In the market for a dobson

[Piero]

@John out of curiosity, how thick is your 12" OO mirror? Do you use a fan to cool it down? Thanks

[scarp15]

While awaiting John's response Piero, I can say that for my 14" OOUK primary, it is quite thin, without knowing the specific depth, a fan is installed which I never use. 

[Ally8446]

2 hours ago, scarp15 said:

While awaiting John's response Piero, I can say that for my 14" OOUK primary, it is quite thin, without knowing the specific depth, a fan is installed which I never use. 

I can concur with @scarp15, my 14" Hubble mirror cools pretty quickly. 30 minutes max and I'm at the eyepiece searching for fuzzies. 

[Piero]

Thanks a lot for the info. Thank thickness for the 14" was my next question actually!

Mm.. maybe they are 1.5"-ish thick.. I have to dig a bit on this... 

Edited March 7, 2019 by Piero

[John]

4 hours ago, Piero said:

@John out of curiosity, how thick is your 12" OO mirror? Do you use a fan to cool it down? Thanks

As Iain says, OO mirrors are a little thinner than chinese equivalents at least (thats the only other ones I've owned to compare them too). I think my 12 inch (11.79 inch to be exact - it's 300mm in diameter) is around 25mm thick but I'd have to take it out of the scope to be sure.

The cell on mine is a simple open back design which seems to help air circulation and cooling. It is fitted with a small fan but I don't use it. The scope can be used at low to medium power almost straight away after being put out from house but for high powers I find that 30-45 minutes are needed to allow all the air currents in the tube to dissipate.

Although simple the mirror cell seems to keep collimation very well. I drove the scope out to an outreach session last weekend which was 30 minutes of country roads, some speed bumps and a rather rough track at the end but the scope was still collimated pretty much spot on when I set it up.

My scope is probably not a fully "optimised" design by todays standards. It was made in 2006 so uses the designs that OO employed back then. Its seems to work very well though

[Piero]

6 hours ago, John said:

 

According to https://www.teleskop-express.de/shop/product_info.php/language/en/info/p1681_Orion-Optics-UK-primary---300-1600mm---f-5-4---1-8-Lambda.html

It should be 38mm or 1.5in for the 12 f5.3.

 

@scarp15

https://www.teleskop-express.de/shop/product_info.php/info/p1684_Orion-Optics-UK-primary---350-1600mm---f-4-6---1-10-Lambda.html

41mm or 1.61in for the 14" f4.5.

[John]

1 hour ago, Piero said:

According to https://www.teleskop-express.de/shop/product_info.php/language/en/info/p1681_Orion-Optics-UK-primary---300-1600mm---f-5-4---1-8-Lambda.html

It should be 38mm or 1.5in for the 12 f5.3.

 

 

It's quite a while since I had the primary out so they maybe right.

[Stu]

I do think a Lukehurst scope customised to your requirements could be a good option Piero. I would go for a Nichol mirror personally, the one in my 14" is excellent.

On the subject of Sumerians, I think they are oftentimes referred to as being one design, but do vary from the quite extreme portability of my Alkaid, to more robust versions like the Canopus. They are still in the light side compared with Lukehurst though.

I do have a 12.5" truss dob which is waiting for a Zambuto mirror and this is much more substantial that the Sumerian so will largely be used at home or just locally.

[Piero]

8 hours ago, Stu said:

I do think a Lukehurst scope customised to your requirements could be a good option Piero. I would go for a Nichol mirror personally, the one in my 14" is excellent.

On the subject of Sumerians, I think they are oftentimes referred to as being one design, but do vary from the quite extreme portability of my Alkaid, to more robust versions like the Canopus. They are still in the light side compared with Lukehurst though.

I do have a 12.5" truss dob which is waiting for a Zambuto mirror and this is much more substantial that the Sumerian so will largely be used at home or just locally.

Thanks Stu!

I was not aware of your Dobsons! Congratulations!

I am actually interested in Lukehurst Dobsons, particularly the 14" f5, but if I decided to get one, I'll need a few structural changes - not just additions - to the deluxe version.

An Astrosystems would come with all those mods plus others. Downsides: shipping, VAT, and Customs...

I already have a moonlight CR2 to be fitted.

Edited March 8, 2019 by Piero

[Paul73]

3 minutes ago, Piero said:

I already have a moonlight rc2 to be fitted.

Good choice. I have one on my Orion Optics UK 16” Dob. It is of a similar vintage to John’s.

Paul

[niallk]

Have you looked at The Dobson Factory scopes?

[Piero]

21 minutes ago, niallk said:

Have you looked at The Dobson Factory scopes?

Yup! Very pretty, but too skinny and fast for my tastes! 

Oh dear.. this sounds innuendo..!

[Piero]

More seriously, I just prefer a more classic design. I'm not looking for a telescope to take on a plane and I'd actually be more than happy to leave it mounted at home. A 12 or 14 truss dob can be loaded in a normal car by one person. 12 f6 ish or 14 f5 are models that allows one to observe standing up and do not require a CC, particularly if one tends to observe on axis. 

I'll have a chat with David and see what he thinks and whether he accepts those changes that I'd like to have.

Edited March 8, 2019 by Piero

[nightfisher]

David is a lovely chap that really knows his stuff, he bought my last newt OTA to make a small dob for a friend

[Piero]

How does it work when one buys a telescope to be imported to the UK from outside the EU? 

I know nothing about the required modules. I believe one must pay VAT and customs for the cost of the item plus shipping, but when? Will the buyer be contacted for a payment request or will the buyer have to fill in and send forms with the payment? 

[John]

I've only bought small items from outside the EU. With those I was notified by the handling agency in the UK (usually Parcelforce I think) that the item is at their depot and will be released when the relevent tax / duty / handling charges have been paid. You then have to roll up, show your ID and their advice note, pay the bill and take away the item.

There may be a way to pay these up front but I've not used that approach ?

 

 

[Mike_S]

Something I learned from getting my first dob last year, my first scope as well. Is get a good focuser. The view keeps bouncing around and nudging out of view with the rubbish that comes with the Orion XT Classic. Maybe it’s uodated now as mine was used.  Hoping to get one with dual speed Crayford next like the GSO delux. 

[Piero]

23 hours ago, John said:

I've only bought small items from outside the EU. With those I was notified by the handling agency in the UK (usually Parcelforce I think) that the item is at their depot and will be released when the relevent tax / duty / handling charges have been paid. You then have to roll up, show your ID and their advice note, pay the bill and take away the item.

There may be a way to pay these up front but I've not used that approach ?

Ah okay, therefore one way is to just wait for the bill and pay. Later this week I will check whether there is a contact number to ask more information on how these taxes are calculated. 

Thank you, John.

 

[Piero]

22 hours ago, MSammon said:

Something I learned from getting my first dob last year, my first scope as well. Is get a good focuser. The view keeps bouncing around and nudging out of view with the rubbish that comes with the Orion XT Classic. Maybe it’s uodated now as mine was used.  Hoping to get one with dual speed Crayford next like the GSO delux. 

 

It should not bounce because of the focuser. The standard focusers are not generally bad as long as one uses light eyepieces. With heavy and / or long eyepieces, that's another story in my opinion. My 8" dob has a moonlight CR2 and it works very well. I could have survived with a single speed focuser at F6 though. Not that I'm complaining eh! 

[Piero]

I played a bit with the maths and figured out that something like 12" F6, 12.5" F6, or 13.1" F5.5 would be a good match with my tastes and eyepieces. Above that, I fear the telescope becomes too heavy to move around and the mirror can take too long to cool down. Those focal lengths / apertures and my eyepiece set are a good match in terms of exit pupils.

• Telescope-Eyepiece calculator: https://docs.google.com/spreadsheets/d/1KtV9Hl0XRQ-IGLiCjxIGSzss-b9TJ_j8ssk0lhh5UpI/edit?usp=sharing
• Approximated eyepiece height calculator: https://docs.google.com/spreadsheets/d/1ZrMBc-Z3crChawd9AbxCDVCI_ZuBo21TI1ZUaycIXhM/edit?usp=sharing

 

Current specs

• 12" F6, 12.5" F6, or 13.1" F5.5
• Primary mirror ~1.5" thickness (maybe with quartz substrate to take advantage of the first 1-2h observing when the mirror is still cooling down)
• secondary mirror CO: 20 % (2.6" or 2.7")
• Astrosystems secondary 4-vane spider
• Astrosystems secondary mirror holder
• Moonlight CR2 focuser - I already have
• Astrosystems upper/lower truss fasteners
• Glatter sling on the primary mirror
• finderscope 9x50 RACI
• clamp for attaching a telrad (TBD in the future)
• wheelsbarrow handles to take it around quickly if needed
• Astrosystems shroud

To be defined

• primary mirror cell
• colour: plain wood (preferred) or cherry
• altitude bearing with 4 "branches" like Astrosystems / Teeters 

Additionally

• 2 step ladder

Edited March 30, 2019 by Piero

[niallk]

Boundary layer fans could be a good feature to have built in (in addition to a rear fan) - if going with a 'classic' style mirror box, like you previously mentioned. 

Quartz substrate may have a lower coefficient of thermal expansion than pyrex for maintaining figure, but it still creates a thermal boundary layer resulting in distortion.

Edited March 16, 2019 by niallk

[Whistlin Bob]

Some wonderful scopes on this thread- it's making me drool over my phone!

My 14" Orion USA is a bit more Johnstone Paint Trophy in comparison, but still brings much joy. The two points I'd make about it that are relevant here are:

- At f4.7 it completely suits me height wise. I'm 6 foot and anything over about 70 degrees I'm standing comfortably. Mid range and an ironing chair is fantastic. Planets ATM have me kneeling in the grass! Mid range EPs are also fine at that speed.

- Totally agree with the posts about the importance of the mechanics. The altitude on mine is adjustable and frankly brilliant- but the azimuth drives me crackers. I have some plans to sort it, but if you can get it right out of the box it will make the experience so much more enjoyable.

[Piero]

@alanjgreen How are the poles attached to your dobson structure? Are the bolts removable or are the bolts/screws part of the poles? Thanks

[fiestazetecmk2]

8" dobsonion gets my vote everytime.excellent scope does nearly everything .here's mine

 

 

[alanjgreen]

3 hours ago, Piero said:

@alanjgreen How are the poles attached to your dobson structure? Are the bolts removable or are the bolts/screws part of the poles? Thanks

You remove nuts from fixed threads and the poles lift off. The bolt thread is a fixed part of the mirror box.

The poles just have holes in to push onto the threads. Each side and pole is numbered so it’s easy to get them on the correct side.

You can see the attachments and the fixed bolts in one of the pictures in this post

http://www.fjastronomy.com/reviews/18-inch-f4-3-david-lukehurst-dobsonian-with-nichol-optics/

(It’s the last picture of the collapsed scope)

Alan

Edited March 20, 2019 by alanjgreen