Captain Scarlet

... OK I can see where a difficulty lies in trying to apply those formulae. Those formulae include a term, "x", which is the ratio of the minor diamaters of the mirror you actually have, versus that of the mirror which just about and perfectly coincides with your primary mirror.

In fact the "x" is an approximate adjustment, only valid for a small difference between your actual mirror and the minimum theorectical secondary. If your actual secondary is significantly bigger than the minimum, the formulae above are wrong.

The correct formulae to determine L1 and L2 are:

Formulae:

L1 = (A – d/2 –t) . sqrt(2)

L2 = (B – d/2 + t) . sqrt(2)

Where

A = 0.5 . (s + M - sqrt(s^2 + M^2))

B = M - A

The offset itself is (B – A)/2 … multiplied by sqrt(2) if measured along the face of the mirror.

L1 is the “short” distance from the rear end of the holder to the end of the mirror;

L2 is the “long” distance from the front end of the holder to the other end of the mirror;

"M" is the minor diameter of your actual mirror; you either know or measure this

“s” is the distance from primary mirror focal point back along its axis to the optical bounce point off the secondary: i.e. how much of the far end of the cone’s axis is bounced off sideways; you measure this.

“d” is the diameter of the secondary-holder;

“t” is the thickness of the adhesive layer used to glue the holder to the mirror (makes a surprisingly significant difference).

Nice.

Although I haven't fully plumbed them yet, wookie1965's reports are surely a great SGL resource for viewing lists.

M

3 hours ago, Pitch Black Skies said:

... I wonder if you would be kind enough to demonstrate the whole sum from start to finish with the measurements included? ...

I've run out of time today, but yes I'll gladly do that.

BTW whereabouts in Ireland are you? I'm between Baltimore and Skibbereen.

Hi Alan,

Imagine there is no secondary mirror, and imagine a line from the centre of the primary mirror to its focal point. Obviously the length of that line is the focal length of the mirror. Now imagine another line, along the centre-line of the focuser/eyepiece tube. In ideal circumstances, these two lines would intersect. The point of intersection is the point at which the line from the primary-mirror-centre would "bounce" off a perfectly-placed secondary along the eyepiece tube.

"s" is the distance from that "point of bounce" to the focal point of the primary mirror. It's also the (sideways) distance from the centre-line of the OTA to the focal point of the eyepiece when focussed.

You can measure/deduce it in a couple of ways.

- Assume the focal length of the primary is correct as stated by the manufacturer. Call this L.

- Measure the distance, on the outside of the main OTA, from the back of the external OTA tube to the mid-point of the focuser assembly. Call this A. (The mid point of the focuser assembly should be the same distance along the OTA as the mid-point of the actual eyepiece tube).

- Measure or deduce the distance from the back of the external OTA tube to the face of the primary mirror, by measuring the depth of the back of the mirror from the back of the OTA tube, and adjusting for the thickness of the mirror which you should know or be able to look up. Call this depth+thickness B

- the distance "s" is then L - A + B.

Worked example using random but plausible values for a 12" f/4.9:

focal length = 1500mm i.e. L = 1500

dist from focuser centre to back of OTA is 1241mm i.e. A = 1241

depth of REAR of primary mirror from back of OTA is 20mm

thickness of mirror is 36mm i.e. B = 20+36 = 56

Thus "s" = L - A + B = 1500 - 1241 + 56 = 315mm .

This pic might help too...

On 11/09/2021 at 21:26, Stu said:

… It’s weird in some ways, the darker the sky, the brighter the Milky Way, the more tempting it is to leave any kit aside and just lie back and gaze up at it for ages.

totally

1 hour ago, RobertI said:

Lovely report Magnus, a fantastic haul of objects. What would you say is the biggest “wow” when moving from a smaller scope to a larger scope? I’ve never owned anything bigger than an 8” SCT,  but always felt that if I got a 12” or 14” dob, the biggest wow would be on globulars?

Certainly in my experience globulars show the greatest improvement. My first scope was (is) a Mak 180 (i.e. 7") which I had delivered straight to Ireland (I was living then near London). Pretty much the first thing I looked at was M13. I remember thinking it was, truth be told, a little underwhelming, 21.8 skies notwithstanding. A brighter smudge than I could see in London certainly (through a friend's Mak 127), but not much starry resolution. Then @neil phillips SW 300p came up for sale, and on seeing M13 through that, the difference was nothing short of amazing. So globs in my experience respond both to aperture and dark skies.

Where I have been seriously and probably most impressed though has been in the "fuzzies". I recall cruising in wonder through Markarian's Chain, losing count of all the galaxies. But again, the 12" I had on tap was in 21.8 skies. My comparisons are tainted by the fact I changed two things simultaneously: bigger scope and much darker skies. In London where I was, 19.1 skies, I had an 8" newt, but that was probably as big as it made any sense to go at Bortle 8. The faint stuff was simply drowned by LP.

If you're near Colchester it seems your skies are a shade under 21.0? 12" aperture should be well worthwhile I think. Globs will be amazing of course and many galaxies should be able to make it through? @Piero I think observes from a similarly dark location to you, and I get the impression he gets satisfaction from a 16". It would be interesting to hear his thoughts.

Cheers, Magnus

2 hours ago, John said:

Great report Magnus

At the risk of leading you and your 12 inch further astray, NGC 404, the "Mirach's Ghost" is a small galaxy close to it's namesake star and well worth a look when in that part of the sky and also you should look out for NGC  604, a small patch of light near a star right next door to Messier 33 - it's significance is that it is an HII star forming nebula within the Triangulum Galaxy, probably the easiest DSO "in another galaxy" to see

Superb thanks John. The list for my next session, whenever that may be, starts here!

M

4 minutes ago, PeterW said:

I’d be investing some pruning kit to keep the shrubbery at bay and improve you views! What f-ratio is the mirror in the cupboard? I’ve got one under a bed still waiting…. 
some good objects in there!
Peter

Mine's f/3.5. I didn't know you had a project-in-waiting ... a big one?

Almost a month after my wonderful first light for this new set-up, the forecast looked clear enough for long enough 2 nights ago (Friday) to risk setting up at the rear of my place here in Ireland. It’s a triangular grassy/scrubby hedge-bordered clearing with a good southerly view from NE to SSW. The forecast said high light cloud until 1am, and heavy cloud thereafter. In the event, I nipped out at 2135 to see how things were, only to find quite thick cloud everywhere! Cassiopeia was barely discernible, Saturn not at all. Oh well, I could look at Jupiter at least, bright enough at that point to penetrate the cloud cover.

When I went back outside just after 2200, it was all magically and majestically clear, and stayed that way all night. My SQM-L reading was consistently a shade under 21.6, pointing the unit directly at the Milky Way at zenith. That translates into a MW-clear 21.8-21.9. Seeing was not too bad. My views of Saturn and Jupiter demonstrated that, of which more below. During the night I saw perhaps 10 meteors, as I generally seem to during these sessions? Is that to be expected even outside a particular “xxx-eids” spell? And the extra effort to run a cable out etc for the hairdryer came in extremely useful, a lifesaver in fact when dew started to impinge after midnight. At one point, I noticed dew on the primary: a quick blast of the hairdryer down the tube cleared that up, and it stayed clear for the rest of the night. I recommend the following experiment: blow the hairdryer down the tube whilst pointing at, say, Jupiter, and nip back to the eyepiece to watch the image get utterly destroyed. Then keep looking, as it gradually settles and clears to reveal startling clarity.

In fact my night was 2 separate sessions, thanks to @John. I did my formal telescope session, the 300mm OO/Helmerichs on AZ-EQ6/Planet + Nexus DSC from 2200 until perhaps 0145 when I finished, dismantled and packed up all the gear. I came inside, ready for bed, quickly checked out this forum, and found John’s account of his Dartmore dark sky trip. His account, there and then, prompted me to go back out again, tired as I was at 2am, to try to see if, like him, I could make out M33 naked eye. And sure enough I persuaded myself that, with averted vision and after about 30 minutes, I could indeed see it, just-about and fleetingly. Perhaps I was imagining, knowing precisely where it was, but I am generally quite self-critical.

The Nexus DSC controlling the AZ-EQ6 was less glitchy than usual, quite well-behaved in fact. Only once did it randomly decide that what it was pointing at was suddenly 157 degrees away and below the horizon, necessitating a complete restart and re-align.

I’d made a rough short list to start off with, some Pegasus galaxies including Stephan’s Quintet plus a couple of PNs, and some globs. Anyway, on to what I saw.

Alignment – Polaris and Markab. I swapped my 18.2mm into my APM finder and its 12.5mm cross-hair into the main scope (at 146x). Using cross-hairs to centre the alignment stars, my GoTo was notably more accurate than usual, until the Nexus glitch.

Jupiter and Saturn 100-305x – My first target was Jupiter (obviously), initially with the 18.2mm for 100x. Nice enough, but no better than I’d seen before. During the night between other targets I kept returning to both these planets, at increasing levels of magnification. By the time I finished, I was on the Delos 6, yielding 305x, and it paid dividends. The GRS was plainly on display. I’ve never had a hint of it before, so I was (in the dark, on my own) jubilant. Just amazing. And the many bands were rough-edged with detail on view. So lovely, like some of the better images I’ve seen. I must learn what these details are called.

Saturn similarly was by far the best I’ve ever seen. The Cassini Division quite clear, most of the time. Stripes on the planet itself just about visible, I think? It was surrounded by lots of more or less faint dots, and I had to resort to SkySafari to see what they were. They were Titan, Rhea, Tethys and Dione. And one naughty star masquerading as a moon. And every now and then there was an extremely faint dot coming and going that may have been Mimas, it was in the right place.

Stu’s suggestions (Fuchsia) + M22 – I had texted my (formerly local) Astro group that the skies were looking good, so @Stu suggested I take in the Eagle, Trifid and Swan nebulae and the M22 glob. Unfortunately there is a close-by tall Fuchsia hedge just where those objects were at the time he suggested them, marking the Western limit of my vista from where I’d placed the scope. I was just about able to detect M22 through the shrubbery-gaps, but none of the others.

NGC 7331 + companions – NGC 7331 is the brightest of a cluster of 4-5 galaxies in Pegasus. So much brighter in fact that it was the only one I could see. NGC 7331 is said to be the MW’s twin, though bigger differences have been found lately apparently. I had a mental image of what I should be looking at, but I simply couldn’t see the much fainter fuzzies where I was expecting them to the left of NGC 7331. But there were some hazy indistinct averted-vision spots on the right. That’s when I remembered I was looking through a Newt, so I can declare that I saw NGCs 7335, 7337 and 7340.

NGC 7320 / Stephan’s Quintet – I’d been primed to not expect too much with only a 300mm scope, even in lovely dark skies. Stu said “Should be possible with your skies”. So my expectations were modest. In the event, I could just about with averted vision and concentration be aware there were a handful of fuzzies. Definitely not imagination, but certainly not very distinct. So, 12” scope and 21.8 skies are the threshold for me for this object, it seems. Quite pleased to get the tick though.

M2 glob – a very nice glob, one can never tire of them in dark skies with medium aperture.

M71 glob (surprisingly faint) – I thought I’d got it wrong to begin with, so I panned around. What it initially “GoTo’d” seemed to be an “on the dense side” Open Cluster, not a glob. I had to resort to SkySafari to check what the neighbourhood looked like, and it did seem I was in fact in the correct place. It was M71. Looking it up as I write, Wikipedia says “M71 was for many decades thought (until the 1970s) to be a densely packed open cluster and was classified as such by leading astronomers in the field of star cluster research due to its lacking a dense central compression”. Haha so I was in good company thinking it was an OC! I think that means I’m a leading astronomer too doesn’t it 😊?

NGC 6781 Ghost of Moon + Oiii – NGC 6781 is a faint circular PN. I’m not sure why it’s so called, as many PNs are circular. GoTo was slightly off, and I didn’t find it with an initial slewing around. I put my Oiii filter into the eyepiece, and found it pretty quickly whilst slewing around again. Very pretty, slightly smeared on one edge, as per the images I’ve seen.

M13 & Propeller – as soon as my mount had finished slewing, and on first presentation of the eyepiece to the eye, the propeller simply leapt out! It was odd, though. When I concentrated, it would come and go. There it was, plain as day, and suddenly, there it wasn’t! Rinse and repeat.

M27 Dumbell with and without Oiii – With the Oiii filter still in I went for M27, the Dumbell Nebula. With the filter, I couldn’t easily make out its “dumbbell-ness” at all. It was simply a large bright homogeneous oval. Without the filter, its eponymous shape returned.

M57 Ring Nebula with and without Oiii – I gave M57 the same treatment, with the Delos 6mm for 305x, and it was similarly impressive with and without the filter.

Pan 31: Ring, M33, M45, cruising - At this point I was deciding it was time to pack up, so I decided to finish off with the big Nagler 31mm for 59x / 1.6 degrees and cruise around. I was still pointing at M57, which now in a much larger field of stars was simply extraordinary. Such a tight extremely bright ring against a velvet studded background. Really beautiful. I also took in M33, which was a wispy smudge with hints of differences of shading. Even 1.6 degrees wasn’t enough of course for this object. I finished off with M45, The Pleiades: mainly Alcyone and most of “Ally’s Braid” (which I call the B2 Bomber), a shallow V-shaped asterism hanging just “below” Alcyone. Again, lovely pinpoints on a black background.

I brought everything inside, then as described above, came back out with my 15x56 binoculars and naked eyes for a second session.

All in all a highly memorable session, especially having two outings in a single night thanks to John. Bed at 3am.

I have, at the bottom and back of a cupboard, a Nichol 20” mirror awaiting the sale of my house to provide sufficient funds to get my dobsonian-build project under way. This session, especially Stephan’s Quintet, has made me all the more eager to get that dob built.

Thanks for reading, Magnus.

1 hour ago, F15Rules said:

Great report John and I envy you 3 consecutive pristine nights...

… though that must come at a cost? How were the sleep/tiredness levels John?

11 hours ago, jetstream said:

… Near neighbours were out with a bonfire, with others across the lake a kilometer away, chatting, most likely enjoying this great night too. Sound really carries across calm water. What a night.

Many years ago when I was a teenager, some cousins and I were crossing a lake in Finland (I am half Finnish) in a little boat with a small outboard engine, to collect some wild strawberries from a secret glade on a small island full of them. We were shouting at each other to make ourselves heard over the engine noise. Many people around the edges of the lake later reported hearing every word of our conversation quite clearly, but not the engine!

Magnus

I’ve just come in from a similarly pristine and totally unforecast clear night, at the place where I’ve recently moved to, a 21.8 location in SW Ireland. I had my 12” out for what was probably my most memorable session yet. I’ll write it up tomorrow hopefully.

I can relate to your wonder. I too imagined I could just about detect a misty patch around where M33 should be.

I’ve just had a quick look through 10x50 Leica bins and it’s now really quite bright again. Comparing it to its closish companion HD220819 it seems about the same, checked against hd 220102 as well, I’d reckon mag 6.6!

Magnus

I too have added it to my list for when the clouds finally part. 12” / 21.8 / Astronomik Oiii should give me a decent chance…

Cheers, Magnus

I have a Trifibre Max520, and store my eyepieces vertically. The case cost a bit under £60 and the pluckable foam a bit under £30 IIRC. Very pleased with it, it quite happily accommodates my monster Nagler 31, a Paracorr2 and an Ethos 13, all rather long units, alongside plenty of other eyepieces, 3 diagonals, a few adapters and my filters.

There is also a healthy market for used Peli cases on eBay if you’re so inclined.

Cheers, Magnus

Wow fantastic Stu! I must admit an LZOS 130 is on my medium-term shopping list too. Looking forward to reading more observations from it!

Magnus

33 minutes ago, tripleped said:

So don’t bury  in rice?

No don’t bury in rice but place it on top of the rice in an airtight enclosure. The idea is that the dry rice has a very high surface area and will eventually capture all the water from the eyepiece. Same principle as silica gel.

I’m sure it’ll buff out

41 minutes ago, Dave scutt said:

Where abouts are you all in Ireland is any one near roscommon 

Near Baltimore West Cork for me. Lovely dark skies on the other side of the clouds

1 hour ago, Buqibu said:

… So a refractor will always have better contrast even if it is much smaller?

For a given like-for-like aperture, yes, because the central obstruction of a Newtonian reduces contrast, and it’s also probably fair to say that most APOs are better collimated than most newts, and mis-collimation also seriously degrades contrast. But it doesn’t take much of an increase in aperture to overcome that like-for-like contrast difference  

Suiter says that the easiest way to increase contrast, as defined by “MTF”, is to buy a bigger scope!

But contrast is but one of many factors, and likely not the most important one in choosing between scopes.

M

I think your calculation is about right. There’ll be VAT to replace whatever UK VAT you got exempted from, Customs, and often an extra “Customs handling fee”. How you pay depends on the courier. From An Post, you will receive a letter saying “we have this delivery for you, ref no ABC1234567, please log on to our website to pay it snd we’ll release it to you”. UPS I think require recipient’s contact details ahead of time and send you an email to pay online in similar fashion; others IIRC deliver it to you with the driver producing a card reader and you pay at point of delivery.

I’ve just moved to Ireland from UK so am just finding all this out!

And my move unfortunately has all but killed off any likelihood of me being able to take much part in the Classifieds Section 😕.

Magnus

1 hour ago, markse68 said:

https://www.cloudynights.com/topic/55289-can-anyone-tell-me-the-history-of-intes/

This below I hadn’t intuited and is interesting. Mine is an M603, and I had assumed the “15” series signified “deluxe”. Not so…

” Anything with "MK" in the model name is made by Intes (e.g. MK66, MK67, and possibly MK91 although there is plenty of heated debate over that). These are typically f/12 instruments. Intes Micro makes the Alter series of Maks with names like the M503, M603, M615, etc. These are mostly f/10, but anything with a "15" in the model name is f/15 (M615, M715).”

(With thanks to rszabo on CN for the quote)

… and where does “Alter” come in?

The manufacturer's website says the film filters out 99.999% of light, i.e. only 1/100,000th of the light energy makes it through (if it affects all wavelengths equally, which is my understanding for this solar film).

A further layer of film will reduce the light making it through the first film by an equal factor, meaning only 1/10,000,000,000th of the original light will get through.

I.e. you will see nothing through two layers.

Cheers, Magnus

Your report on Jupiter and Saturn uncannily resembles mine from Saturday (second light for my OO/Helmerichs 12”) except that whereas your viewing was “advanced” mine might be better considered “beginner”.

I concentrated on Jupiter and saw more belts and bands than I ever have before, including a mesmerizing Io transit. Your description of the planet and its features though mirrored mine quite closely. Seeing was only so-so but with occasional momentary freezes into amazing clarity. Saturn too briefly revealed the Cassini Division, again a first for me. Though I spent little time at Saturn.

M