52 replies to this topic

[starman876]

The power of a big scope is in the delivery of photographic images of deep sky objects.  To see those views, though, requires dark skies.

If you're stuck in brighter skies, and you primarily look at the planets and the Moon, I see no reason to go larger than 10-12" because it is HIGHLY unlikely the seeing

will ever exceed the resolution of that aperture.  The really big dob is simply wasted in that environment.

The 60" at Mt. Wilson, for example, doesn't give any better image of DSOs than a really good 16-18" in dark skies.

 

So back to collimation, which affects high power images most: the ultralight won't hold collimation tolerances you need for high powers on a fast f/ratio.

Yes, it's possible to engineer and make your own ultralight and stiff scope.  But you aren't describing any commercial scope I know of.

So figure you're going to build it yourself.

 

There is a way around it, I suppose: get a commercial ultralight and start modifying it with larger diameter poles or braces between the poles, or guy-wires to stiffen the structure,

and perhaps mods to the cell and trunnions.  That might yield a stiff-enough scope without engineering the entire scope yourself.

I would add more carbon truss rods to stiffen up an ultra light if required.  I have been tempted by the Obsession UC.  Almost bought an 18". but we could not agree on a price.

[Orion64]

There is a possible good reason for an apparently off-center PM appliqué. The optical and mechanical centers of mirrors are not generally/necessarily in the same place. For paraboloidal roundels, the true optical center is (the alignment point) where axial coma becomes identically zero. At work we characterized all incoming parabs in autocollimation interferometry, ran our own macro to back-solve for the optical center... and then dotted them there. Commercial parabs like we guys buy or make --- the that offset is typically within half an inch on slow mirrors and a few mm on the faster ones. For some reason very few people here are aware of that. Probably because it's difficult to locate that elusive point. This also explains why the common Newt alignment tools and procedures give mixed results. So we blame the tools or other stuff... when it may well be that the mechanically-centered feature is simply not where the actual wavefront optical center is.

 

Thankfully, one (should) tip-tilt the Primary Mirror ever so slightly to drive out axial coma in a center-field star. That, as the final gesture/tweak in precision field-alignment. At that point you are done, and what the tools are indicating drops out of the equation.

 

I contrived this graphic to show how that works. The Zernikes morph as evaluation center is moved around. Even some of the optical engineers at work were pretty clueless regarding that unless they were assigned optical testing for a while.    Tom

Thanks, makes sense, thank you

[Orion64]

Orion64

 

 

I put a cooling fan on the side of the mirror box to take off the boundary layer and increased the size of the back fan for better cooling.

Cool down is the only major issue with the Obsession that I have, the 2 inch thick mirror takes a while to stabilize thermally.

Run your fan(s) at all times and give the scope some cool down time before use.  

My 15 inch classic is a 2007 modal and came with an OMI mirror and it works great.

I replaced the secondary mirror in mine (because it was pitted) with an Antares 1/20 wave but did not change the holder

Mine came with the StellerCat and goto but I have never used the system but I hear that it works great

Other than making sure to measure all the poles and making sure they were all the correct length and some cleaning and tune up, I  have not really modified the scope except for the addition of the side fan and getting a larger back fan.

I am very happy with the performance of the scope, as in all endeavors your mileage my vary

 

Robert  

 

edit: I put a variable speed switch on both fans so they could be adjusted as needed and my center spot was etched into the mirror by OMI by a small X 

Thank you for this. Since reading your post, I have done some further research and noticed that cooling can be one of the major things that needs improvement.

How did you mount these fans and which suppliers did you purchase the fans from? Pictures will be appreciated! 

[Orion64]

I had a OMI in my 2004 made 15" Obsession and it could do 1100x on the planets without breaking a sweat. But i also have some super steady seeing.  What brand of mirror do you have?  It should have had a test report with the scope i would guess.

Im pretty sure this is an omi, it also has an x etched into the centre. Here is a test report of the mirror, I posted the report for some opinions in the atm forum and most people concluded that the figures are too good to be true and or accurate. 

Attached Thumbnails

• 

[CHASLX200]

Im pretty sure this is an omi, it also has an x etched into the centre. Here is a test report of the mirror, I posted the report for some opinions in the atm forum and most people concluded that the figures are too good to be true and or accurate. 

I never go by test reports. Just get it out in super steady seeing and jack up the power and see what ya think on Jupiter. It should be more than fine.

[Starman1]

There is a possible good reason for an apparently off-center PM appliqué. The optical and mechanical centers of mirrors are not generally/necessarily in the same place. For paraboloidal roundels, the true optical center is (the alignment point) where axial coma becomes identically zero. At work we characterized all incoming parabs in autocollimation interferometry, ran our own macro to back-solve for the optical center... and then dotted them there. Commercial parabs like we guys buy or make --- the that offset is typically within half an inch on slow mirrors and a few mm on the faster ones. For some reason very few people here are aware of that. Probably because it's difficult to locate that elusive point. This also explains why the common Newt alignment tools and procedures give mixed results. So we blame the tools or other stuff... when it may well be that the mechanically-centered feature is simply not where the actual wavefront optical center is.

 

Thankfully, one (should) tip-tilt the Primary Mirror ever so slightly to drive out axial coma in a center-field star. That, as the final gesture/tweak in precision field-alignment. At that point you are done, and what the tools are indicating drops out of the equation.

 

I contrived this graphic to show how that works. The Zernikes morph as evaluation center is moved around. Even some of the optical engineers at work were pretty clueless regarding that unless they were assigned optical testing for a while.    Tom

Though what you write could be true, especially for home-made mirrors, commercial mirrors are made on tables with automated rotators and the likelihood of an off-center center of the optical axis is very unlikely.

The precision by which center markers are placed, however, is highly likely where the error resides.  It is not that the mirror makers purposely place the center markers off center because the optical centers are off center,

it is that they do a poor job of placing the markers in the centers of the mirrors.

So your "possible good reason" is nearly 100% more likely to be a "probably bad reason".

[Markovich]

Im pretty sure this is an omi, it also has an x etched into the centre. Here is a test report of the mirror, I posted the report for some opinions in the atm forum and most people concluded that the figures are too good to be true and or accurate. 

Yes that's an Torus/OMI serial #. I wouldn't put much stock in numbers tho. Mine came from Torus ( OMI) with stellar numbers but had a terrible TDE.

[CHASLX200]

Yes that's an Torus/OMI serial #. I wouldn't put much stock in numbers tho. Mine came from Torus ( OMI) with stellar numbers but had a terrible TDE.

There were a few bad OMI mirrors that slipped out around 2000 i think. They had tested them wrong and got false info. Far as i know anyone with a bad OMI got it fixed by OMI.  Both of my OMI mirrors were great.

[Orion64]

Has anyone tried placing flocking material on the inside of the cloak to improve contrast a bit? Or rather maybe the flock board? 

[Starman1]

Has anyone tried placing flocking material on the inside of the cloak to improve contrast a bit? Or rather maybe the flock board? 

You mean inside the shroud?

It would be tough to do that, though it might be possible to line a shroud with stretch velvet.

It's easy to line a tube, UTA, or mirror box with darker materials.

[CHASLX200]

Has anyone tried placing flocking material on the inside of the cloak to improve contrast a bit? Or rather maybe the flock board? 

I never have. I just never had a problem getting great views with super well made mirrors. Again i am lucky to have well above normal seeing vs many other places.

[Markovich]

Has anyone tried placing flocking material on the inside of the cloak to improve contrast a bit? Or rather maybe the flock board? 

I have flocked the inside of both the UTA and mirror box. I honestly cant say if I noticed an improvement. The classic design isn't terribly prone to contrast issues.

[Markovich]

There were a few bad OMI mirrors that slipped out around 2000 i think. They had tested them wrong and got false info. Far as i know anyone with a bad OMI got it fixed by OMI.  Both of my OMI mirrors were great.

OMI's attempt to resolve my mirror's TDE was to bevel off the outer .25" of the mirror.  I wasn't happy having paid for a 15" and now had an effective 14.5". Dave stepped in and made it right.

[TOMDEY]

Though what you write could be true, especially for home-made mirrors, commercial mirrors are made on tables with automated rotators and the likelihood of an off-center center of the optical axis is very unlikely.

The precision by which center markers are placed, however, is highly likely where the error resides.  It is not that the mirror makers purposely place the center markers off center because the optical centers are off center,

it is that they do a poor job of placing the markers in the centers of the mirrors.

So your "possible good reason" is nearly 100% more likely to be a "probably bad reason".

Yeah... I'll buy that! In reality, I center dot my purchased mirrors mechanically and accurately. The folded circle of paper technique actually works nicely! But I still do the axial coma thing as a final gesture check. On very slow mirrors that point can be off by a lot --- but it's self-normalizing, because for very slow mirrors it just doesn't matter much. On our (Hubble-class) aerospace PMs we had to characterize and report the offset vector... and it was small... like a mere few thousandths of an inch.

 

My trusty ancient Coulter 29-incher (F/4.5) seemed to be far off, because the tools and axial coma systematically disagreed in the same direction by a noticeable amount. But that was a very thin Jim Jacobsen original that had issues and I stressed the astigmatism out with springs. When it was behaving, it was great... but left to its own relaxed state it was... pretty bad. My Fullum 36-inch F/3.75 is nice and tight with not even a hint of astig. First (good seeing) first light was a profound relief after all the effort to go both Big and Premium!    Tom

[CHASLX200]

I have no clue how to center dot a mirror the right way.

[starman876]

I have no clue how to center dot a mirror the right way.

Follow the directions 

[Starman1]

I have no clue how to center dot a mirror the right way.

Catseye and Farpoint both sell templates to help you do just that.

[skywolf856]

On my 18" Classic I added two boundry layer fans to scrub the mirror surface along with the rear fan.

I added the Glatter sling to the mirror. 

Also added the Astrocrumb heated filter slide to the focuser.

[CHASLX200]

Follow the directions 

I don't have any. I always just guess.. I break off a piece of wood and use it as a guide on both edges. Never was a math guy. Or take a lazy Suzan and spin it with a felt tip pin and when it stops making a circle i got the center.  I am lucky my Galaxy mirror has a center dot already freddie.  Remember the old days when all fast mirrors were bad and mushy with coma filled views?  Every fast scope i ever had in the 70's and 80's were just flat out bad.  This was before the coma correctors , colllimation tools, center dots and good fast mirror makers. Now with the big names like Zambuto and Mike Lockwood they have changed the telescope world for much faster optics than ever dreamed of 30 to 50 years ago.

Edited by CHASLX200, 21 June 2021 - 12:07 PM.

[starman876]

Follow the directions 

https://www.youtube....h?v=xVd-CFUvQ-4

 

here is one video on youtube showing you how to center dot a mirror.  There are plenty more.  There also plenty of sites that provide instructions on how to center dot a mirror

 

just put in the google search

 

how to center dot a telescope mirror

 

it is so simple even a caveman could do it.

[wrvond]

Thank you for this. Since reading your post, I have done some further research and noticed that cooling can be one of the major things that needs improvement.

How did you mount these fans and which suppliers did you purchase the fans from? Pictures will be appreciated! 

I prefer Noctua fans. Here is a 140mm fan that works very well as the primary cooling fan:

https://smile.amazon...,aps,167&sr=8-5

 

Noctua also makes smaller fans that are suitable as boundary fans. 

 

How you mount them is entirely up to you. Most people mount them to the inside of the mirror box with the appropriate sized hole through the wood. A grill or screen like computer cases use works just fine.

Wiring/power for the fans can be a challenge, depending on what you want to achieve. Being able to operate the primary fan and the boundary fans independently is always a good idea.

YouTube videos abound with examples of fan wiring and installation on various classic style dobs. 

[starman876]

I have read the blowing the air across the mirror surface works better than blowing the air from behind the mirror.  I would presume blowing air on both sides would work best.

[Starman1]

I have read the blowing the air across the mirror surface works better than blowing the air from behind the mirror.  I would presume blowing air on both sides would work best.

And you would be right.

Blowing across the mirror disperses the boundary layer, giving better images almost immediately.

Blowing from behind helps cool the mirror.

Blowing on both sides works fast to cool the mirror and get rid of the boundary layer.

My infrared thermometer shows having 3 fans blowing reduces the temperature delta from 20° to 1-2° in less than 30 minutes on my 32mm thick 318mm mirror.

If I had a 32" instead of a 12.5", I might have 4 boundary layer fans and 5 fans behind.

Start the fans running at sunset and start out the dark portion of the night already at equilibrium.  Why waste time with a too-warm mirror?

[George N]

I have read the blowing the air across the mirror surface works better than blowing the air from behind the mirror.  I would presume blowing air on both sides would work best.

That's the theory!

 

With my current Obsession 20 and its 2-inch thick mirror - the single little fan it came with - centered behind the primary - does struggle on nights following being out all day in the sunlight. However -- at home or anywhere with AC power - following DaveK's recommendations - I put a large 24-inch box fan right behind the primary from dusk until observing time. In 'the field' I have a "camping fan" - a small 9-inch box fan that uses either D cells or AC power. I believe the scope could benefit from boundary layer fans.

 

I currently have a NMT 20" with thin Fullum slumped mirror on order. It comes 'standard' with a largish behind the primary mirror fan and three boundary layer fans built into the mirror box. Hopefully it will be a good 'thermal' performer. 

Edited by George N, 27 June 2021 - 10:09 AM.

[bunyon]

I also have a 15" Classic, bought new in 2006. I've lost track of the stuff I've done to it and suspect most owners are the same. So any given 15" Classic is probably fairly different from another, so far as it can be. I definitely had the servocat installed (Argo Navis bought with the scope) and love it. Like any electronics, it's possible for a very small issue to completely inactivate the equipment but, with the servocat, unlike modern gotos, you can disengage the clutch and use it manually. So losing the servocat never kills an evening.

 

I've wired the secondary for dew, I've added a counterweight bar, I've switched out the secondary, changed focusers.

 

The thing I've messed with the most and think I may finally have right is the fan setup. The original fan is okay but I chipped a blade, which made it unusuable while viewing. I removed it and rigged a system to wedge a bigger fan between the two tailgate bars and wedge two smaller fans between the top bar and the box. That worked for awhile - it cooled better and vibration was nil - until heat melted the padded adhesive holding the fans in place. 

 

So, having recently gained access to a laser cutter, I built a tailgate that holds six fans. A center fan and three spaced evenly around it. Two small fans are in the upper corners blowing on a bit of hard paper velcroed to the box to divert air across the mirror. It's recent but seems to work well. I couldn't see any vibration at 380x. I haven't had good enough seeing to go higher than that but I rarely will. 

 

There really is an endless list of potential projects to keep you occupied on cloudy nights.

Attached Thumbnails

•