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Planetery observing with Sky-Watcher 10" Flextube

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#26 stargazer193857

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Posted 21 October 2020 - 09:34 AM

I went out and measurer that a 5.5mm 82 deg eyepiece came to focus 1/8" from a 25mm Plossl, and the Plossl with barlow 1/16" the other direction from the Plossl. Not much travel needed.

The kicker is the barlow lets me near focus on stuff only 20 feet away. Actually I could go closer than that.

#27 Eddgie

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Posted 21 October 2020 - 12:15 PM

Is it also true that a low profile focuser lacks enough range to reach focus with several eyepieces and barlows with a coma corrector while also reaching focus without one?


 

The Orion Newtonian Low Profile focuser is an good example of a Hybrid focuser.  It has a larger focuser tube that is driven by a helical rack and pinion gear system, but it also has an inner "draw" tube that can be extended out as necessary to accommodate some eyepeices that need more outward focuser travel.

And as I mentioned in my original post, one can l also use extension tubes on the eyepieces themselves.

 

But that is why I went with the Orion Hybrid.  It has the built in extension tube. 

 

You can see this focuser here to see how the extension works...

 

http://www.orion-xt1...-focuser-1.html

This is the Orion for Orion scopes, but my focuser and mounting plate, except for the radius in the mounting plate, look almost identical to the one I had on my 12" Orion and to the pictures of other Orion scopes so I am hoping it will be a direct replacement.

 

To be fair though, even with the extension, it only comes to about the same height as the OEM focuser, but of course you needed an extension for the OEM focsuer for many eyepieces anyway.

 

Agena also sells a similar version for less money, but the dimensions on the mounting plate are different and I don't want to have to drill a bunch of new holes if I can avoid it. 


Edited by Eddgie, 21 October 2020 - 12:17 PM.


#28 doug mc

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Posted 21 October 2020 - 06:28 PM

Right or wrong, your post inspired me to take a hacksaw to my focusing tube.

 

I cut off 22mm. Of course, this means that when part way out, the end of the focusing tube drops off the supporting roller bearings.

 

So I cut out some supporting plastic guide-rails, which I promptly glued to each side of the curved metal piece that supports the roller bearings. These rails are at the height that the bearings were.

 

However, there is more friction this way and so now my focuser is of the old-fashioned type - to reach focus I have to pull-push the tube by hand, then lock it in place. The scope still collimates OK and the focuser tube has no slop. Nor does it stick into the light path anymore banjodance.gif.

 

So not the most elegant solution, but it should work (famous last words ...)  I'll know for sure when I test it out tonight bigshock.gif.

 

Edit: Just tested it. Everything looking sharp (see what I did there?) Note that earlier I had also removed *all* the mirror clips and have had no near-death experiences using the scope like this. So, with clips gone and focus tube shortened, the night sky is looking good!

As i stated earlier, to shorten a crayford focusor draw tube, you must first redrill and tap the bearing retainer screw holes first. I placed the new tapped holes 12mm further back on the bearing plate. Screwed the bearings in and then shorten the draw tube by 12mm. This prevented the draw tube from droping of the bearings.



#29 Eddgie

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Posted 21 October 2020 - 06:38 PM

As i stated earlier, to shorten a crayford focusor draw tube, you must first redrill and tap the bearing retainer screw holes first. I placed the new tapped holes 12mm further back on the bearing plate. Screwed the bearings in and then shorten the draw tube by 12mm. This prevented the draw tube from droping of the bearings.

Very good idea.  Again, I considered cutting the focuser tube though I did not think about re-locating the bearings and that is a good idea.

 

I had other reasons for getting a new focuser though, one being the two speed focuser. I know that there is a two speed upgrade available for the factory focuser, but I also liked the extension tube.

 

Good idea about moving the bearings though. 



#30 Notoriousnick

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Posted 21 October 2020 - 06:56 PM

As i stated earlier, to shorten a crayford focusor draw tube, you must first redrill and tap the bearing retainer screw holes first. I placed the new tapped holes 12mm further back on the bearing plate. Screwed the bearings in and then shorten the draw tube by 12mm. This prevented the draw tube from droping of the bearings.

I did not overlook this possibility and may do so at some later stage - I just don't have the gear at the moment to do this. To help me along, are you able to tell me what drill and tap size you used?

 

I will have to set the bearings further back though, since I cut 22mm off the draw tube.



#31 Notoriousnick

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Posted 21 October 2020 - 07:15 PM

Well, that is a radical solution. That would greatly limit your focuser travel.   Hopefully it still has enough for your needs.

As a result of the 'guide rails' I put in, the travel is the same as before. The main difference is that I can't use the focuser knobs as they have no effect. I can however move the focuser tube in and out quite accurately by hand - it was awkward at first but later I could do it at least as quickly as with a focuser knob. In fact, I find that under high magnification, with one hand near the base of the focuser, I can track the target (ie move the scope) more stably while focusing at the same time.

 

The guide rails I made that replace the rear roller bearings support the focuser tube quite well - I've tried my ES 68* 24mm EP stacked with both ES focal extenders and had no problems. I lubricated the guide rails - this makes for smoother motion.

 

As mentioned below, I may also still try moving the rear roller bearings further up at some stage to see how that goes.


Edited by Notoriousnick, 21 October 2020 - 07:18 PM.


#32 doug mc

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Posted 22 October 2020 - 12:18 AM

I did not overlook this possibility and may do so at some later stage - I just don't have the gear at the moment to do this. To help me along, are you able to tell me what drill and tap size you used?

 

I will have to set the bearings further back though, since I cut 22mm off the draw tube.

The bearings in my Skywatcher crayford single speed focusor are held in place with 3mm x .5mm pitch screws. A 2.5mm drill is the tapping size.


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#33 Old Rookie

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Posted 22 October 2020 - 11:00 PM

I know the Skywatcher focuser is an issue.  Years ago I owned the 12" Flextube.  It didn't take me long to replace that focuser with a Moonlite dualspeed.  I did the same about a month ago for my 10" Flextube.  I used the ½" spacer on the mounting plate.  No interference in the light path.  I also use a coma corrector so that provides the extension that I need to reach focus.  In any case, a Moonlite focuser is another option to consider.


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#34 Eddgie

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Posted 24 October 2020 - 03:42 PM

Ok, new focuser is in.  

 

While it is not all that much shorter than the factory focuser (maybe 12mm) even when fully racked in, the end of the tube does not intrude into the light path.  This is fully racked in.

 

XQE_01391.JPG

 

Now I can reach focus with the 1.7 GPC with zero intrusion (I actually have to use the built in extension). In fact, the savings may have been enough to let me reach focus in this position even without using the GPC!  Maybe.  But it is OK if I don't. I can always just shorten the trusses and give up a little aperture, but even here, the lower profile focuser means that I will have saved a little bit of aperture.

 

XQE_01371.JPG

 

This thing is heavy though.  It is a good 10 oz. heavier than the stock focuser (about 32 oz.).  I had removed as much weight as possible from the secondary cage area and happily, even with the binoviewers or with the focal reducer/filter wheel/NV device the balance is almost perfect (though there is a 4 lb magnetic counterweight down there under the bottom of the tube. 

 

So, problem solved and it is a much better focuser than the factory single speed. Very smooth and positive. Lots of bearings. I count eight. 

 

They are positioned at 90 degrees, four on the load end and four on the telescope side, so the rack and pinion does not carry any load other than the traction loading. This means that in any orientation, the load is going to be more evenly spread over more bearings, and while I would have liked to see the bearings riding on steel tracks, this design appears far superior to conventional Crayfords where the pinion shaft can be in orientations where it carries half the load and the load side bearings are not doing anything except guiding the tube. A nice focuser for the money.  Too bad this is not standard on the Flextube but to be fair, the Flextube is very reasonably priced. 


Edited by Eddgie, 24 October 2020 - 03:42 PM.


#35 Eddgie

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Posted 24 October 2020 - 03:54 PM

I also finally got around to re-designing my laser finder bracket.  V1 sported a long tube with insufficient hole size at the radial screws to allow sufficient alignment range. The new tube is only 40mm long (same as the dovetail base) and a few millimeters bigger around, so now there is more than ample adjustment range.

 

XQE_01401.JPG

 

The unusual position was chosen to take the weight off of the secondary cage. Since it is a laser, I don't have to look though it and it is actually in a very comfortable position for me when I am seated in my observing chair, and an observing chair is kind of a must for a short scope like this. While I have done dry run, I have not drilled new mounting holes yet because I want to get under the stars to use it.  The long term plan is to go with Nexus II, but for now I am roughing it, finding things the old fashion way (if a laser can be considered old fashioned, but I mean no DSCs or Go2, which it kind of a shock after having purchased my first set of digital setting circles in 1995.  It is fine during the summer, but in the spring, finding galaxies is very difficult from my location without DSCs. 

 

Next step is a power saw to cut the base square.  It is not that it is heavy because it is not. It is just big and not maneuverable, and when in the back of the SUV, the footprint really makes getting camping equipment in more difficult than it needs to be.  



#36 junomike

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Posted 25 October 2020 - 08:04 AM

Ok, new focuser is in.  

 

While it is not all that much shorter than the factory focuser (maybe 12mm) even when fully racked in, the end of the tube does not intrude into the light path.  This is fully racked in.

 

attachicon.gifXQE_01391.JPG

 

Now I can reach focus with the 1.7 GPC with zero intrusion (I actually have to use the built in extension). In fact, the savings may have been enough to let me reach focus in this position even without using the GPC!  Maybe.  But it is OK if I don't. I can always just shorten the trusses and give up a little aperture, but even here, the lower profile focuser means that I will have saved a little bit of aperture.

 

attachicon.gifXQE_01371.JPG

 

This thing is heavy though.  It is a good 10 oz. heavier than the stock focuser (about 32 oz.).  I had removed as much weight as possible from the secondary cage area and happily, even with the binoviewers or with the focal reducer/filter wheel/NV device the balance is almost perfect (though there is a 4 lb magnetic counterweight down there under the bottom of the tube. 

 

So, problem solved and it is a much better focuser than the factory single speed. Very smooth and positive. Lots of bearings. I count eight. 

 

They are positioned at 90 degrees, four on the load end and four on the telescope side, so the rack and pinion does not carry any load other than the traction loading. This means that in any orientation, the load is going to be more evenly spread over more bearings, and while I would have liked to see the bearings riding on steel tracks, this design appears far superior to conventional Crayfords where the pinion shaft can be in orientations where it carries half the load and the load side bearings are not doing anything except guiding the tube. A nice focuser for the money.  Too bad this is not standard on the Flextube but to be fair, the Flextube is very reasonably priced. 

Eddgie, keep us posted on how you like the Focuser.  I read several reports of the ext. causing huge collimation issues.



#37 TiSaph

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Posted 25 October 2020 - 09:01 AM

If  you want to see the effects of various obstruction sizes, vane configurations and intruding focuser tubes, I can't think of a better place than:

 

http://www.beugungsb...iffraction.html

 

I was about to simulate this myself out of curiosity, but found someone had already done it!

 

Seriously, before people discuss hypotheticals anymore you must look at the mathematical work first.


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#38 Eddgie

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Posted 25 October 2020 - 09:19 AM

Eddgie, keep us posted on how you like the Focuser.  I read several reports of the ext. causing huge collimation issues.

I have read the same thing but I think it could also be a bit of tube wall flex or flex in the truss joints. If I put some weight on the end of the laser with the extension out, I can see the collimation dot move.  The thing though is that even if I just grab the base of the focuser housing, I can deflect the laser on the primary simply by exerting some pressure on it.  So, perhaps a bit of tube wall flex. I think maybe the nuts need some washers behind them to distribute the load a little more. 

 

Also, there is only a single narrow screw used to tighten the extension tube and unless I use extreme torque (which is difficult to apply with the fingertips, I can feel some side to side play.  I think a screw with a thumb knob on the end would aid in getting the the screw tight and eliminate or reduce play. I have not taken it apart but I suspect that there is a compression band on the tube, so getting it tighter should help.

 

So, I think it may be a combo of these things.  I may try putting some washers under the nuts to give a little more contact area but since my configuration does not require the extension tube, I probably won't bother with the thumb screw for the extension. 

 

The focuser worked really well for me though. The motion was super smooth and light with the binoviewer and this is load has a lot of moment on it. With the low speed knob, there as almost no resistance.  A big improvement over the stock Crayford where I had to put a lot of pressure on the pinion shaft to get traction.  Both of my primary configurations require about the same amount of outward focus travel so plenty of room.  I was able to reach focus with both my 25mm and my Nikon zooms without running out of travel or having to use the extensions (the Nikons require a bit more racking out).  From a smoothness of operation standpoint, it is a better focuser.

 

I will try to find some washers to see if that helps the tube wall deflection, but I can tell there is a little.  The base on these is not the same as the factory base and perhaps the factory base does a bit better job of distributing the load. 

 

So, not prefect, but I suspect that some of these issues can be addressed to at least some degree. 



#39 Eddgie

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Posted 25 October 2020 - 11:26 AM

Well, I did find a flex point.  When I put my binoviewers on, I could detect a tiny bit of sag when the focuser was fully racked out.

 

In the focuser there are four screws that hold the radius bearing blocks in place, and just off to the side of each hole there is a second hole with a tiny set screw in it.  My guess is that these set screws are used to push the block inward away from the focuser wall to put more tension on the bearings. 

 

I loosed the four retaining screws and took maybe a 12th of a turn on the four set screws, then tightened the big screws down.  This appears to have made a small difference. But yes, I can see that there is a tiny amount of flex when the tube is loaded.  

 

Also, as I mentioned, the extension screw has to be very tight or it will move. If it is fully extended and made square on its flange, it does not do this, so once again my guess is that there is a compression ring and it is wiggling in the groove that it rides in unless very high pressure is applied to the lock screw. 


Edited by Eddgie, 25 October 2020 - 02:40 PM.


#40 junomike

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Posted 25 October 2020 - 12:42 PM

Well, I did find a flex point.  When I put my binoviewers on, I could detect a tiny bit of sag when the focuser was fully racked out.

 

In the focuser there are for screws that hold the radius bearing blocks in place, and just off to the side of each hole there is a second hole with a tiny set screw in it.  My guess is that these set screws are used to push the block inward away from the focuser wall to put more tension on the bearings. 

 

I loosed the four retaining screws and took maybe a 12th of a turn on the four set screws, then tightened the big screws down.  This appears to have made a small difference. But yes, I can see that there is a tiny amount of flex when the tube is loaded.  

 

Also, as I mentioned, the extension screw has to be very tight or it will move. If it is fully extended and made square on its flange, it does not do this, so once again my guess is that there is a compression ring and it is wiggling in the goove that it rides in unless very high pressure is applied to the lock screw. 

Thanks for the info/update in confirming this



#41 Eddgie

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Posted 25 October 2020 - 02:51 PM

Thanks for the info/update in confirming this

Yes, has me rethinking this focuser.  I will try it out under the stars a couple of times and see if misbehaves enough to be bothersome.  I am using an unusually heavy load on it but still, I don't like that it has some small flex. When I used it last night I collimated with the binoviewer in place and I did not have any issue with it changing enough to see at the center of the field, but it could have been and I maybe just did not test closely enough for it. 

 

Also, the issue is complicated by the fact that the safety undercut on the Baader 2" nose does not match up to the compression ring in the focuser tube. The groove for the ring is machined so that there is an extremely thin lip at the end of the focsuer tube. If I insert the 2" nose all the way, the very narrow ring around the end of the Baader focuser extends just far enough so that the compression band hits it when it is tightened, causing the BV to rock in the tube.  I can't back the focuser tube out any because if I do, the BV compression band tilts it.   The fix for this is probably a 2" nose without a safety cut.  I hate safety cuts. So between maybe a slight sag in the focuser and the trouble with getting the nose piece to lock up square, my test with and without the BV was such that the dots did not line up perfectly.  



#42 doug mc

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Posted 25 October 2020 - 09:24 PM

Out there on the internet is a video about increasing the contact between the focusing shaft  and the flat section on the draw tube. It is done with a  wet stone on the draw tube flat. I have done this and it definitely reduces slipping. Also keeping the flat contact area clean helps as well. 


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#43 Old Rookie

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Posted 26 October 2020 - 02:38 PM

Out there on the internet is a video about increasing the contact between the focusing shaft  and the flat section on the draw tube. It is done with a  wet stone on the draw tube flat. I have done this and it definitely reduces slipping. Also keeping the flat contact area clean helps as well. 

That is part of the tuning up process on GSO focusers.  I've done this in the past on mine.  When the entire process is properly done, a GSO focuser can almost be just as smooth as a Feathertouch. 



#44 Eddgie

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Posted 26 October 2020 - 03:53 PM

Out there on the internet is a video about increasing the contact between the focusing shaft  and the flat section on the draw tube. It is done with a  wet stone on the draw tube flat. I have done this and it definitely reduces slipping. Also keeping the flat contact area clean helps as well. 

This is not a Crayford.  It is a rack and pinion focuser 



#45 Notoriousnick

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Posted 26 October 2020 - 05:23 PM

Out there on the internet is a video about increasing the contact between the focusing shaft  and the flat section on the draw tube. It is done with a  wet stone on the draw tube flat. I have done this and it definitely reduces slipping. Also keeping the flat contact area clean helps as well. 

Another option was to put some heat-shrink tubing around the focusing shaft ...



#46 doug mc

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Posted 26 October 2020 - 08:32 PM

This is not a Crayford.  It is a rack and pinion focuser 

Woops. Sorry.




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