202 replies to this topic

[Roel]

This autumn I will be staying on the island of La Palma for a week of deep sky observing together with some of my observing buddy's. But what kind of telescope should I bring along? After lots of thoughts and some rough sketching I came up with what for me would be the ultimate travel scope...

 

Last week I started building a 10" truss telescope that would easily fit inside a normal backpack and be light enough to be carried to desolate mountain peaks or other exotic observing locations. I have also considered to make it a 12" but in the end I chose a 10" over a 12", because the latter would become just a bit too large and heavy for comfort. I really think a 10" might be the perfect combination of aperture and "compactness".

 

The focal ratio of f/4 is the fastest I can live with without the need for an expensive and heavy coma corrector. Also the viewing height at zenith will be around 3,3 feet - low enough to be used with one of those lightweight low folding seats. And last but not least: a short scope is a lot easier to balance -  I'll be using Vixen LVW eyepieces - pretty capable of handling fast mirrors, but not the lightest pieces of glass...

 

I started building the secondary ring. The 1.25" helical focuser is made out the inner workings of an old camera zoom lens. It has virtually no play, it's extremely lightweight and compact.

 

 

The total size of the transport case will be 28,5 x 30,5 x 9,8cm (11,2 x 12,0 x 3,9 inch), with the whole telescope in it, except for the trusses. I don't know what the total weight is going to be, but it will be well under 9 kg (20 lbs).

[Pierre Lemay]

Roel,

Although it's in French you might get some inspiration from this PDF document. French ATM Pierre Strock designed this 10 inch travel telescope about a decade ago. Since then many ATMs have copied and built his design.

 

Good luck with your project.

[Oberon]

I have similar plans, none rendered on paper. Only I've stuck with F/5 to avoid the Paracorr. And I've eliminated the focuser which is a major nuisance for ultra light portable designs.

[Roel]

Although there will be a lot similarities to a Strock, this design is more like those of Sumerian Optics' Alkaid. (which is by itself an "evoluted" Strock design). I've used this for a previous build, a 8" f/4: http://www.roelblog....-reistelescoop/

[Pierre Lemay]

I have similar plans, none rendered on paper. Only I've stuck with F/5 to avoid the Paracorr. And I've eliminated the focuser which is a major nuisance for ultra light portable designs.

Jonathan,

Good choice using f/5 to avoid the coma corrector. I'm going the same route with the ultraportable I'm building now. 

 

Are you planning on focusing by varying the truss tube lengths by any chance?

 

Looking forward to reading your description of this project whenever you get around to it. 

[hakann]

Hi,

Great, even at 10" if you find clear sky will be good for you.
I was on La Palma under 30 days in December -15. My idea was to see if it was possible to bring a RFT 14" and research of astronomy on island.
I brought a Zeiss 60 mm bino 1.6 kg + box and a carbonfiber Gitzo tripod at 4.5 kg in a box. The bino box I had in my hand bag on plane, and I payed extra for extra luggage. Be prepared on ruff handle on your box..this is aircraft business, not UPS 'glass'.
Idea is to has mirror/ box in cabin. I realise my 14" operation will not work for portable on a aircraft and just 3 Ethos and paracorr, observation chair, collimate tools plus other relation stuff take allot of space.
Be prepared if you going on fall/winter you need European winter clothes !
-I'm mean full geared for alps. Even at +6-8 C it is very cold vs winds.
If you are on volcano at 2400 meters the winds blowing real tuff. If not correct clothes you has no use on telescope.
Island is not as dark as one believe as its light kind of polluted, and I when I was there I had 30 days whit calima ( dust from Sahara ) and no one told me about that, but for people live there they know all about it. Astronomy was not better than bad seeing Sweden.
I would not go there for 1 week, to much work for weather and all travel. Be prepared if you not go direct you come to Tenerife south airport, then bus to north then over to La Palma.
If you come late no busses and north airport is closed at night. All you bags need to go up & on allot. If you are alone it’s not to nice.
Remember pricing on cars will go up 100% as damage/stolen insurance as no one mention. Hotels are pretty high but I rented a small house at decent 650 Euros on west side 600 meters high. Nice ocean overlook.
There are good spots under El Paso on that volcano to ( around 1300 meters ) but you might see Los Llanos that I found irritating ( light )
If you stay on road to ex volcano it come cars and damage your night vision and under observatories they has lights towards the road.
So I was normally in garden at house, but sky was so bad it might be only 2-3 ok night on full moon period. After I came home I heard many had hit the calima there.
Island is special, as a big volcano so there are no flat spots vs winds who make a very unstable weather. Its cloudy/fog many days/year at high places.
One guy who worked at Galileo told me when good, it is as good as Chile but few nights/year vs islands location and winds/fog/clouds.
But a nice island, no sales guys, tourist trekking etc, nice and calm, great trek all over Iland. Normally sunny all days/year and decent weather even winter time.
You can find good place high on the volcano ridge on west side but there is lights almost everywhere from smaller city.
I found a nice spot around 10 minutes from observatories towards Santa Cruz that ridge did hide light from Los Lanos.
Unfortlantly even at night there is cars.

It is a astro hotel whit a great guy on west side, and they has 2 refractors’, one SkyWatcher 16" dob, some eyepieces, and you can go visual or photo and several binoculars, tripods, star map and a office. It’s located around 650 meters near city of Tijarafe.
Most easy way is to go to them and rent a room and astro gear, and you can easy go bus from Santa Cruz to save money vs a car.
You can walk to cafe, bank food, but be prepared on you must like walk plus good shoes and hills..
His place is dark and warmer than to than be up higher. It is way cooler at 650 meters than down beach at Santa Cruz ( east ) or Tazarcotte ( west )
One future idea... The guy from Galileo live on west side at 1350 meters and he has two domes whit photo telescopes, and one whit a Zerodour mirror and when retired he will build two 28" dobs on his dob balcony, it’s a fantastic place he has, and he will rent equipment out !
I has a18" dob but I would not bring it there, and if transport it - weather there is way to unstable.
Ok, as always, you need luck !
My next trip will be Sahara sky hotel now in new moon in April.
-Keep us updated whit your travel, and travel dob.
Good luck.

Edited by hakann, 29 February 2016 - 05:07 PM.

[Oberon]

Collimation by varying truss tube length. Focus by varying truss tube angle.

[Gabor Kiss]

Very good topic, just wanted to start a similar one. I have a 14" premium mirror on the way, and I want to do photos from Tenerife with it, mainly planetary. I am looking for someone actually, who could build me the whole truss dob setup, but my budget is very limited. Can you maybe help me finding someone or can you refer anyone who would do it but not in the 2000 dollars + range? Thank you!

[Roel]

(@Gabor: PM sent)

 

This is the primary mirror box/cell. Because of the thickness of the mirror a three point support is sufficient according to PLOP - which makes things definitely easier to build. The mirror simply rests on the tips of three nylon bolts. Just turn these bolts to collimate.

 

The box is simply a plywood plate with the truss connectors at the corners. Although the birch plywood is rigid enough for a very stiff stucture, I kept two corners attached to eachother for extra rigidity. The two altitute bearings will be bolted to the sides and will also add extra stiffness.

 

(I already applied some paint because I wanted to see what it will look like when finished.... It's a little rough, but in the end, after some sanding and extra layers, it will look a lot better.)

 

Next stop: the altitude bearings!

 

[Roel]

Back side:

 

[Pierre Lemay]

Focus by varying truss tube angle.

OK Jonathan, you've got my attention. Could you elaborate?

[Roel]

@Pierre Lemay and Oberon: to prevent confusion it might be better to discuss other telescopes in a separate thread. Thanks!

Edited by Roel, 01 March 2016 - 06:40 AM.

[Oberon]

Roel, I quite agree.

 

And I like your lightweight compact focuser. I'll be following your build with interest. I'm curious about your choice of F/4 without coma correction. Everything I read says coma is unacceptable at that ratio, but I've not observed faster than F/4.5; have you checked it out for yourself? I too would go with F/4 if I could get away with it.

[Pierre Lemay]

Roel, I quite agree.

 

And I like your lightweight compact focuser. I'll be following your build with interest. I'm curious about your choice of F/4 without coma correction. Everything I read says coma is unacceptable at that ratio, but I've not observed faster than F/4.5; have you checked it out for yourself? I too would go with F/4 if I could get away with it.

Roel, also agree with your request, sorry. 

 

As for observing at f/4 without a coma corrector I do it with my 20 inch (f/3.9) but only with the higher power 1.25" ES 82 eyepieces (11mm and shorter focal length) and I'm quite happy with that.  Note, however, that my 20 inch is on a tracking platform. No coma corrector wouldn't be as fun with a non tracking mount.

 

For wide field observing, a Baader MPCC coma corrector is permanently screwed onto my 2" 21mm ES100 eyepice (1 deg FOV). For wide field views on a smaller telescope, I would recommend picking one widefield 1.25" eyepiece that provides about a 6.5mm exit pupil and attaching a home made coma corrector to it (there have been 1.25" coma corrector designs shown in this forum in the last year or two). For the higher power eyepieces you don't absolutely need the CC at f/4. 

 

The trick to an ultra lightweight is keeping things like the focuser very light. This is no place for a heavy, 2" feathertouch focuser! A 1.25 " helical like Roel is using here is prefered. If you don't have an old camera lens focusing body, you can purchase the Kineoptics HC1. It only weighs about 65 g. 

[Roel]

Yes, the KineOptics is a great focuser. I use a HC-2 on my 14" Dobson and for visual use I would never want another thing than a good helical.

 

About the use of an F/4 without coma correction: all I can say is that the limit of coma one can live with is really a matter of personal opinion. While some use Paracorrs even in F/5 systems, my 14" has a f/4 primary mirror, and I'm using that without a coma corrector for years now. I'm using Vixen LVW eyepieces which are quite expensive, but I think they can handle fast mirrors quite good. A tracking platform indeed helps a lot, but when the scope moves smooth I can even live without that (for powers up to 175x that is...)

 

By the way, yesterday I made the altitude bearings:

 

 

[paul]

I can't wait to see this ! Your designs are always beautiful and well thought out!!

Plus I have a 10" mirror in a box at home.....

Paul

[Roel]

Thanks!

 

This morning I finished the spider. It's a very simple three vane systeem with the well known push/pull collimation.

 

 

Attached Thumbnails

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[Roel]

The vanes are very thin, which can be seen in the picture below. I placed the focuser a little bit more to the "top" instead of the common 45 degrees angle: It is a small telescope and while observing low above the horizon you'll get a much more comfortable viewing position.

 

 

Attached Thumbnails

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[Roel]

Today I've cut the trusse to a few centimeters longer than calculated, and made the upper connectors (I'll post pictures of those in detail when finished). And I couldn't help myself making a quick layout on the floor, to see how the OTA will look when assembled.

 

 

 

 

 

[Roel]

One more...

 

[Pinbout]

what dia tube are your trusses?

[Roel]

The trusses are 10x1mm.

[paul]

Hello,

Looks great so far!! Are you using  1/2 " plywood for the rings and mirror box?

Thanks

Paul

[Roel]

@Paul, I use plywood of different thickness, but mostly 18mm and 12mm (I have no idea what that is in inches...)

For example: 18mm is for the secondary ring and the altitude bearings, 12mm for the rocker box.

 

This weekend I cut the last pieces of plywood, these are for the rocker box/transport box.

 

 

 

[Pinbout]

that's equivalent to 3/4" and 1/2"