Monster New 12" Dob (new to me anyway)

[Moonshane]

Give it a go. If you have a router, try making some larger side bearings too - would be great practice. Just use cheap ply first as it will presumably be painted anyway?

This thread might help. I'd never really used a router before attempting this. The mount is still going strong.

 

[mdstuart]

I found that the mirror on mine just rested on the wood base. I added three blobs of silicon behind the mirror to attach it to the base in a more balanced way and this helped with removing some astigmatism I had initially with my dark star scope.

Mark

 

[AdeKing]

@mdstuart Mark, in mine, there is a layer of bubble wrap between the mirror and the wooden base, which I assume is a later addition.  Thanks for the tip about adding blobs of silicon.  When I put it back together I may employ this approach.

Ade 

[John]

53 minutes ago, AdeKing said:

@mdstuart Mark, in mine, there is a layer of bubble wrap between the mirror and the wooden base, which I assume is a later addition.  Thanks for the tip about adding blobs of silicon.  When I put it back together I may employ this approach.

Ade 

I believe Peter Drew uses bubble wrap between his mirror and the mirror cell in his 30 inch dob so its doing the job !

[Moonshane]

Bear in mind the mirror cell provides three basic functions:

- The clips simply prevent the mirror dropping out (maybe also lateral movement is prevented dependent on the design)

- under the mirror is simply to provide even support

- The cell also needs to allow accurate collimation adjustment

If it does all of this the design is somewhat irrelevant :0)

 

[Peter Drew]

11 hours ago, John said:

I believe Peter Drew uses bubble wrap between his mirror and the mirror cell in his 30 inch dob so its doing the job !

He does indeed , as well as a 17" F5 Dob. However this is not your common or garden bubble wrap (this does work but deflates in time) but a heavy duty version originally intended for covering swimming pools during the Winter. It has bubbles about 10mm diameter which can't be "popped" by finger pressure. Apart from relative cheapness and weight saving, on the 30" the disc of material provides around 750 suspension points with good airflow between the crests and troughs of the bubbles. Each bubble is effectively supporting a 15mm diameter rod of glass 38mm long.

[AdeKing]

@Peter Drew is this the sort of stuff you use Peter?

http://www.ebay.co.uk/itm/Intex-Solar-Bubble-Cover-For-8ft-Easy-Set-Metal-Frame-Swimming-Pool-29020-/182287658908?hash=item2a7130ef9c:g:17IAAMXQfvlSmL0Y

I was having a think about this last night and was trying to remember where I'd seen the really heavy duty stuff until I remembered that it was on a neighbours swimming pool where I grew up.  The bubble wrap currently underneath the mirror is well and truly deflated.

8ft diameter is about the smallest size that I can find. 

[Peter Drew]

3 minutes ago, AdeKing said:

@Peter Drew is this the sort of stuff you use Peter?

http://www.ebay.co.uk/itm/Intex-Solar-Bubble-Cover-For-8ft-Easy-Set-Metal-Frame-Swimming-Pool-29020-/182287658908?hash=item2a7130ef9c:g:17IAAMXQfvlSmL0Y

I was having a think about this last night and was trying to remember where I'd seen the really heavy duty stuff until I remembered that it was on a neighbours swimming pool where I grew up.  The bubble wrap currently underneath the mirror is well and truly deflated.

8ft diameter is about the smallest size that I can find. 

Yes, this seems to be the same. (Same colour at least"!)   

[AdeKing]

Well, I've taken the Primary mirror out of the cell again and have inspected the underside to see whether or not there were any identifying marks.

There is a very small number (2522) etched into the glass near to the edge on the back of the mirror, but no A or B and no other identifying marks.  The only other mark on the rear of the mirror was a scrap of masking tape with ND 72 written on it in black marker, which could be anything and placed there at any time in the lifetime of the mirror.  I've replaced the bubble-less bubble wrap with some new bubble wrap and the primary is now more secure in the cell than it was.

I've cleaned and re-greased the very basic helical focuser with lithium based grease, and it now feels nice and smooth.  I've collimated as best as I can, but to my eyes looking through the Cheshire, the secondary is definitely too far up the tube, probably due to it having been re-attached upside down.

I've managed to get a nice view of the top of an electricity pylon located about 500m away, though I had to rack the focuser out to the pint where it was almost falling out in order to achieve focus. 

Its looking like a clear night tonight, so I've left the Dark Star Dob outside cooling, along with the 8" Dob and am hoping to get first light tonight and compare the views from the two.

Ade

[Moonshane]

Good luck and enjoy!

[John]

22 hours ago, AdeKing said:

I've managed to get a nice view of the top of an electricity pylon located about 500m away, though I had to rack the focuser out to the pint where it was almost falling out in order to achieve focus. 

 

 

Thats quite normal for newtonians. When at sharp focus on an astro target you should find that the focuser will be somewhat further inwards.

[estwing]

Don't forget the report!

[Bruce Leeroy]

I have a Dark Star dob the primary is 220mm and the mirror coating is only a little better than yours I use a moon filter when viewing Jupiter as I think there's alot of light scatter/glow coming from the ageing mirror coating.

Here's 2 iphone images using the Dark star dob...I have a little video footage but it's 28mb so not so sure it will upload, skip to 29 seconds.

 

IMG_3659 - Copy.MOV

[AdeKing]

@estwing things have been a bit flat out today, I hadn't forgotten about writing a report.  

I put the 12" Dob outside for a couple of hours yesterday evening to ensure that it had fully cooled before I started observing just before 21:30.  I set up the 12" Dark Star and 8" Skywatcher Dobs side by side and put an 18mm BST Starguider in the 12" Dob and a 12mm BST Starguider the 8" Dob giving 100x magnification in both scopes as the focal lengths are 1,800mm and 1,200mm respectively. 

The 12" was collimated as well as I could manage given the problems with the secondary mirror, and a quick check of the 8" showed collimation to be pretty much spot on through the Cheshire.

I started out with the 12" and eventually located Jupiter in the scope.  This isn't a lack of knowledge as to where Jupiter was, more the fact that I hadn't set up the finder scope. I did try to align the finder scope, but as its mounted in guidescope rings, no matter how much I tried, I could not get the finder anywhere near to where it needed to be when the target was centred in the FOV.  I guess that I need to play around with this in daylight, but when faced with 6 alignment screws none of which did what  I thought it would, I gave up.

Anyway, I persevered and upon centering Jupiter I immediately noticed that that there were noticeable amounts of coma around Jupiter and the four Jovian moons.  There was also a noticeable ghost image of Jupiter elsewhere in the FOV, but I don't recall where.  I also noticed that there was a general lack of contrast and I thought that there might be some light cloud, but swapping to the 8" showed a lovely sharp image of Jupiter with some faint detail in the cloud belts.

I did try for some faint fuzzies, but as it was a virtually full moon, I couldn't even see the Keystone of Hercules to find M13 or even locate Double Cluster in Perseus, I think that I need to test again under more favourable conditions.

I did however switch to the moon and endured the retina scorching for as long as I could, but to say my eye was watering is an understatement, it was positively streaming.  I do have a variable ND filter, but could I find it last night when  I really needed it, could I hell.

The views of the moon were also not spectacular, they were ok, but I could not get the bitingly sharp imaged that I achieved with the 8" Dob.

The views I got of Jupiter were similar to those posted above by @Bruce Leeroy - thanks for those images and short video clip, they are very similar to the views I was getting last night.

Essentially the 8" Dob wiped the floor with the 12" which was a bit disappointing.  However, given the problem with the secondary, the fact that I couldn't properly collimate the 12" and the rather hazy appearance of the primary I guess that all in all the views weren't too bad.

I think that when funds allow I'll get a new secondary mirror, secondary holder and spider, and see how much of a difference that makes.

I am a bit disappointed, but all things considered the scope shows some promise.  I need to have a good look at the damage on the primary to see if there is any damage to the actual mirror, or whether it is just the coating thats been damaged, then I'll consider getting the primary re-done.

I have decided that the first job is definitely getting a decent focuser, the helical job is truly awful.  I have a spare Crayford from my 8" Dob, and I know that the focuser hole in the tube will need to be enlarged, but I'm unsure whether or not I need anything else in order to fit the focuser to the 12" (14" outside diameter) Dob.

Sorry for the rather long post, but after receiving loads of help in this thread,  I thought that I'd give an honest account.

Ade

 

 

[John]

Good report Ade - very balanced and open

If you get the mirrors re-coated (perhaps replaced with respect of the secondary) the 12" should beat the 8" Skywatcher on deep sky objects. It might still be close on the moon and planets though - the Skywatcher mirrors are pretty good and a 12" gets hit a bit more by mediocre seeing conditions I reckon.

 

[estwing]

Yeah get the 12" sorted it will be worth it

[AdeKing]

@John and @estwing I was bitterly disappointed on Tuesday night but on reflection I figured that Jupiter may not have been the best test as these things are best at faint DSOs so I'm keen to get it out under moonless skies.

I spent an age speaking to Barrie Webb from Beacon Hill Telescopes the other day about primary and secondary mirror cells and assemblies and he made me realise that even if the damage on the primary mirror shows through after recording that it is pretty insignificant relative to the size of the mirror.

I'm definitely in it for the long run so I'll update this thread as and when I make progress.

[John]

While it's worth sorting the scope out within reason, it's worth keeping in mind that you could buy something like that scope, with mirrors in good nick, for around £250-£300.

 

[Astrobits]

I think that the small crescent damage on the primary is very close to being covered by the shadow of the secondary. As it is in the central area it will have minimal effect on the image, any light scatter it causes can be reduced by blacking it with something dark -- black pen,  card mask etc. I think that the first thing to do is to test the mirror then get it re-coated.

The damage on the secondary will not have any visible effect as it will only affect the shape of the area of the image plane which is fully illuminated. It will have no effect on small objects when viewed in the centre of the field. It is necessary to re-mount it so you can collimate the scope.

Nigel

 

[estwing]

14" on ABS for £650...it's a good mirror...Danny K will vouch for it

[estwing]

Big mirrors in dark skies on a moonless night with great company is what it's all about to me

[Alan White]

On 5/7/2017 at 15:26, John said:

I believe it was made by Dark Star Telescopes. I have a catalogue from the mid-1980's which shows their various dobsonians and they look identical to yours.

I think you are spot on John, my first ever scope came from Dark Star and they made Dobsonians, in those colours and in that fashion.
I believe they were very nice scopes to use.
Nice project scope, have fun, take your time.

[AdeKing]

@John I'm in agreement about not throwing a huge amount of cash at sorting this scope out.  I've spent a lot of time this week learning about building your own telescope and after reading the @Moonshane thread linked to earlier and several other self build threads, as well as looking at the stellafane website (http://stellafane.org/) I've decided to try and fettle things together myself and learn some new skills to boot with the aim of keeping expenditure to a minimum.

On 2017-5-8 at 13:08, Astrobits said:

 It might be interesting to test the mirror before getting it re-coated.

The secondary is installed incorrectly.

Nigel

 

@Astrobits I think I misunderstood this comment when I answered it previously.  How would I go about testing the mirror before I get it recoated? Do you mean a star test, or something more involved.  I'm interested to learn more.

I am going to attempt to remove and re-attach the secondary mirror, though I'm not sure what the mirror has been re-attached with, it almost looks like pitch.... gulp.  Once I've removed the secondary, I'm going to make some mods to the holder and replace the current rod with a threaded rod to allow for a greater range of movement, but may end up making up a new secondary holder.

I've found this link (http://conradhoffman.com/secondary.htm) which looks interesting, and wondered whether or not anyone on SGL has had any experience of this type of secondary assembly.

@estwing thanks for the heads up on the 14" Dob, but its a bit beyond my reach at the moment.  I got this 12" to dip a toe in the water and see how I got on with a larger scope.  It cost me nothing, so spending some money to get it up and running is what I expected to do.  However, I suspect (and fear) that once I've got this one back to a reasonable standard that aperture fever may well take hold.  I'm not sure what scares me more, the amount of work I need to do on the 12" or the thought that when I've finished this one that I'll want something bigger .

 

[N3ptune]

I vote for Estwing's solution instead of pouring hundreds of GBP rebuilding the actual 12"

[AdeKing]

@N3ptune thanks for your vote, the problem is that I don't have anywhere near the £650 GBP for the 14" hence wanting to fettle things together to keep costs down.

The other aim is for me to learn more about how the Newtonian works and fits together so I'm less paranoid about adjusting things on the scopes that I paid money for. 

If i'm able to do the majority of work myself the only major costs should be a new secondary (£55-114 depending on retailer) and recoating the primary (£140-160 depending on retailer) plus VAT and shipping unfortunately. With some (hopefully relatively minor) costs for materials.

Well thats the plan anyway.