One for ZS71 owners

[Demonperformer]

Here's one for imagers using the WO Zenithstar ZS71 scope with flattener:

According to this document, the distance between the collar of my ZS71 and my flattener should be 10.5mm. If I set that position, the flattener is barely gripping the internal thread of the scope (by about 30° of a turn). My new FW/OAG (arriving in 5-7 working days) and my new 1600MM are going to be hanging on the other end of the flattener.

When imaging the horizon (not advisable) this may be secure, but if I start shooting towards the zenith (and that's the plan) I can see this putting considerable strain on the tiny bit of thread that is clinging to the scope.

Now I know there is a screw that can be used to hold the flattener tightly in place, but it occurs to me that this is inevitably going to put a skew on the optical path resulting in unpleasant distortion of the image. Or I could screw the flattener further into the scope, but I have no idea what this will do to my images, but I can't imagine it would improve them any. But then, neither would having £1700-worth of equipment falling out of the end of the scope (and onto the floor beneath, if the cables didn't stop them)!

So I am seeking advice from those who have used this system ... how do you tackle the problem?

Thanks.

[Davey-T]

According to the drawing it should leave 35mm in the focuser ?

Dave

[sloz1664]

Hi DP,

I had the ZS71 & 6A FR and I didn't think they were really meant for each other. A bit of a compromise. I set mine up so that they could support my ccd/oag/fw combo. and adjusted the fr to ccd chip spacings until I had nice round stars. Once set, all critical dimensions were noted but it never came apart until the day I sold it.

BTW it took me an age to get the spacings right. 

 

Steve

[carastro]

I have this scope and I am afraid I am not particularly technical when it comes to this sort of stuff.  I usually follow the instructions to start with and then over time (having several scopes, 2 filterwheels and two WO focal reducers often using a dual rig), I tend to just swap the kit around forgetting that there might be differences in spacing.  

At some point I was told the backspace was 55mm and this is what I normally use, I generally don't have any problems with the corners if I get this right except for what I think is a bit of tilt in one corner, so nothing to do with the focal reducer.

Carole 

[Demonperformer]

18 minutes ago, Davey-T said:

According to the drawing it should leave 35mm in the focuser ?

Dave

Yes, but the thread on the inside of the tube is quite a long way in and mine only just starts to catch when the 'shoulder' of the flattener is 10-11 mm from the end of the OTA.

[Demonperformer]

13 minutes ago, sloz1664 said:

I set mine up so that they could support my ccd/oag/fw combo. and adjusted the fr to ccd chip spacings until I had nice round stars.

So were they supported purely by the thread, or did you tighten the scew on the flattener?

Thanks.

[Demonperformer]

8 minutes ago, carastro said:

At some point I was told the backspace was 55mm and this is what I normally use,

Yes, I've (finally) worked out what the distance from the flattener to the chip should be, but it is how far the flattener goes into the ota that I am querying, and if the distance is really 10.5mm, how it is all held in place; the thread on the flattener or the screw on the side.

Thanks.

[carastro]

Quote

how far the flattener goes into the ota 

That's a new one on me.  I thought you just inserted them into the draw tube and then found the point of focus.

Carole 

[Davey-T]

Strange design looks like it fits the Z70 and Z66 as they have thread right to the end and someone decided to fit it to the Z71 with hardly enough thread.

Dave

[Demonperformer]

2 minutes ago, carastro said:

That's a new one on me.  I thought you just inserted them into the draw tube and then found the point of focus.

Carole 

Well, that is quite encouraging. I am just going on what it says on the document to which I linked in OP. My thoughts on getting the scope were 'screw it in until I am happy it is secure', but someone linked that pdf to me in another thread and that is what has got me thinking.

However, if others are not worrying too much about it and still getting decent results, then presumably the 10.5mm distance isn't that critical. Which is good.

Thanks.

[Alien 13]

I am a bit confused by the talk of threads in the ZS71 I dont think mine has any, I just insert the FF into the focusser so that the undercut lines up with the compression rings and tighten, the result then is the 10 mm or so distance quoted in the doc.

Alan

[sloz1664]

42 minutes ago, Alien 13 said:

I am a bit confused by the talk of threads in the ZS71 I dont think mine has any, I just insert the FF into the focusser so that the undercut lines up with the compression rings and tighten, the result then is the 10 mm or so distance quoted in the doc.

Alan

Exactly what I did. I manufactured a spacer, so that if I removed the FR I could set it back in exactly the same position. I never removed the FR so never used it.

Regards the back focus distance:-

 

Ian King & FLO quote - http://www.iankingimaging.com/show_article.php?id=29

This gives a distance of 68.32mm from the FR flange face to the ccd/cmos chip. ZS71.

 

Steve

 

[Demonperformer]

9 hours ago, Alien 13 said:

I am a bit confused by the talk of threads in the ZS71 I dont think mine has any, I just insert the FF into the focusser so that the undercut lines up with the compression rings and tighten, the result then is the 10 mm or so distance quoted in the doc.

Alan

compression rings?

[Demonperformer]

8 hours ago, sloz1664 said:

Regards the back focus distance:-

Ian King & FLO quote - http://www.iankingimaging.com/show_article.php?id=29

This gives a distance of 68.32mm from the FR flange face to the ccd/cmos chip. ZS71.

Yes, I finally got the contradictory figures for the back-focus distance on FLO's page sorted out. But this question is in relation to how far the FF should be inside the OTA (which the pdf link in OP gives as 10.5mm) and how people manage to keep it at that distance, when that is barely enough to engage the internal threads. I think Carole's answer has been most telling:

11 hours ago, carastro said:

I thought you just inserted them into the draw tube and then found the point of focus.

If that works and produces reasonable results, I can just give it an extra couple of turns to solidly engage the threads, and not worry that the 10.5mm will now be shorter than that.

Thanks.

[carastro]

Quote

I am a bit confused by the talk of threads in the ZS71 I dont think mine has any

I wasn't aware that mine had any either, have they altered the design? 

Surely the factor here is the distance between the lens on the FF/FR and the sensor, not how far it is from the telescope, the latter can be altered in a flash when you focus.

As I say I am not particularly technical with this stuff, I just do what works. 

This is an image I took with the WOZS71 & an Atik460EX + WO FR x 0.8 back in 2015.  

However is the OP talking about a FF only, or a FF/FR, though it probably doesn't make any difference to the answer (says non technical me)? 

Carole 

[Demonperformer]

5 hours ago, carastro said:

Surely the factor here is the distance between the lens on the FF/FR and the sensor, not how far it is from the telescope, the latter can be altered in a flash when you focus.

Put like that, it makes perfect sense. And that being the case, I wonder why this page (see image) is so precise about it needing to be 10.5mm.

Anyway, it clearly isn't important so I can secure it with the thread by applying a few more turns and don't need to apply the screw and any resultant distortions.

Thanks.

[Alien 13]

1 hour ago, Demonperformer said:

Put like that, it makes perfect sense. And that being the case, I wonder why this page (see image) is so precise about it needing to be 10.5mm.

Anyway, it clearly isn't important so I can secure it with the thread by applying a few more turns and don't need to apply the screw and any resultant distortions.

Thanks.

With my scope the 10.5 mm ensures that the undercut in the FF/FR barrel is in line with the compression rings in the focusser eyepiece holder so it doesn't slip out if the screws work loose in the cold.

Alan

[Demonperformer]

I haven't got compression rings in the focusser

[Oddsocks]

On 16/10/2016 at 16:59, Demonperformer said:

I haven't got compression rings in the focusser

From your comments it seems you are missing the compression rings for your focuser in which case a chat with the supplier is required.

William Optics have used compression rings in the GT81 / ZS71 focusers for years, in fact AFAIK they have used anti-marring compression rings in all their focusers for years.

Too clarify a few errors that have appeared in this discussion, the "thread" in the main barrel of the focuser tube is not an engineering thread at all, it is part of the baffling system, it is just serendipitous that for ease of manufacture W.O. used a 2" SCT thread to bore the baffles rather than the more time consuming method of cutting concentric rings, and the Flat 6 just happened to use the same thread on the end of the flattener so it could screw to the 2" SCT image back on a standard SCT with the aid of a 2" SCT male-male coupler.

The 10.5mm "Safe Distance" is as has been explained above, the correct distance needed to ensure the compression ring in the main focuser draw tube fully engages with the safety cut-out in the body of the Flat 6 and has nothing to do with the correction distance.

While it might seem a good idea to screw the Flat 6 in further to the draw tube using the "baffle" thread this may not be a good idea since these threads were not cut to any great precision, they should be coated with matt black paint and you may find the Flat 6 will cross-thread, break the threads, jam or scrape the paint off in flakes. The correct distance for the Flat 6, for safety reasons, is with the 10.5mmm safety distance adhered to when used in the ZS71 and the compression ring, which you should have, is then fully engaged to safely support your filter wheel and camera without risk of it falling out.

I certainly would not want to be screwing the Flat 6 into the baffling threads and then putting a large sideways force on the body with the securing screws and the compression ring missing.

If there is an undercut inside the draw tube where the locking screw protrudes then this is where the compression ring should sit and if missing then you need to follow this up with the telescope supplier and not chance threading the Flat 6 further into the draw tube on the baffle threads.

Below are pictures of the GT81 /  ZS71 focuser showing the compression rings in the main draw tube and eyepiece adaptor plus a picture of the same from the W.O. website.

 

H.T.H.

 

 

 

[Demonperformer]

1 hour ago, Oddsocks said:

a chat with the supplier is required.

Unfortuneately, this was purchased earlier this year from another SGL member - with a "reputation" of over 1750 - I guess you can't trust anyone nowadays.

I will contact FLO tomorrow and see what can be done about getting a replacement part.

Thanks.

[Oddsocks]

Just a long shot -  Have a good look down the focuser baffle tube, if the compression screws are overtightened and then the eyepiece holder is inserted it is possible to dislodge the compression ring and push it down the draw tube.

If you can't buy one as a spare it should be easy enough for any good engineer to make one for you, it is just a piece of brass strip.

I think the 2" W.O. diagonal uses the same size compression ring, for imaging you could always borrow the ring from the business end of the diagonal to get you going while waiting for a spare.

[Demonperformer]

Well, there is some good news. Looking for a "piece of brass", I found there is a compression ring, but I was missing it because the one (and only) screw was in the "gap" between its ends. By a bit of brute force and ignorance, I have managed to rotate it so that the screw operates on the opposite side of the ring.

But there is still something weird going on, as there is only one screw (and one screw hole) in the focusser to act on it, rather than the three in the photos. As such I am still a little concerned that it will distort the light path. Still, I guess it will be a case of try it and see.

I'll try & post a picture tomorrow.

Thanks.

[Oddsocks]

52 minutes ago, Demonperformer said:

there is only one screw (and one screw hole) in the focusser to act on it

That is quite possible, the original Z71 only had a single screw and if your ZS71 is an early one it might have the Z71 focuser. W.O. are well known for making only short productions runs and then making small "tweaks" to the design.

Loose fitting of diagonals etc inside the draw tube is not a specific W.O. only trait, there has to be some allowance for pick-and-mix from various manufacturers equipment so the tolerances are quite wide.

Obviously W.O. reviewed and improved the design by fitting two thumb screws and a grub-screw to the later models (or maybe yours is the latest and they have removed them to save money, who knows?)

Long time ago when I began imaging with a SX H9 that had a very loose nose piece fit in a Starlight Feathertouch focuser that did have two thumb screws, standard on a APM-TMB 80, I wrapped two pieces of thin-wall coke-can aluminium around the camera nosepiece to take up the slack, one piece around the tip and one piece around the base, easy to cut with just a pair of scissors and did the job, held in place with a dab of super-glue, yep, actually found a use for the stuff! just needed to be a bit careful inserting the nose-piece into the draw-tube so it didn't catch on the compression ring.

You might find the same will work for you, if thin-wall coke-can is too thick then perhaps a piece of hard polyester packaging film wrapped around, leaving a gap for the compression ring undercut. Otherwise a quick trip to a good engineer could add a second thumbscrew and grub-screw as in the picture of the ZS71 posted above.

[Alien 13]

As above, the two inch bit of the focusser with the 1.25 inch adapter removed should have two thumb screws and a third grub screw to equalize the centering.

Alan

[carastro]

I must get my WOZS71 out of its box and take a look at mine, I have no recollection of a compression ring, but maybe I never noticed.  I think I have two thumb screws though, I am sure I would have noticed that.

Carole