Captain Scarlet

This nice cap arrived in the post today. There’s no doubt it’s a very nice cap, and I paid 12 whole pounds for it, but I was under the distinct impression that it generally came with a free 4” Televue refractor. Imagine my disappointment...

I mostly live in SW London, not far from Hampton Court. I got interested in astronomy around 5 years ago, and since then I’ve been obsessed with trying to be able to see, naked eye, all 7 stars in the “Little Bear” asterism In Ursa Minor. Try as I might, I have only ever been able to see 3: Polaris, Kochab and Pherkad. Not a sausage of the others and I’ve tried really hard and often.

Exactly one year ago when lockdown was imminent, we scooted off to my wife’s place in SW Ireland, to WFH from there, where we could be in closer proximity to her elderly mother. We only ever intended to be there a small number of weeks, into months, but postponed and postponed our return until we really had to come back. Not least to get our leaking roof fixed.

We got back this weekend. Last night was clear. Sitting in my back garden, I tried once again to see the Ursa Minor main 7 naked eye, AND I COULD!!! I couldn’t believe it. I also got my SQM-L out, and recorded 19.20 even without @PeterW’s proprietary stray-light shield. This is “darker” than I’ve ever measured here before. Plus, next door has installed an extra security light in the intervening year.

I’m not sure how or why, but this last 1 year has seen a literally-measured reduction in light pollution both from NELM and SQM-L.

Pleased and surprised,

Magnus

I found a thread on CN about this method. Some big names in the industry recommend the fingertip method...

https://www.cloudynights.com/topic/331653-how-to-clean-your-telescope-mirrors-cheap-and-easy

i too am at the point where I need to give my OO 1/10th 12” its first ever clean

Magnus

32 minutes ago, Gfamily said:

*Pro tip: phone ahead ... and if you can get there before 8pm, you can pick up your tea en-route. 

Is one not allowed to drink tea after 8pm? 😉

Ideally you need a tube that 1. allows the peephole/your eye to be placed at the primary mirror's focal point, and from that viewing position, 2. the open end as viewed from the peephole needs to appear close enough to the edge of the secondary so you can accurately judge its "concentric-ness". That would suggest "long-ish" to me.

(It's only from the focal point that a properly-set secondary appears perfectly circular.)

But also it actually doesn't matter too much if the secondary is not in the perfect position, which is what the circularity-test determines. What's much more important is that the central axes of the eyepiece/focuser and primary mirror coincide: i.e. the tilt of the secondary rather than its position in space. IE that which you might use a Cheshire to do.

Cheers, Magnus

Depending on where I’m staying I’m either already at a lovely dark site or hours and hours away from one. The contrast between the two is fascinating.

In West Cork, where I keep my 12”,  I’m at a nominally 21.8 site though I did once measure 22.0. When weather permits M31 and even M13 are direct naked eye, as are easily the Beehive and Coma Berenices clusters.

Near London, it’s like orange soup. Only Polaris Kochab and Pherkad are naked eye visible in Ursa Minor, I’ve never once had a glimpse of any of the others and I’ve really tried.

Travelling from one place to the other, on the odd occasion it’s been clear and moonless  as I’ve got out of the car at the Irish end, the sight has made me gasp out loud with wonder.

i’m extremely fortunate that my wife hails from one of the few last near-pristine accessible dark places in Europe.

Magnus

Also consider getting a 45 degree correct-image diagonal/prism, so you can use your scope for terrestrial viewing too. I use one a on my medium-sized scopes for fun during the daytime. The one I have is this: WO 45deg diagonal

Cheers, Magnus

£500 price limit screams Berlebach Uni tripod to me...

7 minutes ago, andrew s said:

Not me, unless I was in some way out of my mind.

Regards Andrew 

Ah yes looking back you were a devil’s-advocate contributor to the “show us your berlebach” thread back In April, that’s what stuck in my mind. Apologies for having maligned you

41 minutes ago, andrew s said:

I am a fatalist and work on the principle that if you can do something about it do it, if not don't worry about it. As I said the insurance paid up and I had fun choosing new kit from scratch. Indeed getting it all at once resulted in a better setup. 

Regards Andrew 

I vaguely recall you going on a Berlebach frenzy ... was that the cause?

The first two books I opened, Suiter and R N Wilson, each state that spikes get thinner longer and fainter as vanes get thinner., and vice versa.

as for bahtinov, making the slots wider reduces the number of edges so the two effects will tend to counteract.

Ill have to check on a star as it drifts across...

32 minutes ago, andrew s said:

A dehumidifier caught fire. It was the only thing on as I was about to go on holiday. I had to damp the fire down before rushing to the airport.

Regards Andrew 

You must’ve been delightful company during the holiday

On 07/03/2021 at 16:57, vlaiv said:

 

- thin spider vanes throw same intensity spikes as thick spider vanes. Intensity of spikes depends on length of diffraction edge - rather than thickness of light blockage.

I thought the same until recently, but there seems to be a consensus in the literature that thicker spider vanes do in fact produce shorter brighter stubbier spikes, although same total energy. And thinner spikes stretch out the spikes longer but fainter.

Qualitatively that makes sense to me, as it “allows” the spikes to disappear completely  as vane thickness becomes infinitely thin.

7 minutes ago, andrew s said:

I have mentioned thus before but my observatory burnt to the ground taking a 12" Newtonian, Tak Sky 90, Parallax mount plus lot more with it.

Fortunately,  the insurance paid out.

Regards Andrew 

Oh dear. Did the 12” mirror survive?

My worst so far was similar to one above, I placed my az-eq6 onto its tripod and forgot to clamp it down. Trying to attach the scope I pushed it off. Luckily mostly onto grass but the saddle bolts got bent.

5 minutes ago, wookie1965 said:

I have to buy the wife something for her 60th on the 8th of April I wonder if she would be happy with a sky quality meter? 😁

Your Wife's 60th in just less than a month? OMG. You are at a critical juncture. You must think and act NOW. Your rising sense of panic between now and then will mean that the amount you'll have to spend will double for every halving of the time left.

Good Luck, Magnus

My optical test bench in action ... Intes M603 shown here, having previously done the Skymsx 180 (with the target a good bit further up the lane!)

Amazing.

As it got close ot its zenith at 10km, i.e. stationary-ish, it looked very like the Gerry Anderson Thunderbirds shots from the 1970s with gases streaming slowly off sideways!

This is a great challenge and something I’d been thinking of doing anyway for a while...

The secondary mirror and corrector plate are coated in SiO2, i.e. quartz. It’s pretty hard stuff! After removing the secondary baffle I used cotton swabs paintbrush and acetone to remove glue residues, even (especially) superglue disappears with that. Once the glue has finally dissolved, I treated it as if it was any other lens surface and cleaned again with acetone-cotton swabs and pure water.

however if you’re not going to remove the secondary baffle, then I shouldn’t worry about cleaning the secondary. I only did it because my baffle was seriously off centre so I pulled it off and had no choice but to clean up.

The focus mechanism consists of a threaded Rod rigidly connected to a bulkhead which is bonded to the the back of the primary mirror, both of which are rigidly connected to the outer of two baffle-tubes. The knob turns a threaded nut also on that shaft but fixed to the rear of the ota. As you rotate the knob, the threaded Rod travels in or out through the nut at the knob, dragging the primary mirror arrangement up and down with it. It’s crude but it should work well.

it may be that the attachment of the threaded rod at the bulkhead inside has worked loose, which would explain the behaviour. Unfortunately to fix or check that you will have to do some dismantling.

if you do decide to dismantle, DO NOT DO SO by first undoing the 3 sets of collimating screws at the back. Those screws actually hold the mirror/baffle in place inside the tube, as well as collimate, and detaching them will set the mirror free to drop on to the corrector plate and/or dangle off the threaded Rod.

seerch for “reverse engineering the skymax 180” and you’ll find my thread about dismantling the 180. The way your tube attached to the ota will be different from mine, but the basics of how the focuser works and how the mirror assembly is attached will be similar.

I believe on a 127 the main tube actually screws directly on to the rear lump via a thread.

cheers Magnus

41 minutes ago, vlaiv said:

...

I'll need good laser range finder so I can accurately measure angular sizes. 

I take it you don’t have a mount with encoders? If so you could use the encoder output as a theodolite to measure small arcs? I do that with my ayo2 and dsc.

Or perhaps plate-solve the exact FL of one of your scopes and subsequently use the distance across the sensor between a pair of terrestrial features to derive angle from that?

On 21/02/2021 at 17:18, alex_stars said:

Kind of don't want to let this thread die ...

Have you seen Pulp Fiction? ... "Thread's dead, baby, Thread's dead..."

... actually i hope it isn't. I've been playing with MTFmapper and it is very good. I have a printout of one of the A3 test-chart versions and I'm awaiting the 80kph winds to die down before I can find a sufficiently convenient 50-odd metres target distance for my Skymax 180.

I split it with my 12" last night. It was a little breezy and moving around from time to time. It was definitely there at 304x but to be absolutely sure I ramped up to 522x.

Cheers, Magnus

I set up the 12” OO-mirror SW-blue-tube newt this afternoon in spite of the bright Moon, as I haven’t used it for ages. A bit of a waste of big aperture but I’m a rebel. I’d earlier changed a setting on my Nexus DSC to display alt-az coordinates instead of equatorial and in the field trying to control my ax-eq6 it didn’t like it one bit. At the first opportunity after alignment it wanted to flip azimuth 180 degrees. I ended up doing alignment about 10 times before I realized what might be the problem. I flipped the setting back to equatorial display and all was good again. Must report it to Serge.

Tegmine, Porrima, Decapoda all nicely viewed and split. Beta monosc triple also seen for first time but behind a tree so not perfect.

Moon was able to take 520x magnification and still be occasionally sharp and fascinating.

So a somewhat frustrating session spending more time aligning than viewing but lessons learned and better than raining!

Cheers, Magnus

... just had brainwave (highly abberrated wavefront though) ... it's a parallel-sided pole, I'll measure its circumference tomorrow...