32 replies to this topic

[Rustler46]

I've been a reflector person from the start of my amateur astronomy experience, now extending almost 60 years. Starting with home-made 6-inch and 8-inch 'scopes, I've also employed SCTs (5-, 8- & 11-inch versions) and more recently a nice 4-1/2 inch refractor (AT115EDT). Currently the larger SCTs are carried on a Losmandy G-11 mount with go-to capability. The Celestron 8 is particularly easy to set up and is being used very often.

 

A recent interest in summer globular clusters has had me using a Hardin Optical (GSO) 10-inch reflector for its fainter magnitude limit  versus the C-8 - around 0.4 magnitude. The increased light grasp and resolution really aids in getting the most out of these spectacular clusters. This OTA is what I call my Thirty Dollar Telescope. But compared to the SCTs on the Losmandy mount, it has been less comfortable to use. It had one time sported an early version DSC setup, that had only 250 objects in its catalog. Needless to say this was quite limiting. 

 

So I acquired a Nexus-II wireless module to connect to the existing encoders. This allowed use of my iPad Mini running SkySafari-6 Pro with many thousands of objects (millions really) in its catalogs. With the push-to capability, finding objects is a snap compared to decades of star atlas and finder scope. Still the telescope was at times uncomfortable to use, due to its fixed eyepiece orientation and variable height. So to help with that I recently finished a adjustable height observing chair using plans provided by Sky and Telescope's Jerry Oltion. Jerry was quite helpful answering questions and with advice about adapting the plans to the wood I had available. While I have not had opportunity to observe with that new accessory, I'm sure it will add to the enjoyment in observing.

 

Here are some photos of the fully outfitted 10-inch Reflector.

 

The hand truck allows for quick movement from garage to garden observing site. The HD rubber strap (wrapped around the lower end of the OTA) is used when hand-trucking the telescope to prevent it turning on the altitude axis. It works best traveling with the OTA firmly held against the zenith position of the Dob mount.

 

Here is the telescope with iPad, Nexus-II and Telrad installed, observing chair at the ready.

 

Seen here are the azimuth encoder and a small Lithium-Iron-Phosphate power supply that provides 12-volts for the fan at the bottom end of the OTA along with 5-volts (USB) to keep the iPad going on cold nights.

 

This shows both encoders, the battery and the ventilation fan.

 

 

I'm looking forward to observing some more of the summer's best globular clusters. It may take some time for the weather and wildfire smoke to cooperate. But I'll be ready when the conditions permit.

 

Russ

Edited by Rustler46, 10 September 2021 - 01:09 AM.

[Rockin'Robin]

Ok, you just reminded me that I need to order a fan for my 10" Dob. But one thing I would like to know. How in the world did you arrange to have that fan/shroud made or where did you buy it?? That's exactly what I need for my scope. I'm afraid if I attach the fan right to the bottom of the scope, the vibration will mess up the view through the eyepiece.

Thanks~Robin

 

10-inch Reflector-3.jpg

This shows both encoders and the battery.

 

 

 

[Rustler46]

 

Ok, you just reminded me that I need to order a fan for my 10" Dob. But one thing I would like to know. How in the world did you arrange to have that fan/shroud made or where did you buy it?? That's exactly what I need for my scope. I'm afraid if I attach the fan right to the bottom of the scope, the vibration will mess up the view through the eyepiece.

Thanks~Robin

 

Hi Robin.

 

My wife made the fan shroud. As I recall the fan is suspended by heavy rubber bands behind the shroud. There is elastic around both the telescope tube and around the fan itself. It doesn't look real slick. But my telescope is made to look through, not to look at.

 

As for the fan, I'm thinking a better arrangement would be to have the fan blow across the surface of the mirror, exiting though holes in the tube on the far side. That would scrub the boundary layer of warm air right in front of the surface of the mirror. The rear-mounted fan moves air down the length of the tube, being drawn out around the edges of the mirror. But it doesn't sweep across the entire mirror surface like a side mounted fan. The idea came from a dew prevention in SCT thread. I was amazed at how that method prevented dew without adding heat, at the same time improved seeing by sweeping the corrector lens of the boundary layer of warm air. Seems like it would work on a reflector as well. But that is for a later project.

 

Russ

Edited by Rustler46, 10 September 2021 - 01:04 AM.

[Rockin'Robin]

Hi Robin.

 

My wife made the fan shroud. As I recall the fan is suspended by heavy rubber bands behind the shroud. There is elastic around both the telescope tube and around the fan itself. It doesn't look real slick. But my telescope is made to look through, not to look at.

 

As for the fan, I'm thinking a better arrangement would be to have the fan blow across the surface of the mirror, exiting though holes in the tube on the far side. That would scrub the boundary layer of warm air right in front of the surface of the mirror. The rear-mounted fan moves air down the length of the tube, being drawn out around the edges of the mirror. But it doesn't sweep across the entire mirror surface like a side mounted fan. The idea came from a dew prevention in SCT thread. I was amazed at how that method prevented dew without adding heat, at the same time improved seeing by sweeping the corrector lens of the boundary layer of warm air. Seems like it would work on a reflector as well. But that is for a later project.

 

Russ

That is an awesome set up. Right now I'm still looking for a few eyepieces and a coma corrector for my Dob. And, maybe, a fan. Do you use fans while viewing during the summer month? Or is it an all year thing? I do most of my observing when there are few or no bugs. I don't like the thought of those nasty critters setting up housekeeping in my scope. Plus I have a bad habit of doing my karate moves when bugs are flying around.

 

~Robin

[Rustler46]

That is an awesome set up. Right now I'm still looking for a few eyepieces and a coma corrector for my Dob. And, maybe, a fan. Do you use fans while viewing during the summer month? Or is it an all year thing? I do most of my observing when there are few or no bugs. I don't like the thought of those nasty critters setting up housekeeping in my scope. Plus I have a bad habit of doing my karate moves when bugs are flying around.

 

~Robin

Hi Robin,

 

Thanks for your nice comment. I'm feeling like that old reflector is getting set up most optimally for its genre. I have a couple of dozen eyepieces from WWII surplus to some nice TVs. I also have a GSO coma-corrector, but seldom use it. It would have greater utility if I was more into astro-photography. But it adds 10% to the focal length, impacting the low end, wide field powers. Of all the eyepieces these are the ones I use almost exclusively:

1. 24 mm TV Panoptic - 52X, 1.3° True FOV, 4.9 mm Exit Pupil
2. 11 mm TV Nagler T6 - 114X, 0.7° FOV, 2.2 mm EP
3. 24 mm + TV 2-1/2X Powermate focal extender - 130X 0.5° FOV, 2.0 mm EP
4. 11 mm + Powermate - 284X, 0.3° FOV, 0.9 mm EP
5. 42 mm GSO Superview, 2-inch, 65° AFOV -  30X, 2.2° FOV, 8.5 mm EP

The last one violates the 7mm largest exit-pupil rule. So with my 75-year old eyes' 5 mm exit pupil, it stops down the aperture to something like a 6-inch aperture. But with such a large FOV, who cares - not me. It takes in most all of the Andromeda Galaxy plus the M32 and M110 companions. With an inexpensive eyepiece like that, I don't expect good performance on the periphery. I'm sure the coma-corrector would help with that.

 

As for the fan when I roll out the telescope I pull the dust cover, point the telescope down and turn on the fan. This exhausts warm air from the rear, aiding climatizing the mirror. It is kept running during the entire observing session. Not many bugs here either. Also the air temperature does not drop quickly, so the mirror can be keep closer to ambient temperature this way. This is the same procedure year-round.

 

Clear Skies!

Russ

Edited by Rustler46, 13 September 2021 - 01:47 AM.

[Rustler46]

A couple of nights ago, I had a most enjoyable evening observing with this reflector. The adjustable observing chair was key in this. That way I could concentrate on seeing what I could, rather than the pain of an uncomfortable viewing position. My only regret is enduring without it for nearly 60 years. Even those with younger bodies would benefit from the chair. Jerry Oltion in his 2016 Sky and Telescope article said it would add an inch to your telescope's aperture. In effect that might not be such an exaggeration. 

 

The only expenditure was parts of three days in putting it all together. I had all the needed wood in my scrap wood cache. The majority was from a rejected wooden tabletop insert leaf, that had been given to me by a furniture store. Then it was just working with band-saw, bench-top drill press, & hand drill. Throw in some screws and wood stain and there you have it. I highly recommend the effort. If you have more money than time, then there are a number of commercial chairs available. But I prefer the look of the wooden version. Some two-inch foam and cloth stapled to the seat adds to the comfort.

 

 

One change I'll make is painting the ends of the rear outriggers white so the can be seen in the dark. That way I won't be bumping into these by accident. 

 

Russ

Edited by Rustler46, 13 September 2021 - 01:43 AM.

[New-Old-Guy]

Really nice scope & Chair 

[Rustler46]

Really nice scope & Chair 

Thanks for your kind comment. I really like this latest iteration of the reflector. While its doesn't give as good an image as that provided by my SCTs or refractor, its extra resolution and light gathering power is appreciated. I suspect images would improve with front mounted fan to blow across the mirror surface. In the meantime I'm making best use of what I have.

 

For comparison, here is the same OTA on the Losmandy mount.

 

 

Here's a Moon shot with the same telescope.

10-inch Hardin Optical Deep Space Hunter w/ GSO coma corrector, f/5.5, 1375 mm f.l., Sony NEX-5N, ISO 800, 1/4000-sec.

 

Russ

[Rustler46]

After reading some sage advice on Cloudy Nights, I have implemented some measures to deal with cool-down, acclimatization and tube currents. The first of these is:

• Mirror end fan blowing towards the mirror

Previously the fan had been blowing away from the mirror. But the thinking is blowing on the back of the mirror, around to the front side and out the top end would sweep the boundary layer and bring the mirror closer to ambient temperature. So some limited testing with this in use seemed to help with thermal issues.

 

In some discussions on thermal issues, there are advantages to a solid tube in that it allows for completely sweeping internal air out the top of the tube. This was deemed an advantage compared to open-tube (truss) designs. 

 

But there was one negative with a solid metal tube. That is the metal will cool below ambient temperature via thermal radiation to the night sky. Thus cold tube exposed to warmed air blowing up from below creates turbulence and tube currents. So in an effort to deal with this issue one could cover the tube with cork on the inside. Not having of that on hand I opted to block the night sky as seen from the metal tube. This would act similar to how a dew shield works to slow the cooling of exposed glass optics. Here's my solution:

• Cover the upper (sky side) of the metal tube with a blanket of garden weed barrier.

This blanket is made of a woven synthetic cloth covered on one side with non-woven felt. I chose to put the felt side out. The blanket is 2 feet by 4 feet in size. It required cutting out sections for the focuser, finders etc. So I made a paper cutting guide according to measurements around things that couldn't be covered. Here are some photos showing the thermal blanket.

 

Telescope and soon-to-be thermal blanket, woven side shown 

 

Side view with thermal blanket in place

 

This is a sky side view showing how the metal tube does not "see" the night sky.

 

The hardest part was getting the cutouts correctly located. Once that was done the blanket is held in place by some HD wire ties (16 in all). This entailed using 4 of them daisy-chained together to go around the 3 ft. plus circumference. No - it ain't pretty. But I spend more time looking through my telescopes rather than looking at them. I hope to test this arrangement out tonight. It shouldl improve the image quality by dealing with thermal issues under my control.

 

Edit:

Tested last night, heavy dew. I could feel the warm air blowing out the top of the tube. Everything seemed to work well. Atmospheric seeing wasn't very good. But it was good enough to do a star test on the mirror's figure. So the internal tube currents must have been controlled well enough to clearly see the out of focus rings in the image.

 

The LiFePo4 battery went dead before end of observing. I hadn't recharged it in over a month. But it lasted long enough to keep the iPad battery from dying and kept the fan going for hours starting in late afternoon on past start of observing. So I should regularly recharge the battery, perhaps once a month. 

 

I'm hoping to give another test tonight. ClearDarkSky predicts above average seeing. So I'll check collimation on Polaris, then go on to some double star and globular cluster observing. These are affected by overall seeing conditions, both in atmosphere above and tube currents within.

 

Russ

Edited by Rustler46, 02 October 2021 - 03:13 PM.

[Rustler46]

After testing the recent changes in thermal management under sub-par seeing conditions, it is hard to tell what effect it had. It isn't like I can flip a switch to turn these changes on or off. But the fan blowing towards the mirror plus topside thermal blanket likely improved the inside tube "seeing", not adding much to the bad effects of atmospheric seeing. Time will tell. When I pull the mirror to have it recoated, I will likely add a baffle just above the mirror surface to direct the airstream toward the center of the mirror. This should help scrub the front boundary layer.

 

One definite positive effect from the topside blanket is having a very black surface surrounding the focuser. This is much easier on the eyes than the grey color visible before. I may even get a full black hood attached to the focuser that can be flipped up over my head to exclude outside light.

 

I did notice one thing that is probably related to the primary mirror cooling down. At the beginning of the observing session I checked collimation on Polaris. At that time I did a star test with unfocused star image. The inside focus diffraction rings showed under-correction (bright outer ring). Later in the evening (presumably after further cooling) the star test showed over correction (outside focus, bright outer ring). At least that shows a transition through proper correction. But I'm just learning how to do the star test. So hopefully I can improve in my skills in assessment of mirror figure.

 

With the changes made to my 10-inch reflector, I'm thoroughly enjoying using this OTA. While my Losmandy G-11 go-to mounted SCTs have their benefits, the 10-inch, push-to Dob is much easier to setup. It loses little in comparison to the C-11 and has significant resolution and light gathering benefits over the C-8. At least for now the 10-inch is my grab-n-go setup.

 

Russ

[Rustler46]

In my ongoing quest to optimize my telescope for visual use, I'll be having the mirror recoated with Beral as soon as my 8-inch mirror comes back from Clausing. The existing mirror coating is over 15 years old, long overdue for a refresh.  While the 10-inch mirror is away my goal is to work on getting airflow from the rear fan optimized according to the counsel given in the thread Boundary Layer Fans.

 

In the meantime I've decided to replace the nice stock R&P focuser with my trusty JMI EV-1 Crayford focuser.

 

The smooth two speed focusing will be appreciated.

 

Here it is shown some years back installed on the 10-inch OTA with a digital camera attached.

 

For visual use I'll be using the GSO coma-corrector to get it to come to focus. While this increases the focal length by 10% (f/5.5) it will still give some low power views approaching 2° with a 42mm GSO eyepiece. Even with the 24 mm Panoptic the FOV exceeds 1°. And the comma corrector will no doubt clean up the edge of field aberrations.

 

Clear Skies,

Russ

Edited by Rustler46, 06 October 2021 - 05:49 PM.

[Sarkikos]

Great work.  I see one more improvement you can make:  replace the straight-through finder with a RACI.  

 

You have DSC's, so I suppose you never star hop.  But still, that straight-through finder must be uncomfortable to look through.  You have a Telrad.  Let that be your 1x straight-through finder/pointer.  Get yourself a nice RACI for when you want an RFT view.  Save your neck from cramps.

 

Mike

[Sarkikos]

A couple of nights ago, I had a most enjoyable evening observing with this reflector. The adjustable observing chair was key in this. That way I could concentrate on seeing what I could, rather than the pain of an uncomfortable viewing position. My only regret is enduring without it for nearly 60 years. Even those with younger bodies would benefit from the chair. Jerry Oltion in his 2016 Sky and Telescope article said it would add an inch to your telescope's aperture. In effect that might not be such an exaggeration. 

 

The only expenditure was parts of three days in putting it all together. I had all the needed wood in my scrap wood cache. The majority was from a rejected wooden tabletop insert leaf, that had been given to me by a furniture store. Then it was just working with band-saw, bench-top drill press, & hand drill. Throw in some screws and wood stain and there you have it. I highly recommend the effort. If you have more money than time, then there are a number of commercial chairs available. But I prefer the look of the wooden version. Some two-inch foam and cloth stapled to the seat adds to the comfort.

 

Observing Chair-02529 Small.jpg

 

One change I'll make is painting the ends of the rear outriggers white so the can be seen in the dark. That way I won't be bumping into these by accident. 

 

Russ

Good choice on an observing chair that has a series of rungs you can hook the seat onto, rather than two rails secured only by friction.  I've had two different observing chairs with the friction design.  The seat slipped underneath me periodically on both chairs.  I hate these two-rail friction chairs.  I'll never buy another observing chair that does not have rungs for secure positioning of the seat.

 

Mike

[Rustler46]

Great work.  I see one more improvement you can make:  replace the straight-through finder with a RACI.  

 

You have DSC's, so I suppose you never star hop.  But still, that straight-through finder must be uncomfortable to look through.  You have a Telrad.  Let that be your 1x straight-through finder/pointer.  Get yourself a nice RACI for when you want an RFT view.  Save your neck from cramps.

 

Mike

Thanks Mike for your comments. Yes, one can always look for ways to improve the ease of use of our telescopes.This is particularly so as we get older and our bodies are less cooperative. 

 

As for a finder, I generally don't use one. The only RACI finder in my employ is for my 50+ year old 8-inch on equatorial mount. That finder is older than I am, having been made in 1943 during WWII for a tank sight. For my more modern telescopes (C-8, C-11, 10-inch reflector & 4-1/2 inch refractor) I rely on go-to or push-to object finding. The finder is only used to acquire the 2 or 3 stars for alignment. This has mostly been just using the Telrad. 

 

But in my small-town, humid air, light polluted observing environment the Telrad has difficulty seeing stars fainter than 2nd magnitude. The glass window diminishes the starlight enough to make finding them difficult. Then I thought about using the straight through finder that came with the 10-inch reflector. Yeah, it was packed away on my "astronomy stuff" shelf in the garage. I found that to be superior to the Telrad, easily seeing stars below naked-eye limit. 

 

For that use the straight-through design isn't much of a bother, since I need to sight (naked-eye) along the edge of the finder to ensure I'm on the right star. That would be the case even with a RACI model. So two times straight through and I'm done with the finder for the night.

 

If I was doing star atlas and finder-scope finding, a RACI finder with green laser strapped along side would be a good way to go. That's what I do with my old 8-inch reflector with its finder.

 

 

This telescope's mirror is out being recoated by Clausing. Next will be my 10-inch reflector mirror.

 

Russ

[Rustler46]

Good choice on an observing chair that has a series of rungs you can hook the seat onto, rather than two rails secured only by friction.  I've had two different observing chairs with the friction design.  The seat slipped underneath me periodically on both chairs.  I hate these two-rail friction chairs.  I'll never buy another observing chair that does not have rungs for secure positioning of the seat.

 

Mike

I'm glad I happened on Jerry Oltion's plans for this chair. It was fun to build, costing nothing but some time. If I try to move the chair by pulling on the seat platform it will at times disengage from the rungs. But once I'm seated it is secure. Having a comfortable observing position has been a game-changer for me. My observations have improved by allowing for longer, pain-free scrutiny of the object. This has also made my observing sessions longer before weariness sets in.

 

Russ

Edited by Rustler46, 07 October 2021 - 06:13 PM.

[Rustler46]

The 10-inch mirror has been delivered to H.L.Clausing for re-coating. Howard did a great job on the 8-inch mirror, which gave opportunity for a very nice view of Jupiter recently. The transparency was very poor (thickening clouds), but seeing excellent when I could see the shadow of Io near Jupiter's meridian.

 

I was tempted to buy a new GSO mirror, since re-coating the chipped mirror (with round trip shipping) was over $180. A new mirror would be around $250. But everyone was sold out, due to pandemic circumstances. So when the refreshed mirror is back, it will be returned to service with an optimized ventilation system. This includes a director ring to sweep the mirror's surface of boundary layer air and aid in cooling. Here is the finished ring (AKA slot-nozzle) waiting to be attached to the mirror cell.

 

[Jon Isaacs]

Hi Robin.

 

My wife made the fan shroud. As I recall the fan is suspended by heavy rubber bands behind the shroud. There is elastic around both the telescope tube and around the fan itself. It doesn't look real slick. But my telescope is made to look through, not to look at.

 

As for the fan, I'm thinking a better arrangement would be to have the fan blow across the surface of the mirror, exiting though holes in the tube on the far side. That would scrub the boundary layer of warm air right in front of the surface of the mirror. The rear-mounted fan moves air down the length of the tube, being drawn out around the edges of the mirror. But it doesn't sweep across the entire mirror surface like a side mounted fan. The idea came from a dew prevention in SCT thread. I was amazed at how that method prevented dew without adding heat, at the same time improved seeing by sweeping the corrector lens of the boundary layer of warm air. Seems like it would work on a reflector as well. But that is for a later project.

 

Russ

 

Russ:

 

I think there are issues with a single side fan.  The flow is not uniform across the either surface so the cooling will not be uniform.  

 

What I did with my 13.1 inch is put a full baffle just in front of the mirror basically riding on the mirror clips. The rear fan is fully baffled.  So what happens is that the air flows around the edge of the mirror and then is directed across the surface of the mirror by the front baffle. I saw this technique in another thread.  

 

 

I have not done this with my 10 inch which is very similar to yours but I think I will. It would be very easy and it can be done with thick, black poster paper.  

 

Regarding the chair.. 20 years ago, a friend came over for the evening and he had a Starbound chair.. The next morning, I drove up to OPT and bought one.  It's been a constant companion ever since.  It's the only piece of my asto-gear that I use every night with all my scopes.. I even use it with the 22 inch for objects close to the horizon.

 

Jon

[Rustler46]

Russ:

 

I think there are issues with a single side fan.  The flow is not uniform across the either surface so the cooling will not be uniform.  

 

What I did with my 13.1 inch is put a full baffle just in front of the mirror basically riding on the mirror clips. The rear fan is fully baffled.  So what happens is that the air flows around the edge of the mirror and then is directed across the surface of the mirror by the front baffle. I saw this technique in another thread.  

 

I have not done this with my 10 inch which is very similar to yours but I think I will. It would be very easy and it can be done with thick, black poster paper.  

Yahoo - my 10-inch mirror just came back from Clausing. Now I can put it all back together. I've learned a lot about reflector mirror ventilation in the last two months. The thread Boundary Layer Fans has been an eye-opener for me. 

 

So Jon, your sage advice about front mounted fans blowing across the surface is well taken. Here is the front side director ring waiting for the mirror. My fan will blow on the back of the mirror around the sides on the front as you suggest.

 

Mirror cell waiting for the recoated mirror.

 

Here's a view of the annulus from the mirror side.

 

Here's the director ring (AKA slot-nozzle) after painting flat black.

 

 

I've occupied myself while the mirror was away by further refurbishing my old home made 8-inch RFT telescope. It has been likened by one forum member as of the "steam-punk" genre. I agree. It will be fun using my old friend to observe this week's near total lunar eclipse. I'm hoping the weather holds between west coast hosings with atmospheric rivers.

 

Russ

[Rustler46]

Now that my focus has returned to my Thirty-Dollar telescope, I have a few tasks to complete to get it back in service.

1. Re-attached the mirror cell to the telescope's rear casting
2. Reinstall the newly recoated mirror in its cell
3. Fashion a seal for the director ring between the annulus and the telescope's metal tube
4. Install the director ring above the mirror at the correct spacing
5. Attach the mirror assembly to the metal tube
6. Reinstall the ventilation fan assembly

These steps should be fairly quick to accomplish. For step #2, I need to make some changes to the mirror cell.

 

 

The four cork pads that contact the rear of the mirror will need to be covered with some glued on pieces of plastic sheet. This is to prevent cork to glass friction, that was warned about on Cloudy Nights. This will be easy to do. Also the four cork pads around the edge of the mirror cell will be removed. The gap between the mirror and its cell will be taken up with a thin wrapping of expanded foam sheet. This is the sort of material that is wrapped around fragile items being shipped. One or two layers of this should be enough to take up the slack and also insulate the edge of the mirror from the cooling air stream. This was a recommendation from the Elmira video in the boundary layer fans thread.

 

As for the fan, I've found another 80 mm fan that has a higher flow rate than the one previously used. The new fan has a 0.20 amp draw compared to the other with 0.12 amp. I'm surmising the flow rate will increase from around 25 CFM to over 40 CFM. The lessor fan will be available for use on my 8-inch RFT. If 40 CFM is too much for the 10-inch reflector I have the option of running it from the 9-volt output of the LiFePO⁴ battery supply. The actual air flow derating for an axial fan is a big guess. Free air flow rate is reduced by any filter material in use, along with resistance in the air path around the side of the mirror from behind. But depending on performance I have the option of increasing the director ring clearance above the mirror. Since I've already made supports for the ring, I'll stick with that clearance for now. But I have plenty of 8-gauge aluminum wire to use for increased clearance if needed.

 

For step #3 I'll get some fabric from my wife to attach around the annulus' edge to provide the seal. This will ensure all the ventilation airflow is directed across the surface of the mirror.

 

I'm really looking forward to getting this 10-inch reflector back in service. This weekend is supposed to be clear after over 2 weeks of rain. The improved cooling and boundary layer scrubbing will help it produce as good an image as is possible, particularly for higher power double-star or lunar/planetary observing.

 

Russ

Edited by Rustler46, 17 November 2021 - 09:52 PM.

[Rustler46]

I've started working on step #3 - the seal between director ring and inside of telescope tube. The tube's inside diameter is 298 mm, or 149 mm in radius. The annulus O.D. is around 290 mm, 145 mm radius. The difference in radius is 4 mm. So that is the gap that needs to be filled by the seal. It doesn't need to be perfect. It just has to reduce the amount of air flowing between director ring and tube. I want the majority of the air flowing over the mirror front surface. 

 

So my wife supplied me with a strip of dark cloth wide and long enough to go around the annulus' circumference. This will be glued on along the edge of annulus to close the gap. Having that flexible seal will allow for any changes in the gap brought about when the mirror cell and attached director ring move during collimation. I'll post some photos later.

 

Russ

[SteveG]

I've started working on step #3 - the seal between director ring and inside of telescope tube. The tube's inside diameter is 298 mm, or 149 mm in radius. The annulus O.D. is around 290 mm, 145 mm radius. The difference in radius is 4 mm. So that is the gap that needs to be filled by the seal. It doesn't need to be perfect. It just has to reduce the amount of air flowing between director ring and tube. I want the majority of the air flowing over the mirror front surface. 

 

So my wife supplied me with a strip of dark cloth wide and long enough to go around the annulus' circumference. This will be glued on along the edge of annulus to close the gap. Having that flexible seal will allow for any changes in the gap brought about when the mirror cell and attached director ring move during collimation. I'll post some photos later.

 

Russ

You don’t really need to seal the outer edges of the front baffle. When I did this 10 years ago, I did smoke tests. Once you seal up the back of the scope, the air will flow through, then directly over the face of the mirror. I suggest the front baffle be very close to the mirror. I placed mine right on top of the mirror clips.

[Rustler46]

I've started working on step #3 - the seal between director ring and inside of telescope tube. The tube's inside diameter is 298 mm, or 149 mm in radius. The annulus O.D. is around 290 mm, 145 mm radius. The difference in radius is 4 mm. So that is the gap that needs to be filled by the seal. It doesn't need to be perfect. It just has to reduce the amount of air flowing between director ring and tube. I want the majority of the air flowing over the mirror front surface. 

 

So my wife supplied me with a strip of dark cloth wide and long enough to go around the annulus' circumference. This will be glued on along the edge of annulus to close the gap. Having that flexible seal will allow for any changes in the gap brought about when the mirror cell and attached director ring move during collimation. I'll post some photos later.

The seal around the director ring has been finished as shown in the photo. I found that contact cement applied to the ring and to the fabric worked best. I let the cement cure for 1/2 hour before bringing the two into contact. When inserting the entire mirror assembly into the bottom of the tube, it was a snug fit. But all of the fan's output will be directed over the mirror surface. And wonder of wonders, tonight is supposed to be clear for some testing of the 10-inch reflector, now with improved ventilation and new mirror coatings.

 

 

My old Thirty-Dollar Telescope is now "fully outfitted" and ready for action. Future improvements will be replacing the 0.12 amp axial fan with a more capable 0.18 amp one of the same size. That will free up the lesser air flow fan for use on my old 8-inch RFT.

 

Some improvements:

• ​The cork pads supporting the back of the mirror were covered with thin plastic sheet to reduce cork to glass friction (as recommended in the thread boundary layer fans).
• The cork pads in contact with the edge of the mirror removed.
• The edge of the mirror was covered with thin foam sheet to reduce excessive edge cooling.
• The 4 rubber mirror hold-downs were left less than fully tightened down to prevent pressure on the mirror in those areas. Previously when these were fully tightened down there was significant pressure on the mirror.

I'm hoping these minor improvements along with the improved ventilation and boundary layer scrubbing will result in optimized performance. Time will tell.

 

Clear Skies!

Edited by Rustler46, 23 November 2021 - 08:13 PM.

[SteveG]

The seal around the director ring has been finished as shown in the photo. I found that contact cement applied to the ring and to the fabric worked best. I let the cement cure for 1/2 hour before bringing the two into contact. When inserting the entire mirror assembly into the bottom of the tube, it was a snug fit. But all of the fan's output will be directed over the mirror surface. And wonder of wonders, tonight is supposed to be clear for some testing of the 10-inch reflector, now with improved ventilation and new mirror coatings.

 

10-inch with Director Ring-02639.jpg

 

My old Thirty-Dollar Telescope is now "fully outfitted" and ready for action. Future improvements will be replacing the 0.12 amp axial fan with a more capable 0.18 amp one of the same size. That will free up the lesser air flow fan for use on my old 8-inch RFT.

 

Some improvements:

• ​The cork pads supporting the back of the mirror were covered with thin plastic sheet to reduce cork to glass friction (as recommended in the thread boundary layer fans).
• The cork pads in contact with the edge of the mirror removed.
• The edge of the mirror was covered with thin foam sheet to reduce excessive edge cooling.
• The 4 rubber mirror hold-downs were left less than fully tightened down to prevent pressure on the mirror in those areas. Previously when these were fully tightened down there was significant pressure on the mirror.

I'm hoping these minor improvements along with the improved ventilation and boundary layer scrubbing will result in optimized performance. Time will tell.

 

Clear Skies!

The boundary layer scrubbing will be far more efficient if you lower that ring closer to the mirror.

 

Mine sit right on top of the mirror clips:

 

[Rustler46]

The boundary layer scrubbing will be far more efficient if you lower that ring closer to the mirror.

 

Mine sit right on top of the mirror clips:

Thanks Steve for your suggestion. Yeah, there has been a lot of discussion about the use of fans for cool-down and boundary layer scrubbing. As expected there are differences of opinions on the best route to go. 

 

I have learned a lot from the recent thread boundary layer fans. Much was added to the discussion by forum member phonehome, who gave the results of investigations with his awesome Elvira reflector - see a forced ventilation scheme for a fast Newtonian. 

 

After reading much useful information on those threads I chose to follow Ed's recommendations, which are based on his research. His design cals for having a 2 feet per second airflow across the front of the mirror surface. This is achieved with a specific fan cubic feet per minute air flow and director ring spacing.

 

What follows are the specific recommendations based on the Elvira project. I chose to follow these in my reflector ventilation system. Following those guidelines I ended up with a 16 mm (0.63 inch) clearance between director ring and mirror surface.

 

Clear Skies,

Russ

 

Design of Mirror Ventilation Airflow System

From:

 Cloudy Nights > Equipment Discussions > ATM, Optics & DIY Forum

A forced ventilation scheme for a fast Newt. - #60

 

 

Here are the 2 pages from the video that describes how to easily determine the five values required for the system.  They are:  system CFM, scrubbing/outflow velocity, distance outside the mirror edge horizontally, gap length (circumference) and distance from mirror face vertically (gap height).  These values should be roughly adhered to for a successful implementation:

 

How are the CVS airflow parameters determined and where is the ring
or nozzle precisely located? Five easy pieces of information are required:

 

# 1. System CFM

The first requires an approximation or estimate of the air-flow volume
in Cubic Feet per Minute (CFM) through the system to efficiently remove heat. As of the making of this video a precise formula has not been established as there are many variables. Therefore I suggest as a starting point roughly 1 CFM per inch of circumference for 6” to 24” mirrors and roughly 1 CFM per inch of circumference plus radius for 26” to 34”. Above 36” there is little info available so the best I can suggest is roughly 2 CFM per inch of circumference times 2. Unusually thick mirrors or massive support systems should err on the higher side and thinner low-expansion mirrors with lighter-weight support systems may err on the low side.

 

When selecting your fan or fans for system volume a CFM de-rating of 30% minimum should be subtracted from each fan’s free-space specification to compensate for air-path turns and filter restrictions. HEPA filters should be chosen for optimal filtering velocity which is usually indicated by a filter’s recommended CFM range. Caution: under-sizing the filter will greatly restrict the airflow, have reduced filtering and lead to more frequent replacement. As to particle/ percentage performance I’ve had good luck with filters rated HR-14 or MERV14 or equivalent. Related design note: any turns, passages or ports in the CVS such as the mirror cover’s central vent should have a minimum cross-area that’s at least 4 times that of the fan(s) aggregated exhaust ports to avoid excessive resistance.

 

#2. Scrubbing/outflow velocity

The second also requires an approximation of the velocity at the gap at the highest speed setting. For 6” to 14” mirrors it is about 2 Feet Per Second (FPS). For 16” to 24” about 3 FPS, from 26” to 34” about 4 FPS, and above 36” estimates roughly at 5+ FPS. These velocity guidelines ensure vigorous scrubbing to the center of the mirror and development of the outflow column.

 

#3. Distance outside the mirror edge horizontally

The third represents the distance horizontally the ring should be outside the mirror as to not vignette. About 1 to 3MM is sufficient but no more.

 

#4. Gap length (circumference)

The fourth is the gap (or ring length) which is the circumference of the mirror.

 

#5. Distance from mirror face vertically (gap height)

The fifth is the vertical distance of the director ring above the mirror. This can be determined from the above information. This is most easily performed using on-line air- flow/duct calculators. If offered, select a rectangular duct shape, not round (the gap presents as a rectangular duct). Enter the circumference of the mirror into one of the duct dimension fields. Enter the proposed height of the gap into the other dimension field. Enter the adjusted system CFM rate and check against the required velocity at the
gap.

 

Example:   https://www.engineering.com/calculators/airflow.htm

 

This will help in determining the vertical distance and for most this will be between 0.20 and 0.75”. To be clear this is the height of the gap from the mirror face to the bottom edge of the director ring.

Edited by Rustler46, 25 November 2021 - 10:22 PM.

[Tom Stock]

https://m.youtube.co...h?v=UbH4AS-0ViE