is a sticky tape it should be more resistant to tearing. Another possibility is the self adhesive copper RF shielding tape that can be found from electronics components suppliers, it is usually available in quite a wide range of thicknesses and widths.

http://uk.rs-online.com/web/c/adhesives-sealants-tapes/tapes/copper-tapes/?sra=p

 

[Oddsocks]

Forgot to add the link to the Hotech data sheet.

https://www.hotechusa.com/v/vspfiles/assets/images/manual - sca ffl.pdf

 

[AlistairW]

Much appreciate you help Oddsocks

[ollypenrice]

The aluminium tape was always recommended by Astro Engineering for drawtube fit correction but it went scarce. Good to know that Screwfix sell it.

I don't think that the tightness of the screws has any bearing on the the problem. As Oddsocks says, they will still be pivots. Go easy!

Olly

[AlistairW]

Thanks everyone, got a few things to try, will let you know how I get on.

(All this push fit and thumb screw stuff makes me realise that it not the best solution, - actual screw on adapters and SCA seem a better way to go)

[Davey-T]

12 hours ago, AlistairW said:

WO twist lock I don't think will work for me as I have switched to a Moonlite.

Perhaps someone should give Ron a nudge to point out that compression rings aren't ideal for modern astro imaging and get him to produce Moonlight focusers with self centering twist locks.

He always seems open to suggestions.

WO seem to producing many of their scopes with twist locks as standard now

Dave

[AlistairW]

Just wondering about tilt again.

Say there is some tilt at the point where the focuser assembly touches the OTA. But let say it can be countered by messing about with thumb screws on the FF/FR. So  by having two opposing amounts of tilt, - then I am wondering if that actually really cancels the tilt. Kind if like having a tilt adjuster. Wondering if this messes up the light cone.

Thanks

Alistair