My Beginner's Experience of Setting Up a 300PDS 12 Inch Newtonian Reflector

[JSeaman]

About 6 months ago I saw an image of M101 taken with a 300PDS and it was fantastic. Since then I've had my heart set on one and spent a few months looking for one to come up on the second hand market. I read a lot about them and their limitations but eventually managed to get one for £300 so I was very excited about getting started.

Size

I knew it was big so it wasn't a surprise per se when I collected it. I had a 2M observatory which I had spent a long time putting together over the last year or so and I planned to slot it in and start using it immediately. Imagine my disappointment when it hit the roof and walls of the observatory simultaneously.

So yes, it's huge and didn't fit. I tried cutting my pier down and re-welding it as well as various positions of the scope where it would have been dramatically out of balance just so I could squeeze it in. I had one fairly nasty bump and decided enough was enough so knocked the observatory down. I replaced it with a temporary gazebo so I could get started.

You need something in the order of 2 metres internally by my reckoning to have a fighting chance of using one in a dome, mine was 2m externally. 

As well as the length/width and angles you have the weight to contend with. Ile for a fixed installation. Those among us that like a challenge can no doubt take them out on a mobile set up/tripod but I wouldn't fancy this myself so a fixed pier is certainly a good idea.

Parking

With the scope in the normal 'Home' position pointing up at Polaris it is very tall indeed. I thought the >2M high gazebo would be adequate but no! To deal with this I have now set the telescope's home position to be a custom defined one, this is very easy to do in EQMod and is a good idea too. It allows you to easily look inside the scope and check everything, it also means if you do drop anything in the tube it's less likely to damage your primary so I recommend this.

Mount

I knew it was going to end up being a problem attaching all my equipment to my mount but I was very keen to make it work. I built up a mini rig for my ED80 (previous imaging scope and superb for astrophotography work) as a guide camera. This time around I attached my power supplies, USB hub, scope, focuser, dew heaters etc. all to the guide scope so I didn't have lots of trailing bits hanging between the mount and scope.

I made up my own mount for the guide scope rig to attach to the 300PDS and bolted that in place.

My mount is an NEQ6 Pro and this is the minimum recommended to carry the weight of the 300PDS. Whilst an NEQ8 would be lovely, it's also about £4k so has a limited audience. I managed to get the scopes attached to the mount and set about balancing it. My experience was that I need tens of kg of counterweights to have any chance of balancing it out. I used standard weights I had kicking around and this worked well.

I managed to balance it fairly nicely in RA, I can move to any position and it will hold there well. That said, when I reach the extremities in either direction it will pull slightly. Balancing in Dec was no problem but the mount creaks and groans when I move it, obviously on the ragged edge of what it can do.

 

Guiding Test

OK so we have a ridiculously large telescope mounted and relatively well balanced with all the equipment attached, time to move on to making it move. My first night consisted of trying to find focus and see if it can move. I left my Lakeside focuser and Moonlite on the ED80 so I managed to focus with that on the moon to start with (a nice helpful bright object when you're first finding focus I find). 

Once the guide scope was focused I moved onto the 300PDS, this only has the stock Skywatcher focuser but I managed to focus pretty well. I have an Atik 314L and EFW2 in the chain and the focuser was almost all the way in to get round stars. 

At this point I had a quick go at guiding and it was obviously considerably worse than my previous set up with the ED80 imaging and an ST80 guiding. That said, the mount was moving and I took some test shots to show guiding was working fundamentally.

Collimation

After proving the concept of movement I tackled the subject of collimation. It is a word I had heard before and I was loosely aware of what it was but I hadn't really paid much attention because I was using a refractor (lenses not mirrors) previously.

I immediately bought a laser collimator then discovered a lot of people don't recommend them! So first my experience with the laser - you simply put the collimator in the focus tube and look down the end of the scope. There is a circle in the middle of the big mirror on the 300PDS and you tweak allen key screws in the secondary to align this. I found this process very easy indeed.

Once you've done this you move to the back of the scope and twiddle knobs on the primary mirror to get the laser in the centre of the bullseye which is sat in the eyepiece holder. 

So what's the issue? Well, if you touch the laser collimator or rotate it you can make the laser dot wander. A lot of people have issues with the collimator not being collimated! So basically, put it in a V block (or similar) and rotate it, see if the dot points in the same place. I checked this and it was perfect. The other problem is the focuser isn't amazing at holding things in the same place - 2 grub screws mean you can actually angle the collimator in a number of ways. I tried to snug it up flush against the draw tube and just gently tighten them so it was held but not moved which seemed good enough. I did also find that the collimator (which was a 1.25 inch) had movement in its 2 inch adaptor so I tried to do the same alignment with that.

Ultimately I found the laser collimation very easy but I would recommend you look at collimation caps and Cheshire eye pieces before investing in one as you may have a different experience.

Once I had a clear night I tried defocusing on a star to see if I got a black dot (the secondary mirror) in the centre of the star to check my collimation. If it is not concentric (off to the side) then collimation is bad. This seemed good to me but I'm not imaging yet so time will tell.

Mirrors

During the collimation process I was getting more and more concerned about the mirrors, they were really quite dirty. I read a lot again and found people that clean their mirrors all the time and others that suggest you shouldn't touch them until they're thick with dust. I decided they were not clean enough to leave so wanted to address this.

Again, you will find a lot of methods if you read around but I wanted the easiest and least likely to damage anything and settled on this. Buy some deionised water (DI) - it's literally a couple of quid and you can get it from any car spares shop or online. DI water will leave far less in the way of deposits or streaks than standard tap water, it's well worth getting. Also I recommend a sprayer which is also about a pound, it means you can fire a jet of DI at the mirror which won't have the power to damage anything but will move the dirt.

I did 2 steps, one was to bring the main mirror inside from the cold (just a few posi screws around the outside of the tube, don't drop it as it is heavy!), and then lots of nice condensation formed on it. I dragged some cotton wool pads over the water and pulled off loads of dirt. Note that you don't "wipe" it, you're just dragging across the top to avoid scratching. 

This removed a lot of dirt and then I sprayed a good coating of DI water on the primary mirror and let it dry naturally with the mirror stood upright (well leant at a large angle). This cleaned up very well indeed.

For the secondary I didn't want to remove the mirror as I knew I'd be messing with the spider and alignment angles which I wasn't ready for yet. Instead I just sprayed my magic DI through the focuser eye hole and dragged the cotton wool over it from the front of the tube. I then sprayed again with the DI and it dried perfectly clear.

So no fairy liquid or elaborate cleaning routines but a very effective way of removing the dirt.

Slewing

With my set up, the challenge is the very first time you use it, finding a star to set up the go to. Basically I tell it to slew to the brightest thing I could find and then see if it is in the field of view (FOV). I slewed, the FOV was blank. Darn it! I tried moving about with EQMod to see if I could find the star nearby - nothing. With hindsight I should have probably used eye pieces now but instead I just released the clutches and started hunting. Moving the massive scope around by hand really let me know how hard the mount was working but I eventually found my bright star and locked the clutches off.

Previously, once my clutches were locked I could lean heavily on the scope with no movement at all. With all the weight it is carrying now, the mount can easily be moved in RA or in Dec with a knock. I guess there is only so much force the clutches can deal with - I now have to be very careful moving around the scope to not bang it or I have to go through the slewing process all over again.

I did discover a problem with my set up at this point - if I meridian flip it doesn't go back to the target. That is an issue for another day though

Other than finding the first star, the NEQ6 Pro slews around with the 300PDS just like it did with the ED80, smooth and with fine control.

Guiding and Dew

I spent about 2 hours taking some bias and darks and a new dark library for PHD2 on the ED80 as a guide scope. I popped the covers off and made the most of a small gap in the clouds to try things out, I managed to use my Bahtinov mask on the 300PDS to get focus and the same on the ED80 guide scope.

I started guiding and it was pretty poor to be honest. My best guiding with the ED80 was a total RMS error of 0.67", with the 300PDS I have managed 1.93". This is a problem because I am now working at a 1,500mm focal length rather than 600mm and I am 3 times worse than I was. My gut feel is that there is improvement to be made in this but the polar alignment looks really good visually and I think it's going to be twiddling PHD2 settings and possibly PEC algorithms rather than hysteresis/resistive switch.

So, although the guiding is poor, it guides. I spent several weeks getting to this point and I finally hit my first 5 minute exposure on the horsehead B33 ... nothing. I mean, I got some stars which let me plate solve by eye and see my goto was working well but there was no sign of a horse head. I tried a 10 minute exposure and similar results, I could make out the stars but no sign of a nebula.

I was scratching my head at this point but clouds rolled in (the story of this year for me) and I went to wrap things up for the night. I decided to have a look at the mirrors thanks to my park position making them so accessible and the secondary was covered in dew! The primary was fine but the secondary was very bad. A quick search for dew shield will show that people like to cut camping mats up and make an even bigger telescope! This is the next job for me then I'm hoping I can have a go at actually taking a proper image.

So that's the journey to date, I basically took a fantastic rig in a nice observatory and ruined it Do I regret it? Nope. I don't think I'll necessarily get the best images in the world, they probably won't be as good as the ED80 all things considered but it has been a heck of an experience already. My hope is that I will end up leaving these Bortle 5 skies and one day do some visual with the 300PDS but for now I'm just learning as I go and I'm hopeful I'll get a working AP set up in the next couple of months.

I hope that helps anyone considering a 300PDS like I was, it's a beautiful monster but it certainly isn't the easy choice!
 

Edited December 14, 2020 by JSeaman

[Jiggy 67]

Wow, you’re obviously a very determined individual, overcoming each problem as it arises. Good luck, I hope you get it all up and running 

[JSeaman]

Thank you very much, I've just had a mini disaster in that the gazebo has blown over and knocked the telescope out of position so starting from scratch again - planning out a new roll off roof observatory as I type!

[Jiggy 67]

These things are sent to try us!!

[JSeaman]

Indeed they are, just placed two orders for wood so another project has just begun!

[JSeaman]

Busy few days, nearly done, just the roof left to finish

[Paz]

Great report, full marks for attitude and perseverance!

[tooth_dr]

Very interesting and a cool project.  I did briefly run a 300PDS on the same mount as you.  I agree it was very large and it also hit one wall of my Obsy at a certain point, so I was never comfortable letting it work away at night.  I went down to the 250, and found it much more usable, and the 1200mm FL wasnt much of an issue.  You can see I also used weights, and have an ED80 in the background.

You might struggle with differential flexure using a guidescope - the tube is very thin and the mirror is very heavy.  Also your mentioned than your guiding is now 3 x worse.  We need to note that your imaging scale is now also 2.5 x finer, so you really need to be guiding even better than before.  To get the most out of it, 0.45"/px RMS would be the target!  I never managed it and subsequently sold it on.

Edited December 19, 2020 by tooth_dr

[JSeaman]

Flex shouldn't be a problem because I made the mount attach to the tube rings so no load on the tube directly

Agreed on the guiding though, there's a lot of work to do there if I'm going to get anything decent out of it. Hopefully we'll have some clear skies over the next few weeks and I'll find out

[tooth_dr]

2 minutes ago, JSeaman said:

Flex shouldn't be a problem because I made the mount attach to the tube rings so no load on the tube directly

Unfortunately this doesnt remove the differential flexure.

[JSeaman]

How do you figure? The tube rings aren't likely to flex

[tooth_dr]

1 hour ago, JSeaman said:

How do you figure? The tube rings aren't likely to flex

Im only sharing what I learnt the hard way!  I’m not sure how you have attached your scope to the mount, I was only assuming the standard way of tube rings and a dovetail bar!

The big issue is the tube itself flexes - its made of the thinnest steel with a huge piece of glass hanging at one end.

I had my kit down to 30kg to balance. Looking at your photos with the ED80 on the top you look like you are using 45kg to balance?  The EQ6 is sold with this scope but not with another telescope on top!  If you are serious about giving this a go, in my humble opinion, you’ll need to shed as much weight as possible, and the only way to do this is a) guide with a finderscope or b) guide with a OAG!

I hope it works out at the light gather/resolution/imaging scale is phenomenal, and some of the best visual sights I had were through this scope.

 

All the best

Adam  

 

[Cup of Tea]

When using a laser collimator, if the dot moves when you rotate the laser, then I suspect the collimator needs collimating.

[JSeaman]

1 hour ago, Cup of Tea said:

When using a laser collimator, if the dot moves when you rotate the laser, then I suspect the collimator needs collimating.

No I put the collimator in v blocks and it is collimated, it's the alignment of the focus tube to the collimator that's the problem but I did jump to the same conclusion as you initially too

[JSeaman]

No I've welded together some 8mm steel, it's quite robust Fully custom

Yes I'm something in order of 47kg, I'm definitely well above a comfortable weight for it but I thought I'd try and make it go nonetheless. 

Thanks for the comments about visual too, I'm looking forward to ultimately using it for that

Managed to make some headway on the roof now too ...

[JSeaman]

I finally got my proper first light last night. My guiding was hovering around 0.9-1.9" which is more than I am aiming for (0.88") but it was good enough to take a few subs. Full moon and bad seeing but I managed 1:40 in Ha on Barnard 33 / Horsehead which I've thrown out as red, enough to say that the rig works in principle at least which is great news

 

 

[tooth_dr]

18 minutes ago, JSeaman said:

I finally got my proper first light last night. My guiding was hovering around 0.9-1.9" which is more than I am aiming for (0.88") but it was good enough to take a few subs. Full moon and bad seeing but I managed 1:40 in Ha on Barnard 33 / Horsehead which I've thrown out as red, enough to say that the rig works in principle at least which is great news

 

 

That’s a great result.  Stars are surprisingly round too. 

[JSeaman]

TBH I'm amazed at that given last night's conditions and the amount of time on it. Interestingly I found 2 min and 5 min subs wouldn't  stack in DSS but 10 mins were fine. I was a bit worried about subs that long but it didn't seem too much of a problem. Hoping for a better night tonight so I can improve the guiding a little further and get some proper data

[JSeaman]

Seeing was still a little poor last night but got some 20 minutes subs in their now. Still guiding a little worse than I need but I can safely say it is possible to image with a 300DPS. Fairly happy with the outcome although I know I have a long way to go

[JSeaman]

Now this is more like it ...

[banjaxed]

I think we would all love one of those.

[JSeaman]

It took about a week to build all told but it's not cheap! I sorted the pulleys and winch today, just waiting for my limit switches which arrive tomorrow and then I'll take over the movement with an Arduino