27 replies to this topic

[Daveatvt01]

Well, let me first say that stepping up from 10" to 17.5" is a big jump! Many objects have much more detail... the Veil nebula looks like a photograph, M101 and M51 have easily visible spiral arms, and parts of NGC 7000 have an incredible amount of nebulosity, etc etc etc.

 

 

So it hasn't been as monsoon-y as other years here in Tucson, and I've already had a chance to get the new scope out twice in the driveway and twice in darker skies. The driveway sessions (with a full and nearly full moon) were mostly for testing/troubleshooting, though I did get to spend some time on the planets and brighter objects like M13. The strangely positioned red dot finder that required a screwdriver to adjust was quickly replaced with a better positioned telrad and 50mm RACI finder I had already. It was a little top heavy before new finders went on and now it needs a more permanent counterweight solution. 

 

The good:

-Big mirror (obviously) compared to my other scopes

-Has a nice feathertouch 2 speed focuser

-The trusses have been modified for faster assembly to the UTA (bicycle wheel quick release style)

-Breaks down into parts that don't weigh more than 50lbs

-Fits into my old Cherokee

-Has a separate wooden box to transport/store the mirror 

 

The bad:

-Secondary really needs to be recoated

-Takes a bit to get set up and collimated (much faster with a second person though)

-Something is shifting and it needs to be collimated I'd say every 1-2 hours or so, based on my limited use so far. 

-And my least favorite part- when it goes down to about 25 degrees elevation the weight of the mirror shifts far enough forward that one of the wooden triangles that make up the mirror cell slips from it's position between the mirror and the collimation bolts and falls. 

 

The primary mirror has some scratches and scuffs, but overall the coating is holding up nicely. It is adhered to a piece of wood for some reason. It also also has no mirror cell clips or anything holding it in, and I am concerned it might fall if the scope goes all of the way down to the horizon.

 

 

Any advice on how to determine what is shifting when it loses collimation? If it is the mirror, trusses or UTA? I'm not sure where to start.

And any thoughts or advice on the mirror sling slipping/falling? and the primary mirror itself possibly falling? 

How should I care for the secondary mirror? Because of the truss design it's pretty exposed.

 

Thanks!

-Dave

[havasman]

An appropriate size bottle of Chivas Regal will leave you with a nice soft drawstring bag you can cover your secondary with. They're the hobby standard.

 

You might want to look at upgrading that mirror cell. Aurora precision does good work.

[Astro-Master]

One of the problems with the Sky Designs is the closed back of the mirror box, it takes a long time for the mirror to cool.  Havasman"s idea for a new mirror cell is sound.  A good 18 point floatation steel mirror cell with a fan will allow you to open up the back of the mirror box to allow the mirror to cool, and will probably solve the shift in collimation, as well as keeping the mirror in place when the tube is horizonal. 

 

In the mean time check the sling to see if it stretches when you lower the tube, or if it just needs to be tighten to keep the mirror in place.  You should be able to make some mirror clips out of wood to keep the mirror from tipping. but first fix the problem of the mirror sliding and the wooden triangle falling off.  Just work on one problem at a time, and before long everything should be working fine.

[KBHornblower]

I cannot imagine what is going wrong between the mirror and the supports.  At 25 degrees the weight of the mirror should still be holding it against the supports, with the sling holding it in place.  That cell could be my homemade one before I sawed a hole in the center and installed a fan.  I have had it as low as 4 degrees when getting a glimpse of Omega Centauri, and horizontal when measuring the focal length in my basement.  I do have clips for safety in case it starts to topple in that position.

[Daveatvt01]

An appropriate size bottle of Chivas Regal will leave you with a nice soft drawstring bag you can cover your secondary with. They're the hobby standard.

 

You might want to look at upgrading that mirror cell. Aurora precision does good work.

Thanks! Yeah, it may get a real mirror cell upgrade in the future, but I’d like to use it for a while without putting too much more money into it yet.

 

One of the problems with the Sky Designs is the closed back of the mirror box, it takes a long time for the mirror to cool.  Havasman"s idea for a new mirror cell is sound.  A good 18 point floatation steel mirror cell with a fan will allow you to open up the back of the mirror box to allow the mirror to cool, and will probably solve the shift in collimation, as well as keeping the mirror in place when the tube is horizonal. 

 

In the mean time check the sling to see if it stretches when you lower the tube, or if it just needs to be tighten to keep the mirror in place.  You should be able to make some mirror clips out of wood to keep the mirror from tipping. but first fix the problem of the mirror sliding and the wooden triangle falling off.  Just work on one problem at a time, and before long everything should be working fine.

How do I check the sling? I am new to big dobs. Clips sound easy to make.

 

I cannot imagine what is going wrong between the mirror and the supports.  At 25 degrees the weight of the mirror should still be holding it against the supports, with the sling holding it in place.  That cell could be my homemade one before I sawed a hole in the center and installed a fan.  I have had it as low as 4 degrees when getting a glimpse of Omega Centauri, and horizontal when measuring the focal length in my basement.  I do have clips for safety in case it starts to topple in that position.

I’ll take some more photos of the cell and mirror in case that helps and upload them soon

[KBHornblower]

I cannot imagine what is going wrong between the mirror and the supports.  At 25 degrees the weight of the mirror should still be holding it against the supports, with the sling holding it in place.  That cell could be my homemade one before I sawed a hole in the center and installed a fan.  I have had it as low as 4 degrees when getting a glimpse of Omega Centauri, and horizontal when measuring the focal length in my basement.  I do have clips for safety in case it starts to topple in that position.

Addendum:  I am guessing that the three triangular pieces are resting against collimation screws, and that the strips keep them from rotating or moving laterally, while allowing them to rock slightly to provide equalized 9-point support.  With the sling to keep the mirror from sliding off, an exercise in entry-level physics tells me that the mirror will remain firmly against the supports at any possible viewing angle.

[Astro-Master]

An Aurora precision mirror cell might be expensive, I'd contact Discovery Telescopes, they sell 17.5" Dobs. and might have a mirror cell to fit your mirror box.

 

I would contact the Astronomy club in Tucson for help to figure out what's going on with your mirror cell and sling.  Its hard to advise without being there to see what is going on.

[Daveatvt01]

You are correct about the setup, except the mirror is also glued to a circular piece of wood. I’d imagine the glass is much heavier than the wood, shifting it’s center of gravity 

[KBHornblower]

You are correct about the setup, except the mirror is also glued to a circular piece of wood. I’d imagine the glass is much heavier than the wood, shifting it’s center of gravity 

That is still not enough to cause any trouble when leaning back 25 degrees unless the sling is horribly out of whack.  Once again I would have to see a side view of the mirror in the cell to have the foggiest idea what is happening.

[Daveatvt01]

That is still not enough to cause any trouble when leaning back 25 degrees unless the sling is horribly out of whack.  Once again I would have to see a side view of the mirror in the cell to have the foggiest idea what is happening.

Hmmm... I’ll be the sling is not supposed to be that far off the bottom of the box. I adjusted it to be flush with the floor, but haven’t had a chance to test it yet.

 

An Aurora precision mirror cell might be expensive, I'd contact Discovery Telescopes, they sell 17.5" Dobs. and might have a mirror cell to fit your mirror box.

 

I would contact the Astronomy club in Tucson for help to figure out what's going on with your mirror cell and sling.  Its hard to advise without being there to see what is going on.

Thanks for the Discovery recommendation. I have been considering joining the local club for a while... contacting them is a good idea if we can’t figure it out 

[Retsub]

Dave, too bad for your troubles but even from a long distance (Tx) I think that scope can be made to work pretty well but it needs a close eye on things first. Maybe I and others can straighten all/some of it out. The pic of the sling doesn't look right at all and the wood behind the mirror ??? it only needs the 3 triangles with pads on them to make up a 9 point cell. It can work well too. One reason for the wood behind the mirror could be to raise the focal point out farther since it has that type of shorter focuser but of quality that probably was not available when that scope was built by Bob Combs. He built a 20" f4 for my late wife Barbara that she used for many nites believe me. This is taxing my memory a bit but thats ok and with your help I think it can work out much better for you. I have made several mods on ours. One was to make assembly easy for just me to do. Lets go from here now and sort it out. Hang in.   *BW*

[Mark Strollo]

My version of the Sky Designs 17-1/2 mirror “cell” is an 18” diameter paper-thick piece of plastic. The three triangles are stapled to it, an then it is stapled to the base of the mirror box. The three collimating screws just push up from the bottom of the box against the triangles. My sling is flush to the wood like yours. 

See attached photo, taken when I installed a computer fan to promote cooling. 
 

[Darren Drake]

That looks a lot like what I did to mine.  I cut a hole in the mirror box and put in a strap.  Later I removed  the strap as I didnt think it made a real  difference..

Attached Thumbnails

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Edited by Darren Drake, 24 August 2020 - 03:39 PM.

[KBHornblower]

I cannot imagine what is going wrong between the mirror and the supports.  At 25 degrees the weight of the mirror should still be holding it against the supports, with the sling holding it in place.  That cell could be my homemade one before I sawed a hole in the center and installed a fan.  I have had it as low as 4 degrees when getting a glimpse of Omega Centauri, and horizontal when measuring the focal length in my basement.  I do have clips for safety in case it starts to topple in that position.

I have rotated the image from the opening post so it appears in a normal operating position, as seen looking into the scope with the mirror removed.

Is it the top triangle that is slipping at low elevation?  If so, it may be because those two strips are not stiff enough to hold it.  I would try putting them above the triangle rather than below it.  I still cannot imagine a weight shift that would lift the mirror off of it without a lot of violent motion.

[Augustus]

I would really just ditch that cell. You could make one yourself pretty easily out of aluminum with screws and pop rivets, NMT has a nice tutorial on YouTube

[KBHornblower]

I would really just ditch that cell. You could make one yourself pretty easily out of aluminum with screws and pop rivets, NMT has a nice tutorial on YouTube

On the basis of my own experience I would keep the cell, with at most some minor debugging.  I do the same thing he does, which is packing the disassembled scope in my truck to haul it to dark sites.  I also take the mirror out and pack it in a separate box for safe keeping.  This pure Dobsonian sling arrangement makes it easy to slide the mirror back in place, with only some touch-up of the collimation needed to be ready for observing.

[Daveatvt01]

Dave, too bad for your troubles but even from a long distance (Tx) I think that scope can be made to work pretty well but it needs a close eye on things first. Maybe I and others can straighten all/some of it out. The pic of the sling doesn't look right at all and the wood behind the mirror ??? it only needs the 3 triangles with pads on them to make up a 9 point cell. It can work well too. One reason for the wood behind the mirror could be to raise the focal point out farther since it has that type of shorter focuser but of quality that probably was not available when that scope was built by Bob Combs. He built a 20" f4 for my late wife Barbara that she used for many nites believe me. This is taxing my memory a bit but thats ok and with your help I think it can work out much better for you. I have made several mods on ours. One was to make assembly easy for just me to do. Lets go from here now and sort it out. Hang in.   *BW*

 

CED9F3EF-BAB3-40CE-B0BD-8842869896F2.jpegMy version of the Sky Designs 17-1/2 mirror “cell” is an 18” diameter paper-thick piece of plastic. The three triangles are stapled to it, an then it is stapled to the base of the mirror box. The three collimating screws just push up from the bottom of the box against the triangles. My sling is flush to the wood like yours. 

See attached photo, taken when I installed a computer fan to promote cooling. 
 

Nice. That looks simple and effective!

 

I have rotated the image from the opening post so it appears in a normal operating position, as seen looking into the scope with the mirror removed.

 

Is it the top triangle that is slipping at low elevation?  If so, it may be because those two strips are not stiff enough to hold it.  I would try putting them above the triangle rather than below it.  I still cannot imagine a weight shift that would lift the mirror off of it without a lot of violent motion.

Yes, the top triangle is the one slipping. It might not show up well in the photos but the strips are flimsy tape. 
 

 

There are tiny holes in the cell pieces and the bottom of the mirror box that were probably for holding them in place.

 

I would really just ditch that cell. You could make one yourself pretty easily out of aluminum with screws and pop rivets, NMT has a nice tutorial on YouTube

I’ll tuck that away for the future...

 

On the basis of my own experience I would keep the cell, with at most some minor debugging.  I do the same thing he does, which is packing the disassembled scope in my truck to haul it to dark sites.  I also take the mirror out and pack it in a separate box for safe keeping.  This pure Dobsonian sling arrangement makes it easy to slide the mirror back in place, with only some touch-up of the collimation needed to be ready for observing.

That’s the current plan. I’ll see if I can tweak the current setup. 

[KBHornblower]

I can see now that those strips of tape are unsatisfactory for keeping the triangles in position.  It's like trying to push a string.  Did you get the scope used?  If so, it appears that a previous owner changed things for mysterious reasons.  I cannot imagine a reputable manufacturer doing it that way.

 

My guess now is that when setting up the scope, the top triangle was too close to the center of the mirror and just barely resting on the collimation screw.  If so, it could slide off as the weight of the mirror takes up any slack in the sling.  That would certainly blow out your collimation.

 

I have my triangles held in place laterally by studs on which they are free to slide up and down when adjusting the collimation screws.  The holes in the triangles are just enough larger than the studs to let the triangles rock freely the little bit that is needed to equalize the axial loading properly.  When I set it up I tilt the scope to about 45 degrees and give it a good shaking to be sure there is no slack in the sling, and to be sure the weight of the mirror is firmly against the supporting pads.  Then I touch up the collimation.

 

I will post some photos when I get a chance.

[Retsub]

   I found a old pic of our very early 20" f4 Sky Designs scope on a Polar Roller drive platform at a small star party with the 107" looking over us. Yes, I think your scope can be made to work pretty well again with your help and from others and at not much cost to do it which can be important to many. Barbara used this scope year 'round many nites and for more years than I remember until the wood had gotten so bad it just looked ugly. I had replaced the upper tube ring several do to being exposed to the dew so many times. Usually the sling needs to be not wider than the mirror width. Years back someone offered Kevlar matl that was the right thickness and width and I installed that. Later on came the cable sling too and the calculations for placement were done too. In one pic of your scope it clearly shows that the two retainer blocks and pivot clips that were above the mirror are completely missing for what ever idea someone had but you need them for safety and to help keep the mirror from being able to lean forward when looking very low like Barbara many times did as low as the weeds would allow. Observing at its best, get it while you can. Pic of Marks cell looks very normal and with the hard wood pegs on the triangles. I have never seen a Sky D with triangles like yours. You mention "wood" on back of the mirror ? A pic would help. It could be a attempt to get the focal point farther out the tube or... ? I made some changes to our truss tubes to make it much easier for one person to assemble the upper tube parts. We can get to that later. Remember too that the floor of the mirror box is double thickness and if many changes like a new cell will quickly change the balance point rapidly. A quick way to remake the two missing mirror retainers is to use long bolts with washers and nuts to be able to adjust the eye bolts height that have the threaded ends covered with thin plastic tubing to protect the mirror as they will finally be adjusted to barely be above the mirror edges. I have seen a dob nearly turned completely over and the mirror retainers kept the mirror in place with no damage to it. Think of what else could have happened. I have never tried but I'll bet our scope can go quite low before the mirror tries to tip forward, at least down to 10 deg. Sorry for my bad writing as I didn't make a list for arrangement of my thoughts. Let us know.    *BW*   https://webmail.eart...ory&x=965131801

[Retsub]

Sorry but I can't get the pic to go.   *BW&

[KBHornblower]

I can see now that those strips of tape are unsatisfactory for keeping the triangles in position.  It's like trying to push a string.  Did you get the scope used?  If so, it appears that a previous owner changed things for mysterious reasons.  I cannot imagine a reputable manufacturer doing it that way.

 

My guess now is that when setting up the scope, the top triangle was too close to the center of the mirror and just barely resting on the collimation screw.  If so, it could slide off as the weight of the mirror takes up any slack in the sling.  That would certainly blow out your collimation.

 

I have my triangles held in place laterally by studs on which they are free to slide up and down when adjusting the collimation screws.  The holes in the triangles are just enough larger than the studs to let the triangles rock freely the little bit that is needed to equalize the axial loading properly.  When I set it up I tilt the scope to about 45 degrees and give it a good shaking to be sure there is no slack in the sling, and to be sure the weight of the mirror is firmly against the supporting pads.  Then I touch up the collimation.

 

I will post some photos when I get a chance.

Here is a photo.

 

This shows the studs on either side of the collimation screw, with the triangle a bit beyond them.  The other two triangles are in place with their studs recessed into the holes.  Each stud consists of a piece of 5/16" wood dowel with a hole drilled in the center, and held in place by a wood screw with a flat washer.  The large holes in the triangle go down halfway through the wood, with a diameter of 11/32" the rest of the way.

 

I may have over-engineered this a bit.  If I had not had the tools needed for this, I could have done well by simply mounting the triangles on a thin sheet of plastic or heavy paper, as a previous poster demonstrated.  That would have been pure Dobsonian.

[Retsub]

Pic #12 looks very much like the same as our early Sky D and aprox 6 others I have seen. Articles in old TMaking said to use a sling that wasn’t wider than the mirror thickness as to wrap around the edge and not distort images. That would make the bottom of the sling off the floor the same amount as the mirror is. Cable slings use a formula for edge placement to be at the mirror center of gravity to also not cause distortion. Hang in.  BW 

[Daveatvt01]

I can see now that those strips of tape are unsatisfactory for keeping the triangles in position.  It's like trying to push a string.  Did you get the scope used?  If so, it appears that a previous owner changed things for mysterious reasons.  I cannot imagine a reputable manufacturer doing it that way.

 

My guess now is that when setting up the scope, the top triangle was too close to the center of the mirror and just barely resting on the collimation screw.  If so, it could slide off as the weight of the mirror takes up any slack in the sling.  That would certainly blow out your collimation.

 

I have my triangles held in place laterally by studs on which they are free to slide up and down when adjusting the collimation screws.  The holes in the triangles are just enough larger than the studs to let the triangles rock freely the little bit that is needed to equalize the axial loading properly.  When I set it up I tilt the scope to about 45 degrees and give it a good shaking to be sure there is no slack in the sling, and to be sure the weight of the mirror is firmly against the supporting pads.  Then I touch up the collimation.

 

I will post some photos when I get a chance.

Yes, I did get it used, about a month ago. The previous owner had passed away, and the scope had been sitting for years. His wife said he had also bought it used, and I actually found an astromart ad from a sale of it at some point in the past (same mirror markings and focuser, also semi-local in Phoenix):

https://astromart.co...-by-sky-designs

 

In the ad the focuser has been upgraded from the original already and the mirror is already backed with wood. No photos of what the sling and cell look like. 

 

In the years since that sale the trusses have been modified with quick release bicycle clamps:

 

The collimation bolts stick up really high too. They are about 1/2" off the bottom of the box with the cell perched on top (see photo below), and if you lower the bolts eyepieces start not being able to focus anymore. I think I'm going to lower the UTA by about 1/2" to get everything lower.

 

Thanks for the info on the studs, that is a really neat idea. I'm not sure if I'll try that or the plastic sheet for my cell.

 

   I found a old pic of our very early 20" f4 Sky Designs scope on a Polar Roller drive platform at a small star party with the 107" looking over us. Yes, I think your scope can be made to work pretty well again with your help and from others and at not much cost to do it which can be important to many. Barbara used this scope year 'round many nites and for more years than I remember until the wood had gotten so bad it just looked ugly. I had replaced the upper tube ring several do to being exposed to the dew so many times. Usually the sling needs to be not wider than the mirror width. Years back someone offered Kevlar matl that was the right thickness and width and I installed that. Later on came the cable sling too and the calculations for placement were done too. In one pic of your scope it clearly shows that the two retainer blocks and pivot clips that were above the mirror are completely missing for what ever idea someone had but you need them for safety and to help keep the mirror from being able to lean forward when looking very low like Barbara many times did as low as the weeds would allow. Observing at its best, get it while you can. Pic of Marks cell looks very normal and with the hard wood pegs on the triangles. I have never seen a Sky D with triangles like yours. You mention "wood" on back of the mirror ? A pic would help. It could be a attempt to get the focal point farther out the tube or... ? I made some changes to our truss tubes to make it much easier for one person to assemble the upper tube parts. We can get to that later. Remember too that the floor of the mirror box is double thickness and if many changes like a new cell will quickly change the balance point rapidly. A quick way to remake the two missing mirror retainers is to use long bolts with washers and nuts to be able to adjust the eye bolts height that have the threaded ends covered with thin plastic tubing to protect the mirror as they will finally be adjusted to barely be above the mirror edges. I have seen a dob nearly turned completely over and the mirror retainers kept the mirror in place with no damage to it. Think of what else could have happened. I have never tried but I'll bet our scope can go quite low before the mirror tries to tip forward, at least down to 10 deg. Sorry for my bad writing as I didn't make a list for arrangement of my thoughts. Let us know.    *BW*   https://webmail.eart...ory&x=965131801

Sounds like you two got a lot of good use out of yours  ! 

I noticed the damage on the bottom of the box, but didn't realize that was where clips had been. I guess they weren't useful or were in the way once the mirror was moved. Thanks for the info! It's harder to put something back together if you don't know what it looked like in the first place! 

I suspected the triangles were not original or at least modified, because of what is on the underside when I bought the scope:

Nickels! Some added dented nickels to keep the thin collimation bolts from digging in to the wood. An affordable idea I guess, but it doesn't help the cell stay in one place, if it's supposed to(?)

Here is a photo of the wood mounted on the back of the mirror... I wonder if I should try removing it at some point or if that would cause additional problems:

It is about 3/4" thick and the same diameter as the mirror. If it came off easily, the sling would be well positioned to support the mirror (from the comment below).

 

All of that added height (bolts +  wood backing) make the mirror sit so high that the top 1.25" of it is above the metal sling:

Only 1/2" of the mirror is supported by the sling(!)

 

 

Pic #12 looks very much like the same as our early Sky D and aprox 6 others I have seen. Articles in old TMaking said to use a sling that wasn’t wider than the mirror thickness as to wrap around the edge and not distort images. That would make the bottom of the sling off the floor the same amount as the mirror is. Cable slings use a formula for edge placement to be at the mirror center of gravity to also not cause distortion. Hang in.  BW 

Good to know, thanks! 

[KBHornblower]

My educated guess is that a previous owner changed focusers and then could not reach focus without raising the mirror an inch or more.  A better way than changing the mirror mounting would have been to shorten the struts.  That may or may not be practical for you depending on the design of the struts and whatever tools you have.

 

The wild card here is whether or not that piece of wood can stress the mirror enough to deform its figure.  It certainly defeats the purpose of the 9-point support system.

 

My support triangles were originally mounted on a sheet of thin cardboard that came with the mirror, with the optimum pattern for 9-point support printed on it.  That gave up the ghost over 30 years and I replaced it with the stud system I showed earlier.  If I had not had the tools needed for that, I would have used another sheet of suitable cardboard or plastic.

[sixela]

One of the problems with the Sky Designs is the closed back of the mirror box, it takes a long time for the mirror to cool.  

Drill a hole, install a fan, and optionally add a baffle a few centimeters above the mirror to direct the airflow over the mirror again on that side. Beats open cells any time in my book.