bobmoss

4 hours ago, pete_l said:

There is no single reading that defines your sky darkness for all time. I take SQM readings from my site occasionally and can say that it varies a lot. Mostly I get values in the mid-21's 21.4, 21.5, 21.6

However the value changes with the seasons, with the sunspot cycle with the time of night and with many other factors. Also SQM can be very high when the place is completely clouded over! ? While SQM reading also give some idea of transparency - with little in the way of particles / water vapour to reflect back light from sources of pollution, it tells us little about steadiness, which seems to me to be a more valuable metric.
The best ever was at the beginning of January this year when it was -8°. Here's the photo

Interesting, thanks!

5 hours ago, Skipper Billy said:

I am just across the way from Skye !

 

Hello! I'd give you a wave but there are some big mountain's between us!

5 hours ago, Stub Mandrel said:

Skye has its own challenges. Right at the start of getting into this hobby I tried to get some photos with the limited long exposure on my bridge camera. Parked up somewhere on the coat road I didn't do very well, and when I started hearing large animals breathing loudly and moving around on the rocks nearby (I guess they were seals) it did get more than a bit spooky...

I've only been here a year and the darkness still gets me. We live in a small township with not a lot of houses so I can literally walk out of our house at night and not be able to see the car parked 20 ft away! One your eyes get light adapted after a minute you can see but its scary at first. Its also sooooop quiet.....

10 hours ago, Saganite said:

Skye is class 1 Bortle.

Some parts are. Unfortunately I live near-ish to the main town of Portree so get a little bit of light pollution from there but it only knocks it down to a 2.

6 minutes ago, Stub Mandrel said:

On Skye?

I would suggest getting a scope out (a) it's actually going to get dark (b) it isn't cloudy and (c) the sheep are able to remain upright.

?

So true! We got one out of three last night - it got dark...

36 minutes ago, scarp15 said:

You do simply need a SQM - L. I have just checked the actual coordinates on the light pollution map based on a dark sky session last Saturday: 55' 13' SQM 21.73. The reality is that for this session based upon a frequency of readings, periodically taken throughout, the average reading and repeated log concluded decisively at 20.96, the top reading on this particular night was 21.1. This is based on the circumstance for that given night, which was very cold with snow and ice, its probable that a combination of ice crystals, some moisture resulted in good but not excellent transparency. It is not the darkest area I could get to, but this particular location is convenient, being less than an hours drive from home, visited frequently over a number of years when occasionally gaining readings of 21.3.  

A SQM-L will record readings at a 40 degree point of sky you observe in. Logging the variations becomes quite addictive, it is not a chore more so a pleasurable task that is complementary to the outcome of your deep sky observing session. The online map resource as said, is just about OK as a rough guide indicator, but it is on a specific night that counts, we should each of us have a Unihedron SQM-L in our tool box.

Interesting, thanks. I guess you take a reading and then decide if its worth getting a scope out?

1 hour ago, vlaiv said:

You can actually check other people's readings on that website as well - just turn SQM overlay to get recorded measurements and other info.

One can add their own measurements as well.

Just checked, no other people's readings near me unfortunately. Only 2 on the whole of the Skye!

1 hour ago, vlaiv said:

I see Canon 600D listed in your kit in signature, so I'm guessing you take odd picture now and then with that camera and the scope?

If you are, then there is simple way to measure SQM your self (approximately). Take an image of sky near zenith (where you want your SQM value measured) and another one of bright reference star. Use green channel from your image and measure star intensity - photometric measurement - you can use AstroImageJ for this. Then take empty patch of sky and measure average value there (in first image). Make sure you subtracted dark frame from your image (you can do flat calibration as well but you don't need to do it for this rough measurement).

Divide average background value with pixel scale squared (so if you image at 1.5"/pixel  - divide with 1.5*1.5 = 2.25). Divide total star brightness with background value and calculate magnitude difference based on this number

-2.5*Log(star_intensity / background_intensity) = magnitude difference between star and sky brightness. Subtract this value from star magnitude.

This should give you SQM reading.

Excellent, thanks for that. Will give it a go next time I get a clear night.

23 minutes ago, Beulah said:

AWESOME!!! I managed 21.75 in Mid Wales a few nights ago....your 21.9 is nothing short of breathtaking! 

Would love the get hold of a proper SQM to see how accurate the website is. It is pretty dark here though, must get around to trying to properly estimating how dark it actually is.

Pretty happy with my results...  Shame it doesn't take into account the wet weather though.

Great post - some very useful hints and tips in there.

Thanks for all the kind comments everyone.

As soon as we get some clear weather (ha!) and I can get a scope set up I'll post some pictures of the completed setup.

I have thought about doing that as I am looking at getting into imaging. As you say, it could be quote useful to leave it our for a few days, especially if I've spent time getting the polar alignment just right.

Is the idea to carry out the scope and attach it each time you use it? Or are you planing some kind of removable shelter so you can leave the scope set up?

Unfortunately I'm going to have to carry everything out each night.

Nice work Bob,  using the premix concreting service is the way to go, I did similar for the base of my observatory back in the summer there.  I mixed the concrete for the pier foundation by hand and that alone was enough.  I like your idea about the channel to drain the water water from the base.  If that doesn't work out what about laying down some self-levelling compound -  or maybe another thin skin of cement shaped so water  runs off away from the pier.  I had my pier installed today and what a joy to see the telescope mounted - like Christmas for grown up kids.  Good luck with the rest of your build.

Jim

Thanks. I did think about self leveling compound as a solution. I think it should be fine as it is though as its all getting gravelled over so I won't see it anymore!

Nice job & doing the concreting the easy way.  I was really surprised how solid a well mounted pier is after using a tripod, enjoy!

Thanks. It feels really solid and I'm really looking forward to actually getting a scope put on top of it.

Fitting the pier

I ended up leaving the concrete about 6 weeks before trying to bolt the pier down. This was mainly down to weather and time constraints.

Altair Astro supply 4 x M12 300mm long threaded rods with their piers for attaching to concrete (or you can ask them for L bolts for setting directly into wet concrete). I went with the threaded rods as I want to be able to unbolt and remove the pier in the future.

The idea with these is that you drill a hole slightly bigger than the bolts (14mm), put in some chemical anchor resin, push the rods in and then they will set firm. The key to using the chemical resins is preparation. The holes need to be very clean and free from dust. Also, once you open that resin, you have about 15 minutes until it starts gelling and then you can't use it any more.

I placed the pier in position, marked the holes and then drilled the 4 holes using an SDS masonry drill down to a depth of 200mm. Don't try and use a normal hammer drill, they are just not powerful enough. I cleaned the holes out using a bottle brush.

I then cut the rods to about 260mm so that 60mm will be showing out of the top of the hole. I also chamfered the bottoms as I had been recommended to do that to stop them unscrewing once the resin sets (also, the off the shelf chemical anchor bolts you can buy have this chamfering so there must be a good reason for it).

With some help from my wife we were ready to start!. I filled each of the holes about 3/4 full with resin as my wife followed me round and screwed the rods down into the resin. Once the rods are in, clear up any overspill resin as quick as you can.

Next we put a few thin pieces of wood on the ground and offered up the pier to make sure all the rods were lined up - they were! Remove the pier and wait for the resin to set.

The next day, the resin had set solid, those rods are not going anywhere!

I then placed the pier into position and tightened the nuts down. Do them up tight but not too tight! They will shear off before the resin gives way and you won't get much warning.

Here is the pier in place:

After a few days, 2 things became apparent. Firstly, water was pooling around the bottom of the pier and secondly the rods were a bit too proud and I was going to end up catching my foot on them.

So, I removed the pier and got out the angle grinder. First off, I used the metal cutting disk to cut a good amount off the rods, leaving just enough for the bolts. Next, I put the masonry disk on and cut out a big channel to hopefully let the bulk of the water drain away:

I then bolted the pier back down. The last thing I did was to mask off the bottom of the pier and spray the top of the rods with cold galvanising spray. The rods are zinc coated but where they have been cut off they will start to rust. Hopefully the cold galvanising spray will slow down the corrosion.

Concrete Day

The day finally arrived for the concrete. A big yellow truck pulled up outside our house:

The 2 blokes were really helpful. I showed them where the concrete needed to go and told them I wanted a strong mix erring on the wet side. He set the machine for 0.6 cubic metres and set it going. A few minutes later we had some concrete ready. He took a barrow off the truck and told me he and his mate would barrow it round for me if I stayed with the hole and tamped it down as it went in - result!

Here is the hole part full:

Once we used the first lot of concrete up, he came and had a look and made up a small batch just to get to the top of the shuttering. It only took about 10 minutes for them to fill the hole.

Here it is filled to the top:

Once the hole is filled, you need to start tamping the top of it with a large piece of wood to flatten it and get rid of any air bubbles. With hindsight, I could have done with slightly more concrete because as it all settled the middle has gone down about 2mm (which doesn't sound a lot, but it means that water pools in the middle). This was also caused a bit by having a slight shelf around the edges.

I spent the next 3 or 4 hours checking in the concrete. Each time all the water had totally gone from the top I smoothed it over a bit more with a metal float to try and get a nice smooth finish.

You then need to leave the concrete to set hard for a few weeks. I took the shuttering off after about a week:

Groundwork

And so the hard work begins. I marked out a 3ft x 3ft square and decided to go to a depth of about 3ft (to end up with about 0.75 of a cubic metre).

Do NOT under estimate how much work is involved in digging such a hole. It looks easy. I thought I would do it in about an hour. I was wrong... I ended up doing it over a couple of days and probably took about 5 hours to do.

The hardest part is trying to keep the walls from falling in and trying to keep them straight. Oh, and keeping the water out when it rains.

We are fairly lucky as the soil is quite chalky and therefore crumbly. That is until you hit the solid chalk layer.... And don't forget you can't use a pick-axe as there isn't room to swing it.

We bought a roll of 20 heavy duty rubble sacks to put all the waste into as we didn't have any room for it in the garden. This all had to go to the tip - don't try and get it all in your car at once, it's very heavy!

Anyway, this is the hole part way through:

And this is my dog Max helping out:

Once you've got the hole dug and straightened, you need to make some shuttering around the top for the concrete. You need to make sure that this is perfectly flat and level as you will use this as a guide to tamp down and smooth the concrete.

Here is the hole with the shuttering in:

Decisions, decisions...

So, before any work started I needed to make a lot of decisions. Where to put the pier, what pier to get, what to mount it on etc.

What base to go for was the first decision. Obviously I wanted a solid concrete base but what size? Everyone seems to have a different take on what size to go for but I knew I wanted it to be pretty substantial.

I first looked at hiring a mixer and mixing it myself. This looked like a lot of time and effort. I could buy sand/cement/aggregate in 25Kg bags but it surprising how much you actually need! For 1 cubic meter, you need around 13 bags of cement, 24 bags of sand and 48 bags of aggregate! You can buy "bulk" bags of ready mixed aggregate but you have the problem of what to do with the leftovers. Also, I've never mixed concrete before and really didn't want to get it wrong.

I next rang a few "readymix" type concrete places. The first obstacle I came across was that most places wanted to deliver at least 1 cubic metre and they wanted to deliver it all (so if you've got your measurements wrong you either end up with not enough or you have 200Kg of wet concrete spare!). The second problem is that the observing area is round the back of the house so it would need to be barrowed round, and they have a maximum "waiting time" - if you haven't got all the concrete barrowed round in 10 minutes then they will go! They did suggest dumping it all in the middle of plastic sheeting, but I didn't like the idea of that. I got some prices of between £160 and £260 for 1 cubic metre.

I was starting to wonder how the heck I was going to get concrete delivered when I stumbled across something called "Mixamate". This is a company who have big yellow concrete machines. They carry the sand, cement, aggregate and water separately and mix as little or as much as you want right there on site! I gave them a call and they were very friendly. They have a minimum charge of 1 cubic metre (£177) but they can make up less than that if required so there is no excess concrete to get rid of. Also, they have a mini tipper truck you can hire for £20 which is really narrow and you can use that the move the concrete around.

So I decided on using Mixamate as it was the easiest option. Also, seeing as I was going to have to pay for 1 cubic metre whatever happened I thought I may as well aim for a hole around that size.

The next decision was where to put the pier. The plan is to use the "observing area" as a patio area in summer so I couldn't have the pier right in the middle. I therefore put it off to one side but made sure there is enough room all around so that once a scope is on the pier you can still walk around. Oh and don't tell the wife, but I also placed it so you could *just* squeeze a dome into the space it needed in the future

The last decision was which pier to get. This is the one that has caused me the most grief and has taken the longest. I originally wanted the Astro Engineering pier as I think it's a great design and really substantial. However, it seems that AE are no longer in business - the website is still there but you can't buy from them, none of their stockists have any piers and AE don't answer e-mails.

I then started looking for alternatives. The 3 I narrowed it down to were the Altair Astro Pier, the Pulsar Optical Rigel Pier and getting my own design built.

I spent quite a while designing my own pier, doing CAD drawings etc. I sent it off to 5 local engineering firms asking for a quote. One company came back and said they were too busy. One company came back with a quote of £432. The other 3 never even bothered to reply. I decided against building my own as it was a little too expensive (as I would also have to have a top mount machined).

Next up was the Pulsar Rigel Pier. From the small photo on their website it looked quite a good design, very similar to the AE one. I originally e-mailed them asking if they still did the AE piers and they replied saying "Our piers are identical to AE piers and are made by the same engineering company, we just have a different top plate mounting system." which sounded promising. I emailed them back asking if they had any further pictures and any dimensions as I wanted to see how it all fitted together and how sturdy it was. They replied saying "There are no other pictures at the moment. ....They are slightly heavier than Astro Engineering piers, we use heavier steel in some areas.". Unfortunately they didn't seem to want to give me any more details. I want to be able to swap between my EQ6 mount and my LX200 Wedge so I would need 2 mounting plates. I e-mailed them back asking how much it would be for a 2nd mounting plate, I'm still waiting for a reply a month later...

Last up was the Altair Astro pier. I had originally been wary of this design as I'm not a fan of the "rats nest" top. However, I found a few pictures around the internet and it looked to be well designed - it seems they use 4 M16 bolts to hold the top plate so I'm sure that isn't going to move. I fired off an e-mail to them asking about wedge mounting and they replied promptly and answered my questions.

In the end, I decided to go with the Altair Pier as I was already buying some other stuff from them and they seemed to actually be interested in selling me a pier. I've since had a few more conversations with them (both through e-mail and on the telephone) and their customer service has been fantastic.

I've always wanted an observatory but where we live at the moment we don't really have the room. So for a while now I've wanted to put a pier in the garden but have been struggling to find a good spot that doesn't get in the way.

I brought up the subject with my wife a while ago and she said "Why not clear out the vegetable patch and put a pier there?". I'd not dared think about doing that before, or even suggesting it, but if the boss says dig up the vegetable patch, who am I to argue...

So a DIY project was born, and in case it helps anyone else, I'm going to document it the best I can. I'm actually quite far on with it now but just haven't had the time to post any details.

This is how the vegetable patch looked before I started doing any work: