Ships and Stars

The small mirror blanks are expensive, but as you go up they get much cheaper and I don't know anyone who lists prices for blanks that large. The curve generation is sold as an option it appears.

I came across this website after asking around for large mirrors. Definitely agree on the Italian styling on the dob there! Did you see they sell up to 32" mirror blanks with the curve generated!! Price isn't too bad either.

It might be out of date by the next time the UK gets a clear night with no moon 🤣 Seriously, @Neil H I'm sure it's fine. Another handy one for me has been Sky & Telescope's Jumbo edition Pocket Sky Atlas. I have lots of stuff online and on my hard drive, but that one always goes outside with me.

Thanks Gerry! It's been a handful. From Dec 17th until three days ago my wife and newborn daughter were in hospital twice with only a few days at home in between due to complications and a chest infection for the little one right out of the starting gate - that was touch and go for a bit. They are back and resting comfortably and our daughter is alert and I dare say happy most of the time. Hoping by next new moon I'll be able to at least get away once they are asleep to my dark sky site and return before sunrise (I honestly sound like some kind of vampire now 🤣). Just need the weather to hold. We've had four really clear nights here lately, high humidity, but lunar with the 8" has been my only option. Hope to file another DSO observing report soon! I'd like to turn my attention to more galaxies now after reading your last report. And play with binoviewers on the brighter DSOs of course! Sorry for going off piste with the thread GTom! Go for the big scope second-hand!

PS I looked at making a very large mirror, something in the 25"-28" range (I truly have severe aperture fever), and decided there was very little chance I could do a 25-28" mirror within the next two to three years. Or five. I've read a lot about mirror making, but never ground or polished one, nor have I tested one. My plan was to buy a 8" blank to start with and then move up in size, but after reading some other posts, even they are spending loads of time and more money on grinding supplies than a second-hand 8" scope costs. Therefore an 18" or 20" dob with GOTO that's ready to rock under say £3000-£3500 suddenly seems like a bargain. I missed out recently on a 25" dob for only £2400 but it was on Tenerife and couldn't find a reasonable way to get it back. I think if I ever get something larger, I'll most likely have to have something shipped from the US where 24"-32" is relatively common.

It's a lot of money, even used, so don't blame you. There's probably a genuine reason for the sale, maybe the current owner has difficulty moving it around or decided they aren't using it enough to justify the money just sitting there. My problem is time, we have a new family member and two of us (not me) were in hospital for a little bit, so I couldn't even think about getting out to my dark site. I'm hoping as the next new moon approaches in a week or so I'll get another chance. It is a bit of a beast to lug around, but all 20" dobs are except maybe a small handful of ultralight ones costing 10k or more, and they still won't be light!

Hi @GTom, I have the 500p with GOTO, I love it, it's mind-blowing, and to replace it with something better/larger will likely cost 2x to 3x more. I bought mine second-hand but unused, sadly the previous owner fell ill before he could use it. Surprisingly, most of the reviews are critical, but I think people were trying to use really high magnification or we are just hearing from a select few who had a bad copy. Mine is tack sharp, have used it up to 444x so far in the UK under a jetstream with probably 80%+ humidity. The mirror doesn't require any cooling. I can load it in my van in under ten minutes and set up in about 15 min. The base and mirror/rocker assembly are just over 30kg each, about what a big suitcase weighs when going on a long holiday, etc. The truss rods and top end weigh next to nothing. The GOTO system works great when the scope is properly balanced and levelled, and once on target, will track objects without fuss for long periods of time without correction. I rarely use a coma corrector, mainly only with the 21 Ethos, and even then I usually don't bother unless I'm concentrating on star clusters. I can also set mine up easily by myself, I'm 6" and a bit, but no weightlifter. I only had to do four simple things to mine: 1.) Cut small notches in the edges of my mirror cover to remove it more easily when the truss rods are in place. 2.) Use a 12mm socket on a 1/4" ratchet to tighten the base truss rod ends, a lot faster than the spring-loaded handles that can only turn 180deg. 3.) I put some tape on the end of the GOTO drive cable to cover the sharp cable end. 4.) I had to add a little more weight, like 1.5-2kg. That's it. I've been into serious astro for less than a year and have already seen so many things, including the Horse Head. The Veil and M42 are amazing. Viewing the moon without a good ND filter hurts and I wouldn't recommend it, it's that bright. The biggest *complaint* I have with mine is the fact that it has so much light-gathering ability, you can point it anywhere in the sky and the eyepiece will show so many stars that aren't on a sky atlas it's really easy to get lost/overwhelmed if not using GOTO. I couldn't tell which stars were my alignment stars, they were all so bright. The only other complaint is that the GOTO drive cable can be fiddly to mess with in the dark if it pops off when setting up/taking down without a torch. It's not insignificant, you will need a certain level of fitness, but to me it is the grab and go of 20" dobs. I mean a lot of people stick 5m long sea kayaks on their cars or load up for the car boot every weekend, so to me it's no big deal. For the aperture it's quite transportable in my small van. I looked at getting a 350p or 400p for something more portable, and decided the difference in weight and volume when loaded into a car was negligible, the 500p is perhaps even lighter in individual component parts. 👍👍 One thing you won't need to do after all the moving and setting up is completed, is view some faint object thousands or millions of light years away and wonder what it would have looked like if you had bought that bigger scope!

Hello everyone, sitting around dreaming, this time about making a second set of shorter truss rods for binoviewing without a glass path corrector / OCA so I can get the widest field / lowest mag possible with my WO binoviewers. I thought about this some time ago, but didn't chase it up much. I simply can't afford new Binotron 27s with power switch at the moment, or some crazy Sieberts, or I'd just go for those and make life easy. Someday soon perhaps. But in the meantime... My dob has 6 truss rods that are 1775mm long from end to end, including the ball ends (see photo). The ball ends are a simple design, a 25mm ball end sits on a 16mm dia shaft that has a 30mm pedestal which sits atop 30mm tubing. There are flats machined on the 16mm shaft for a 14mm spanner, presumably they screw in but are on really tight, possibly glued. I haven't really tried to unscrew one yet but did briefly pop a spanner on one. They are tight. A few questions if I may: 1 - Are the 25mm ball ends on a 30mm dia pedestal something someone could 3D print out of hard enough material? I'd need a dozen in total and there must be some threads hidden in the end of the tube where it attaches. Alternatively a machine shop might be able to turn some out of alloy, or I could find something online from some industrial application etc and use those. 2 - If my WO binoviewers have a light path of 100mm, then presumably the new truss rods would need to be 1675mm instead of 1775mm to focus without a GPC or barlow in an f4 dob? I should be able to find 30mm alloy tubing fairly easy. 3 - Would I still lose some brightness by not capturing the full light cone from a fast dob? For instance, the Binotron 27 SS has an optional 45mm OCA which captures the 'fast' light cone from a fast dob as explained here. It seems to me the bog standard WO BV nose without a GPC would be in the same position as any 1.25" eyepiece and thus capture the full cone of light... 4 - What detrimental effect(s) would moving the secondary closer to the primary have on reduced aperture, i.e. would the secondary mirror block more light presumably? Would a smaller secondary be the 'perfect' set-up for a truss dob modified for binoviewers? I doubt I'd ever replace mine, just curious what impact there is upon performance. Any thoughts welcome, or if by some chance someone has some Binotron 27s with the powerswitch they want to part with, please drop me a line. Cheers.

Just out of curiosity, I took the second image and inverted the colours in GIMP to show how far you can manipulate images with a few clicks. Interestingly, though of course it is unrealistic in the visual sense, the individual stars and some of the dust clouds seem easier to point out this way.

Good report! I'll have to live vicariously through observation reports for some time, virtually no chance of me getting to a dark site for a while, maybe February. Argh! Just got a pair of new low magnification eyepieces at last for the binoviewer and a proper dew control system as well. Hey ho...

I'm a bit obsessed with Bortle and SQM readings, nearly ordered a SQM-L meter online a few days ago but hesitated after reading a few articles. If you take SQM-L readings, one caveat apparently is that the sensor doesn't deal well with LED light pollution, my understanding is that is doesn't register it well and will give a darker reading that what it actually is. I don't have enough experience to really rely upon visual limiting magnitude with the naked eye, but those really good nights are readily apparently as soon as I step out of the van at my dark sky spots and glance at the Milky Way - you know it's going to be great when everything just pops out. I can flip-flop between trying to view from home (not very good) and 21.5-21.9 sites, the 21.5 site is 20 minutes away but half of the sky is restricted by trees. Still, can be quite a good site, that's where I got my first proper views of the Veil and Dumbbell, etc. my true dark sky site is supposedly around 21.92-21.94, is about an hour and 15 min away, and is remote. On a really dark night from there I can detect faint sky-glow from the central belt of Scotland/Dundee some 60-70 miles away to the south, if I go north, I start picking up skyglow from Aberdeen so it's in a sweet spot. If you have the time I agree with your approach - go for the darkest site when possible instead of trying to do visual from light polluted areas, I find that is often an exercise in frustration. Fewer nights under darker skies seems to work a lot better for me than frequent ho-hum sorts of nights battling light pollution. I've family in hospital so don't really have a choice now, viewing is on hold for the time being, not much I can do about that. Hopefully by mid to end of January I'll have another chance and things will improve health-wise on the home front.

Amazing views, thanks for posting these! What an incredible piece of kit.

That's very good actually, was looking online at them after reading this thread and the technology has come a very long way from a simple 12V charger. My next one will have this!

I currently am the happy owner of a 20" dob, but was looking to move up to something larger someday, simply for more aperture to view the fainter DSOs, mainly nebulae and galaxies. Financially the only real option is making my own mirror in the 24"- 28" range, ready for coating. This morning, I was combing the web and found an older 25" dobsonian for sale at what is initially, a very attractive price. Some of you may have seen the listing. The primary and secondary mirrors alone are potentially worth close to the asking price (to me anyway), with one small caveat - it's in lovely Tenerife - which makes a lot of sense with their world-class viewing conditions and climate. I've placed a few enquiries about shipping, but will be surprised if it's under £1000 to any UK port. That pretty much ends things right there. Just a return ferry trip from Cadiz to Tenerife is £800 if I wanted to collect the scope in my van, not to mention driving to southern Spain (hurts my head thinking about the cost). It would be a proper struggle, even by my dogged, persistent standards, to get my hands on this scope, unless a shipping miracle happens or they are willing to part with just the mirrors and cell, which admittedly is a shame for the rest of the scope and the effort the builder has put into it. This got me to thinking if anyone out there in the UK or EU had a finished mirror or mirror blank 24" on up they might want to part with? - if not, who might sell them in the UK or EU? - do you think the Tenerife 'giant' is better off finding a new home in Tenerife? (Certainly better viewing conditions, let alone clearer skies!) My closest other option - I've priced a finished Nichol Optics 24" mirror and secondary, and whilst Nichol is certainly reasonable and very helpful in correspondence, the 'ready to roll' 25" giant is looking a real bargain if I can sort reasonable shipping. Any ideas? Many thanks.

Ah nice! This is exactly what I want to do with my 200p Newt, currently on an EQ5 - I don't do AP and don't get on with EQ mounts, can't even see Polaris from obstructions around home. The 200p is begging for a dobson mount. Great job with your Orion newt/dob.

I have (or had) a lot of different eyepieces that had different focal lengths. Some needed an extension tube, some didn't want anything, some would only work with a coma corrector because it provided a very narrow range (1.15x) not duplicated by my other extensions. If it doesn't seem to focus, the quick and dirty method I use to determine a rough idea of focal length is to just take out all the adaptors etc until you have a bare focusser tube and hold the eyepiece up to your eye and move your head in and out towards the focusser until you get a hint of pointy stars (my highly technical term). I'm assuming all caps and covers are off, and you are looking at the brighter stars like Capella, not a lorry parked over the road. Even if it's not properly collimated, you should get a decent idea of how far in or out the eyepiece needs to be.

I was going to suggest the Torx method as well. That's the first port of call for me when a hex is stripped out. Can't think of a time it hasn't worked!

I have a truss dob but since your lower OTA is solid it might be relatively close overall. My 500p came with 4.5kg (3x 1.5kg discs). Too little, so I added another 1.5kg which seems about spot on. It changes slightly depending on eyepiece/barlow etc but mostly on how much dew or frost accumulates on the fabric shroud - it gets quite heavy if saturated and adds a lot of weight. One time I left my counterweights at home and used a tie down strap to secure a small tool bag to the underside. That's one of the best alignments I've had! I thought if I tried that again and got it spot on, I could take my small digital luggage scale and see precisely how much counterweight it liked. Someone on here with a SW flex tube (or Orion UK solid tube?) OTA had a clever idea to use a magnetic weight they could slide up and down the OTA to fine tune, that or a velcro strip? Might have been John? A nice solution I thought...

That's what I need for my secondary. I've been going the 12v hairdryer route, works in a pinch but had to stop and clear the mirror four times last observing session. Did affect views briefly, although it puts out very little heat (it would never actually dry hair). A proper set up is much easier.

When a lead acid battery isn't kept fully charged, sulfation occurs on the lead plates. This happens to various degrees depending upon level of discharge and a little sulfation is normal. If left unchecked and the battery is in a discharged state for 'some' time (the period of time can vary widely) it will eventually form a hard crystal sulfate coating over the lead plate system and render the battery unable to take a charge or hold voltage. Disconnecting the battery does not prevent this. I use a 70ah deep cycle leisure battery for my setup and fully charge it after every use, then check it at least monthly if not more. Best option is to leave a small trickle charger on it, but I've a small child who loves to investigate and dismantle everything, so that's not an option for me! It stays locked up until I check it again. Hope that helps. PS a borderline weak to moderately bad battery with higher sulfation can sometimes be almost fully rejuvenated by applying a slow 200mA overcharge of 15-16v (for 12v batteries) for around 24hr which dissolves the sulfate formations inside and brings the internal temp up to 50 or 60C. However this has to be very carefully controlled in a well ventilated area for obvious reasons! 💥

It was unmistakable in the 130pds! Something ridiculous like 14x as well, ha. Just a test though. I kind of wish I'd kept the 55 plossl, it still had excellent views and lunar in particular was exceptional.

Perfect set up for California nebula! Good find on the 41 Panoptic.

That article on exit pupil in excess of 7mm was on the TV myths website - apparently with refractors it's not an issue but indeed with reflectors the secondary can cause a black spot at really low magnification. For laughs I tried the 55mm plossl in a 130pds - no way, just a giant black spot, never had it in the 500p though. I would still have some overall light loss with a 40mm I suppose... I'm keeping the 21E forever unless I become completely destitute however! It's an absolute winner.

True, you can't have it all. However, I had a 55mm TV plossl and it had something like a 13.5mm exit pupil at f4. Still had bright, razor sharp views. That was going to be paired up with a night vision tube, but would have been too bulky. 10mm exit pupil wouldn't be quite so bad but suppose the 30mm makes much more sense. There was an Ed Nagler article on the TV website discussing exit pupil, if I remember correctly he didn't seem too fussed if it was over 7mm. I'll dig that up.

Oooh, that 40mm sounds wonderful! (My non-technical contribution to this thread 😉 ! I've always been a Pentax camera and especially lens fan, never had the pleasure of trying out their EPs - soon I hope).