Transforming the 28-inch f/4 into a 30-inch f/2.7

The photo above - taken by Joe LaCour - is my recently completed 30-inch f/2.7 at the 2022 Alt-Az Workshop in Portland Oregon. I had already been observing with it for four months (June through September 2022) by this point. If you have 53 minutes you don't know what to do with, you can see the presentation I gave about this scope at the workshop on YouTube below - my remarks begin at 4:15 into the video. If not, all the same points and photos are on this page.

Why transform a perfectly good 28-inch f/4 into this 30-inch f/2.7? Not for the extra aperture - there's little difference in the brightness and resolution between the two scopes. The answer is weight. The 30-inch, 5/8-inch thick meniscus mirror weighs 38 pounds compared to the 92 pounds of the 28-inch, 2-inch thick mirror, a difference of 54 pounds. That translates to making the 30-inch mirror much easier to lift out of the mirror box to re-silver every year or so, effectively extending the window of time I have to do this by many years. Having a bad back, and being 67 years old (as of July 2022) this is a huge benefit.

I could still lift the 28-inch mirror, but it's right at the limit of what I can do safely. As fine a mirror as it is - thank you Steve Kennedy - it was time to let it go. I sold it, along with its 5-inch diagonal and holder, and the 28-inch cage in November 2022.

Not that I planned any of this! I was observing with Mel Bartels at his place in October of 2021, which was soon after he had completed making two 30-inch f/2.7 meniscus mirrors. He had nearly finished the scope for one mirror, a beautifully made piece of telescopic art, and we were talking about this when a light bulb turned on over my head. I asked him how much each mirror weighed - not for the first time I'm sure - but now when he said "38 pounds" I did the mental calculation on how much lighter that is than my 28-inch mirror.

So I asked if he had any plans for his "extra" 30-inch mirror, and when he said he would sell it to someone who could give it a good home, I told him I could but I needed to talk to Judy (my loving spouse). Which I did as soon as I returned home. She was enthusiastic about this idea because she had worried about my back every time I lifted the 28-inch out of the mirror box. Mel and I soon agreed on a price and I picked up the mirror in December 2021 when I went back to his place to assist with silvering his mirror. Woohoo!

In the meantime, I'd been working on a SketchUp design for my 30-inch, which is a re-worked version of the 28-inch structure.

The ground ring, rocker and mirror box (not to mention the drive) didn't need any modifications - the 30-inch mirror fit inside the mirror box with 5/8-inch to spare - so that gave me a great head start. Amazingly, the existing 18 point mirror cell needed surprisingly small modifications. The position of the 18 support points matched Mel's PLOP calculated points to with a tenth of an inch, but the support triangles needed more room to conform the highly curved back of the mirror. The central support beam of the cell needed to be cut out for the same reason.

I used magnetic spheres to replace the standard nuts and bolts triangle connectors, and this has worked out wonderfully. Because the triangles are aluminum, I first had to attch steel washers to each triangle to the magnetic spheres would stick. The arms the triangles are attached to are already made of steel, and once connected, each triangle not only had plenty of room to pivot and conform to the back of mirror, but the magnetic spheres held them with more than enough force to keep them in place. Win - win!

I also needed to make a sling. Mel found this was one of two good ways to support the edge of the mirror, and given the other option was somehow holding it from the back center of the mirror, I thought this would be a good way to go. Plus, this is what Mel went with, and I knew it worked. I added a bracket to each side of the mirror cell to hold the sling - a 1/2-inch wide, 1/16-inch thick aluminum strap. I added a self adhesive Teflon tape to the inside of the sling to reduce friction, in hopes to reduce or even eliminate any astigmatism from the mirror getting stuck on the sling - it's the light brown material seen in the photo on the left below. So far so good.

The sling brackets are attached to the mirror cell so collimation adjustments don't induce any flexure into the sling - the cell, mirror and sling all move together.

I made a new front plate for the Complete Ventilation System (CVS) and now all I needed to do was make the new cage and shorten the truss poles. As it turned it, the cage became a more involved project than I imagined up front, but it turned out pretty well in the end.

I used 3mm thick Dibond for the cage, which is an aluminum/poly carbonate composite material that is used as cladding for building and signage. If I had to do it over, I'd use the 6mm version, called Alucobond, or better yet, 3mm aluminum and weld the cage together. But the 3mm Dibond is working just fine, but I needed to use epoxy and add a number of internal supports that are bonded and screwed together. Not surprisingly, the cage needs to be perfectly stiff to hold the 7-inch minor axis diagonal mirror in place so it will hold collimation. The bare cage, as shown the photo below, weighs only about 4 pounds. It's much heavier with everything attached to it, but it is stiff!

The design of the cage is based on Mel's 30-inch. I'm not as inventive as Mel, so why not copy the best?

Note that the Aurora Precision filter ring - partially seen below the focuser - is accessible through the circular cutout just to its left. The shroud covers the filter ring so this is the only way access the filter ring while observing, and works quite well. Sometimes a great idea does work out as expected!

The very first thing I purchased after Mel agreed to sell his spare mirror to me, was the 7-inch minor axis diagonal from Ostahowski Optics. My timing was excellent because they had one ready to go, needing only to be coated. It arrived a week after ordering it. The second thing I ordered was a diagonal holder from AstroSystems. Unfortunately, this took months to arrive and it set back my completion date by about 5 weeks later than it could have been.

Oh my, was it ever stout! It was way too heavy, and the first I did was drill dozens of holes in the both the back plate and the adjustment plate to reduce the weight. I was able to reduce the weight by 2.5 pounds, and covered the holes with Protostar self adhesive Flockboard to prevent the holes from becoming traps for moisture, bugs and grime. The diagonal mirror is one-inch thick, and is held to its back plate with three blobs of Dowsil 732 RTV. There are three metal clips that will catch the diagonal if the RTV ever fails.

This is the first time I've made a wire spider, and I have Mel to thank for this as well. I was really impressed with the wire spider in his 30-inch scope the first time I saw it - even without the wires being fully taught, the spider held the diagonal perfectly rigidly. So I copied this too. You can see how the wires are attached to the hub I made for the diagonal holder - plus the wires for the AstroSystems dew heather - in the photo above, and it all looks so simple.

Installing a wire spider takes a bit more than it seems though. First you have to make a jig to hold the diagonal assembly in place, collimate the optics, and then install the wires.

I used a half-inch thick board I already had, made a bracket to hold the diagonal assembly and cut two large holes in the main board so I had access to the attachment points for the wires on the diagonal assembly. That all took way more time than I anticipated, but then this was the first time I've done it. Attaching the wires was straightforward, but collimation was lost when I removed the jig, and I needed to adjust the tension on all the wires to regain it. Good experience though.

The tension bolts for the outer ends of each wire are seen in the far right photo, and serve as the only collimation adjustments for the diagonal mirror.

For the first month after the scope was ready for action, I was certain that I needed to remake the cage because collimation would shift all over the place while observing. Then I thought to double check the nut that held the diagonal stalk to the hub - and it was loose. Tightening that nut solved the wandering collimation issue.

Silvering the 30-inch mirror went well, although I ended up with areas of slight haze, which I think are a result of not completely rinsing off the calcium carbonate used for cleaning the glass. No worries though, using Rob Browns Poor Mans Reflectivity Test (PMRT) I measured the coating at 99.5% reflective in the red, 97.3% in the green and 93.7% in the blue - not bad.

By the way, the black light baffle around the inside of the cage that protrudes back about 14-inches is also Protostar Flockboard, as is the front end light baffle. The 80mm finder and the Quinsight reflex finder complete the scope, and both are holdovers from the 28-inch along with the Feathertouch focuser.

I had a good idea how the scope would work in advance - I had the 28-inch version for 18 years, and I had one evening of sharing views through Mel's 30-inch, so putting those two experiences together prepared me for what my 30-inch would do. If anything, it's even better than I expected

The photo above was taken at Mel's place near Sisters Oregon, and his 30-inch is set up to the right, out of the picture. While we observed, I kept catching myself wanting to say "Mel, come look at this!" but then he has essentially the same telescope...

The thermal management of the 30-inch mirror is significantly more important than it is for a traditional 2-inch thick mirror. It's surprisingly easy to over or under-correct the apparent figure of the mirror by turning on the CVS fans at the wrong time. The rule of thumb I gradually figured out is that if the mirror appears over-corrected, turn on the CVS fans. If it appears under-corrected, turn off the fans. I'm still learning how to best use the CVS fans to keep the mirror's figure in the sweet zone, but then Mel doesn't have a CVS on his scope and does just fine with only a small fan on the back end of his scope, so there's more than one way to skin this cat.

The photo on the left above shows the insulating wrap I added to the edge of the mirror. Ed Allen, the inventor of the CVS has shown this keeps the edge from over correcting as the airflow from the back comes around on its way to the front surface. It works! I used synthetic leather held on with a few small pieces of gaffers tape.

The gap between the bottom edge of the front air baffle of the CVS and the mirror - photo on the right - is 5mm.

My last dark sky observing trip of the 2022 summer was at Logan Valley, in eastern Oregon. I met Chuck and Judy Dethloff there - they're standing in the mid-distance, just right of covered scope in the photo - and we had two excellent nights of observing. This is where the better than expected came up. The views of M33 and M42/43 were just staggeringly beautiful, significantly more so than I remember with the 28-inch. The difference is the combination of the wider field of view I get with each of my eyepieces, and the astounding quality of the 30-inch f/2.73 silver coated mirror. A sweet spot was the 17mm Ethos eyepiece and Paracorr 2 at 140x with both M33 and M42/43, but that's not to say that higher power views weren't also exquisite.

This eyepiece sketch of Stephan's Quintet was made in August 2022 from Chickahominy Reservoir in southeastern Oregon. Stephan's Quintet was near the zenith, the SQM was 21.75, and even though the seeing was mush, this view at 396x was too good not to sketch. I feel like I'm just scratching the surface of what this scope can do though, and I'm excited about its potential.

If there are silver linings to growing older with a bad back, this is one of them!

So, what's next? I plan to make a shroud that fits the scope, rather than continuing using the shroud from the 28-inch and rolling up the bottom two feet. I'll use a material that's completely opaque to light. The current shroud is lycra and not only stretches out over time, it lets in more light than I prefer.

But then, all I really need are more clear, dark skies.