Perseid's Truss Tube

AGMI · Post by **AGMI** » Thu Nov 18, 2004 10:31 pm

hmm someone told me a perseid truss is cheaper ann better than a hardin DSH 8

ariefm71 · Post by **ariefm71** » Thu Nov 18, 2004 10:41 pm

definitely.
a dsh 8 will cost you around us$600 shipped to singapore. that's about s$100 more expensive than perseid's 8" truss dob.
both are using the same mirror from guan sheng optical

AGMI · Post by **AGMI** » Thu Nov 18, 2004 10:43 pm

Cool... how shoud I order it? I'm dying for a new scope T_T

good night going to study the pleiades

kayheem · Post by **kayheem** » Thu Nov 18, 2004 11:19 pm

[quote="AGMI"Cool... how shoud I order it? I'm dying for a new scope T_T [/quote]

Hi,

All of us have been through this phase of 'dying for a new scope' :-)

However, I strongly suggest that you try to control the urge and wait for a working model to come out first. Just looking at the photo, the upper cage is supported at 4 points by the 4 truss tubes.

http://www.singastro.org/viewtopic.php?t=1559

I may be wrong, but I think this configuration may be prone to parallelogram shift.

Try to do a hands-on first on a final production model before jumping in.

Kay Heem

Post by **Airconvent** » Thu Nov 18, 2004 11:45 pm

Yes...the optics are the same but for Hardin, you need to pay the overseas shipping to Singapore. Perseid's shipping is about $50 (or is it RM50?).

Kay Heem : Is there really some concern on the design used? I'm seriously considering getting one and CK did promised that the final product would be more refined than the prototype. Vincent did give some good feedback on it... what is this parallelogram error you referred to? Does it have a huge impact on the image? I noticed the rods are placed into holes in the frame. It is normally not secured although grub screws are provided. I was told once the front end is in, it should be quite sturdy.... any opinions is appreciated...
pity you did not see the scope in the flesh...looks impressive and the price is also within our range....

rich

kayheem · Post by **kayheem** » Fri Nov 19, 2004 12:11 am

Airconvent wrote:
Kay Heem : Is there really some concern on the design used? I'm seriously considering getting one and CK did promised that the final product would be more refined than the prototype. Vincent did give some good feedback on it... what is this parallelogram error you referred to? Does it have a huge impact on the image? I noticed the rods are placed into holes in the frame. It is normally not secured although grub screws are provided. I was told once the front end is in, it should be quite sturdy.... any opinions is appreciated...
pity you did not see the scope in the flesh...looks impressive and the price is also within our range....

rich

Hi Rich,

No, it is not 'parallelogram error' but 'parallelogram shift'. If you fix the mirror box and apply a lateral force to the upper cage, does the upper cage shift or move? If it does, there is parallelogram shift. If you can visualize it, the upper cage is displaced laterally over the mirror box and forms a paralleogram with the truss tubes instead of forming a rectangle.

That is why most truss dobs utilize the standard serrier truss configuration. I believe the teleport also suffers from a bit of this, but is not as bad, as the truss tubes extend further down the deep mirror box. Elton, please correct me if I am wrong.

I am NOT saying the Perseid's design won't work, but it is better to have a hands-on test on the final model before deciding. I have some experience in this as my first prototype truss dob (6" F/8) suffered from it. Some of you will remember seeing this dob at the very first Singastro equipment fest at Eunos field a few years ago.

Chris' small dob does not have significant parallelogram shift as the C-channels are secured at 2 places each on the upper and lower segments. This prevents the shift.

Rgds,

Kay Heem

ariefm71 · Post by **ariefm71** » Fri Nov 19, 2004 12:25 am

kay heem, is this shift related to the use of four poles instead of eight? if this is the case, how bad was it when you're using the two poles last time for the 10" dob? i'll be using four 1" poles at the corners and tighten them using ball and socket clamps ordered from moonlite.
btw, my kriege&berry book just arrived, reading the first chapter now ...

Post by **Airconvent** » Fri Nov 19, 2004 12:27 am

ariefm71 wrote:kay heem, is this shift related to the use of four poles instead of eight? if this is the case, how bad was it when you're using the two poles last time for the 10" dob? i'll be using four poles and tighten them using ball and socket clamps ordered from moonlite.
btw, my kriege&berry book just arrived, reading the first chapter now ...

hi arief...
mmm...what's this kriege & berry book about?

Post by **VinSnr** » Fri Nov 19, 2004 12:30 am

Airconvent wrote:
ariefm71 wrote:kay heem, is this shift related to the use of four poles instead of eight? if this is the case, how bad was it when you're using the two poles last time for the 10" dob? i'll be using four poles and tighten them using ball and socket clamps ordered from moonlite.
btw, my kriege&berry book just arrived, reading the first chapter now ...
hi arief...
mmm...what's this kriege & berry book about?

It's a book about building the truss tube dobsonian.......

Post by **VinSnr** » Fri Nov 19, 2004 12:31 am

ariefm71 wrote:kay heem, is this shift related to the use of four poles instead of eight? if this is the case, how bad was it when you're using the two poles last time for the 10" dob? i'll be using four 1" poles at the corners and tighten them using ball and socket clamps ordered from moonlite.
btw, my kriege&berry book just arrived, reading the first chapter now ...

Hey, let us know how you find the book. is it easy to follow? Can I take a look at it next time?