I’ve recently picked up a new Newt to add to my collection… a 6” (150mm) GSO f/4 model. This is to complement my existing 8” (200mm) f/5 Newt.
(NOTE: Technical details are up front - hopefully some of which may help anyone working with this Newt. Imaging results in post 3 and 4).
I started looking at adding a new OTA to my collection in December, when an elbow injury made it very difficult to safely mount the 8” Newt for a while. I ended up sticking to using my small refractor for a few weeks while I recovered, but that got me wondering whether it was worthwhile adding a new imaging OTA to my collection that slotted in between the nimble 72mm refractor (435mm at f/6) and hefty 200mm reflector (1000mm at f/5). I, of course, immediately started looking at 4” triplets, and mentally justifying the need for one, but then I spotted that a local GSO distributor had a “double discount” on their GSO Newts, and I could pick up a 150mm f/4 unit for an astonishingly cheap AU$350 (US$230). Brand new.
This seemed like it was worth a shot at that price. I’d paid more for my coma corrector. The focuser, tube rings and dovetail included with the unit added up to more than half that price if bought separately. However, I also knew this unit was going to be a bit of a project. I’d spoken to a local reseller about it previously, and despite being a huge GSO fan, he’d openly described the 6” unit as “challenging”. I figured I was up for that challenge, particularly when the cost of getting it wrong and making a total mess of things was minimal. I’d enjoyed tuning my 8” Newt, and was satisfied with the results. Worst case, I’d have a spare dovetail, focuser and finderscope, and learn something along the way.
Figure 1: Product image of the GSO Newt. Note how far the focuser is down the tube on the new model!
So I placed the order and got ready to get to work on it. I read Gianluca Rossi’s article on tuning his GSO Newt, but noted it was an older model, with a shorter tube and the old-style GSO secondary holder. Some of his issues may not apply here. I read bokemon post on the issues with the mirror cell, and started noting down what I’d copy from his ideas.
The unit arrived, and I immediately went open it up, just to loosen off the primary mirror clips. They’re always tight on Newts when shipped, but these were tighter than I thought was possible. Tight enough to prompt me to do a web search for “can overly-tight primary mirror clips damage a mirror?”. Relieved and reassured by the results from that search, I went to reassemble it.
Reassembling it was easier said than done. The primary mirror cell for the 6” Newt is entirely unlike the 8” unit, and has some “interesting” design choices. Unlike the 8” unit, which has collimation bolts, the 6” cell has collimation nuts. To remove the mirror, you loosen off these nuts, then pull out the mirror, which is simply sitting on a thin backing plate, like this:
Figure 2: The mirror backplate. A really simple design.
Once you want to re-insert the mirror “cell”, you push it, mirror and all onto some bolts that protrude from the rear of the tube. Hopefully avoiding scuffing the mirror surface on these bolts in the process. You have a millimetre or two around the edge of the mirror to avoid scraping it. Also, not shown in the image below, there are springs that operate as the “push” of the push-pull alignment mechanism. There is so little clearance for these that they scrape on the side of the mirror as you insert it. During my tuning of this OTA, I’ve had the mirror on and off a dozen times, and those bolts worry me every time. I’ve taken to mentally calling these bolts “the spikes of doom”.
Figure 3: The spikes of doom.
Really, this is a bizarre design choice. Inserting the mirror into this OTA is the closest I’ve ever felt to Indiana Jones avoiding deviously-designed hazards… warily avoiding spikes and springs, ready to trap me and impale me (or the mirror) if I lose concentration for a second. It’s like a game of Operation except the sound of failure isn’t a buzzing, it’s the sound of your mirror coating being scraped away. But so far, I’ve successfully navigated this hazard.
Once the backing plate is on, it’s all fine. Except the backing plate has 7mm holes. Yet the bolts are 5mm. So there’s a huge amount of play and scope for lateral movement:
Figure 4: Not exactly fitting like a glove.
Tightening each collimation bolt ends up moving the centre-spot in an arc as the mirror shifts laterally while the bolts are tightened. Dialling in perfect alignment is an exercise in frustration. So the mirror cell came off again, and I followed the tip from bokemon’s post above of adding sleeve washers to bring the holes down to 5mm. Except due to machining tolerances in the backplate, I could only do this on two of the three holes if I wanted to get the plate back on again (so, for the third, I stuck on a plain nylon washer to equalise heights). This was sufficient to stop lateral movement and make collimation straightforward.
I also inspected the cell for other issues while doing this. I noticed when rotating the mirror in its cell that it was scraping on the backplate. The supports for the mirror clips had some rough edges that meant they made contact with the mirror from the underside, rather than it simply being supported by its cork pads. This small amount of contact probably wouldn’t cause any issues with astigmatism, but I didn’t like the sound of it. So I propped up the mirror slightly by adding a few thin layers of non-stick tape to the cork pads. This resulted in silky smooth rotation of the primary in its cell.
I then took the opportunity to replace the comically weak springs on the spikes of doom with new ones. These needed to be very thin to avoid scraping on the mirror’s sides, but rigid enough to actually provide some “push”. I found some fairly rigid ones, but they still felt a bit squishy when installed, and I doubted their ability to hold the mirror perfectly in place without using locking screws (which I loathe). So I replaced the locking screws with some nylon alternatives I could easily cut down to short stubs, and pushed an extra set of springs onto the ends of these. Not ideal (it would be better to have just three strong springs, each located on the bolts supporting the cell), but it does seem to help collimation stability by adding a bit more “push”.
Figure 5: The abused and modified primary cell with sleeve washers and springs
I also replaced the secondary holder adjustment screws with thumbscrews. I’d rather not be sticking a screwdriver down the front of an OTA, no matter how cheap it is.
In adjusting the secondary, I tightened down the spider vanes as best I could, and noticed the tube starting to deform around the spider vane adjustment nuts. The fact that the spider is so far down from the front end-ring meant that the steel tube didn’t have much support at that location and flexed easily. I added some washers to the spider vane adjustment nuts to help avoid dimpling, but figured I’d need to think of a better solution. This flex didn’t bode well for focuser stability either, but that was a problem for later.
Figure 6: I keep some cheap M4 thumbscrews (purchased in bulk from eBay) for replacing the usual Phillips head screws.
To round things off, I attached a ZWO EAF to the GSO Dual-Speed Crayford, using BuckeyeStargazer’s excellent 3D-printed attachment.
During this process, I’d been assessing collimation stability and doing tests with an artificial star. I wasn’t really happy with the star tests (particularly the astigmatism and turned-down edge that dominated the test), but this Newt was now at least stable and I could collimate it. So it was time for first light.
... to be continued.
Edited by AaronH, 26 March 2023 - 05:36 AM.