Getting a big Dob' ?

[Fraunhoffer]

Hi all, well after returning to astronomy 9 months ago after a break of too many years I'm having a fab time.

Wow, how things have moved on.

I picked up a nice s/h 4" refractor and a nice SH SCT-8 on a nice EXOS-2 mount which ive modified with goto2 and serviced (I could only dream of these before).

Ive dabbled a bit with the photography side using my DSLR and a gpcam and come to the conclusion that I really prefer observing and taking the odd photo (30 mins max). I might keep the gpcam for some EAA for a bit or faint subjects but other than that setting up the laptop blah blah takes too much time out of my observing window. Id rather sketch what I see and dial up a robo-scope if I want to take a picture of something difficult.

As I live not far from near Heathrow and the skies I suppose are Bortle 6 and (maybe 5 when the lights go off). The refractor and SCT arrangements make nice portable rigs when its worth travelling an hour to a darker place. I like looking at reasonably accessible DSOs, clusters, nebulae, coloured doubles, galaxies etc.

Anyhow to the subject of my question.
I'm considering saving up and getting a big dob' as a semi-permanent arrangement in the garden - with a roll off cover. We have a reasonable view of most of the sky and a flat-ish concrete patch where a hot tub used to be many years ago.

Id be interested in comments about the SW 300P Flex tube goto Dob (or similar) from people that have one. 

A. Will it actually reveal much more that the SCT8 ?
B. Why not go all out for a 14" er ?
C. What are the shortcomings, must-do mods people have done ?

Re C:
-I read that its a good idea to put onto a half barrel/water butt stand to access lower elevations and make more comfortable viewing.
- rigging up some anti-dew device on the secondary
- I'm about 185 cm tall anyway so standing or use a bar stool might be ok.
- many people change the focusser or use an electric kit?
- get a shroud ?
- lacquer the MDF stand to make it more weather proof.
- what to look for in a used one?
- is this going to involve additional expense with 2" eyepieces ?

Thanks for any input. I'm not in a hurry to get one although if a good used one pops up I might bite.

Clear skies all.
PS - sorry for the looooong post.
 

[Geoff Barnes]

Welcome back to the fold Fraunhoffer!

As you can see from my signature I have the very same scope and I love it to bits, it is good at just about everything you want to observe, great for moon and planets, wonderful for DSO's, globulars etc, and splits doubles easily.

First thing to note is, as a whole unit it is HEAVY, a 2 person carry or 1 person if it is split in two parts.

I have only made 3 or 4 mods;

The SW focuser was quite good but I replaced it with a 10:1 two speed Lacerta for extra critical focusing at high powers.

I found I had to adjust the primary mirror collimation too regularly for my liking so I installed much stronger springs to hold collimation better - haven't had chance to see if it has improved yet due to 2 months of cloud!

I also have it permanently mounted on a dolly board with rubber castors so I can wheel it around the lawn smoothly which works really well.

I built a very basic marine ply box house to store it in outside which also works very well, keeps the weather off it and keeps it at ambient air temperature so no cooling needed, wheel it out of its box and away I go.

I also invested in an Astrozap shroud which is fantastic as once it's on you can leave it on and it collapses down with the flex

I love the Go-To feature for finding faint objects quickly, and the tracking is wonderful for keeping your subject in centre field of view, especially at high power, no annoying nudging required! But also just release the two clutches and use it manually for speedy slewing to brighter objects.

I've not looked through anything other than an old 4 inch refractor which just doesn't compare.

I've never found dew a problem so far but we don't get really freezing cold much here. 

Thoroughly recommend it as a near perfect all round scope, but a 14 incher? Hmmmmmmmmm……  

 

[Ships and Stars]

Hi there, if you are able to shift a bit of weight around and have a car/van with a little room for the odd visit to dark sky sites, I'd go for the biggest dob possible within reason and budget. Why not? I bought all my gear used, but it took a lot of waiting and watching for various bits to come up. Had I bought new, I would have ended up with a much smaller scope (which I do have - a 130PDS and a 20" Stargate dob, no middle ground here, haha). I came from a photography background, so wanted to take a break from stressing about exposures etc and just enjoy the views, so dob it was.

Anyhow, in my brief experience, a dew heater for secondary - yes, yes, yes. 

Raising the scope - buy a huge one so you don't have to!

Shroud - if a truss dob, a definite.

2" eyepieces - sure some are insanely expensive (21mm Ethos, I'm looking at you) but on the other hand the TV plossl line is great for the money I think. Lots of other options.

The base - try epoxy resin used for wooden boat building. Non-toxic, no VOCs, super durable. 

Used - old dobs might need mirror recoated. 

Focuser - mine is fine, but it will have more weight hanging from it this autumn, so might upgrade. If you are not in a hurry, just keep an eye on the second hand market, or put a wanted advert out there?

Happy dob hunting!

[scarp15]

Question A, Will it reveal more than the SCT8,

Yes, from a dark sky location, I formally had an SCT8 which I took to dark sky places, very good, but then I bought a 12" Flextube, the manual version and it was completely transformational. As mentioned there are mods and upgrades to benefit this.  From a personal perspective, I was entirely happy with the simplicity and relative lighter weight of the manual 12". 

[Paul73]

If you don’t need to move it. Go big!

I started with an SW 250 flex tube which got quickly modified with light shroud, dew heaters on the secondary, duel speed focuser, Telrad Finder to complement the RACI finder ...... all worthwhile upgrades. I now have a 16” which makes observing the faint stuff far easier.

Paul

[Ships and Stars]

PS - a wheelbarrow attachment will make life much, much easier with a big dob with fewer visits to your GP. Moving the 500p around by taking the mirror assembly off the base is still a proper chore and eats into my viewing time. It's not so much the absolute weight, it's the way I have to lean forward to carry it due to the bulk and worry of tripping with the mirror, argh! Worst nightmare. A skyliner should take a wheelbarrow attachment quite easily. I'm still trying to sort one for the Stargate, but work has been full on. I'm surprised no one seems to have come up with a wheelbarrow attachment for the Stargate yet. It needs it. Some blistering views of the heavens however!

[Fraunhoffer]

Great comments, thank you.

- Im thinking it will mostly stay in one place with a roll off shed/ lock down cover over the top so unless there is a month of storms it will have to stay outside anyway. A platform with the big lockable wheels sounds a good idea though. 

- then just thought, If I sell the gpcam cameras I could go up an aperture or two  :-0

= time to flog some of the loft contents I think

 

Edited August 6, 2019 by Fraunhoffer

[Fraunhoffer]

How well does the go-to bit work on these please?
Is it a type of friction drive, or does it have a worm and gear with a clutch?

[NGC 1502]

 

You have lots of good advice given already, but this may help.....

The mounts on most commercially made Dobs are made from coated particle board as you will know.  It make them heavy of course but another issue is if particle board continually gets wet it can swell and break up.   Most of us have found our scopes and mounts dripping wet after a good session.

Before assembling the mount best to give all raw edges a coat of PVA wood glue/sealant, let it dry overnight then give a second coat.  Following assembly seal all joints with a silicone sealant.   This simple treatment will give the particle board a very long life.

Great to hear of your return to astronomy 👍

Ed.

 

[Geoff Lister]

4 hours ago, Fraunhoffer said:

How well does the go-to bit work on these please?
Is it a type of friction drive, or does it have a worm and gear with a clutch?

I have the 250mm Skyliner truss-tube version with Synscan GoTo. The mount has a pair of heavy-duty gearboxes, (I think they use all spur gears, but there may be a worm somewhere in the train), and the handset has the option to calibrate the drive to remove backlash. There are dual encoders on each axis, so you can move the mount manually, with suitable fixed clutch friction, and the mount still knows where it is pointing, for excellent tracking.

I often do "Brightest Star" alignment at dusk, and the tracking will keep an object near the centre of a 32mm eyepiece whilst I go in for tea and wait for the sky to get fully dark. You only need to be able to see 1 star to do a manual slew to the first star, and when centred, the mount performs an automatic slew to close-to the second star, usually well within the field-of-view of my 32mm eyepiece.

Geoff

[Fraunhoffer]

Just in case someone is following this thread Ive found some v useful threads here:

 

[Fraunhoffer]

Thanks for everyones comments. I took the plunge and took the opportunity to pick up a 3rd hand 12" flextube goto.

First light:
I set it up last night even though the weather was a bit iffy, aligned no problem and had a lovely view of M57 and Alberio before everything got dripping wet with dew.
Lovely to be able to stare directly at M57 to see it. The colours of Alberio were nice and clear. Vega (one of my alignment stars) was a dazzling light blue - lovely.

It isn't too heavy in the 2 parts - more slightly awkward to get the base through the door (tuck that az wire in).
Its very quiet - I was surprised.
Collimation (not done that before) was dead easy with the collimation laser ep that came with it.
The stock single speed focusser was ok and there wasn't much wobbling when in use, so Ill probably leave it as is for now. (I guess the weight helps here).
It was quicker to set up than my equatorial mount. Base down - tube in - extend tube - set horizontal and collimate - connect up battery - align - go.
Quite like the shove-to save time when changing from one side of the sky to the other.
At 185 cm tall, the ep is at a nice height too.
Well pleased.

It was so wet yesterday that the finder and secondary dewed up massively after about an hour so I will make up some simple dew heaters over the weekend.
Ill also be making up a stand to keep the base off the floor.
The existing particle board has swelled slightly near the base so I want to control that and will take the suggestion of silicone and some light PVA.

Thanks all, clear skies.

 

 

[Limerick John]

I have a Meade 8" SCT and I picked up a 12" Lightbridge mainly for camping and for taking to dark sky sites. It easily fits in the back seat of a small car and only takes a few minutes to pack away.

Visually it blows the 8" SCT away on pretty much everything, especially on the planets as it doesn't have the long cool down time and tube currents to deal with. It is a bit heavy but the fully assembled OTA can be carried outside by one person and set up. I leave mine fully assembled in the house and carry it out in two trips (Base and OTA). The flextube would be similar except you can add goto. I personally prefer the truss tube design for a telescope of 12 inches as it can be broken down into smaller pieces for transport.

Enjoy your new toy !!

Edited October 6, 2019 by Limerick John