Astrotech 8" RC for 1395USD

[netwolf]

Looks like AT in the US is cutting prices due to cheaper prices from GSO. If the dollar were higher this would be a incredible bargain. I wonder if local retailers of this GSO 8" RC will also drop the price.

http://www.cloudynights.com/ubbthrea.../o/all/fpart/1

[Tilt (Michael)]

As you mentioned, if our dollar was better that would be a very good deal. At current rates it puts this around the $2200 AUD mark, so its still kind of exy. Andrews has them for $2800, should be interesting to see what happens to the price of the GSO RC's here.

Michael

[Kal (Andrew)]

If you add GST to the $2200 then it brings it up to ~2400ish

I think if you rang Lee Andrews and negotiated then you would probably have a good chance of getting a little bit off that $2800 too.

[gregbradley]

The key question is how accurate are the mirrors.

I read one post on the net that quality checking of mirrors (not sure what type of scope and not necessarily these at all) was simply a guy putting the mirrors on a wall and shining a torch at them from a distance and then checking if they passed or not!

So I would want to see some images from them. RC optics are hard to make and Star Instruments have been making 1/24th wave mirrors for a long time. I believe it takes a long time. I know the Chinese industries pay their workers practically nothing but even so it takes skills and I doubt they have these. Not that they can't learn but it may take a decade or two.

Greg.

[Kal (Andrew)]

If GSO has a guy with a torch doing the QC on their dobs then he must have a pretty decent eye

[marki]

simply a guy putting the mirrors on a wall and shining a torch at them from a distance and then checking if they passed or not!

:rofl :

With the massive US investment into Taiwan, they have some of the most advanced production facilities on the planet. I am sure they must at least have an inferometer lying around some where .

RC optics are hard to make and Star Instruments have been making 1/24th wave mirrors for a long time. Greg.

I agree and I think many of us are waiting to see pics from these scopes. I remember making a similar comment on another thread about the 1/24th wave optics of the star instrument mirrors. I was shot down in flames by our expert optician and it turns out that the primary and secondary combined only provide 1/4 to 1/8 wave optics which is ok for an instrument with such a large obstruction .

Mark

[Gama]

Originally Posted by gregbradley

The key question is how accurate are the mirrors.
...... I know the Chinese industries pay their workers practically nothing but even so it takes skills and I doubt they have these. Not that they can't learn but it may take a decade or two.

Greg.

Where do you think the computer your using, and monitor your watching, and most everything in your house, office is made. Hmmm, U.S.A, NOOOOOOOOOOOOOOOOOOOOOOOOOOO

Its 1/24 wave CHINA baby !!


Theo

[Satchmo]

Originally Posted by marki

...and it turns out that the primary and secondary combined only provide 1/4 to 1/8 wave optics which is ok for an instrument with such a large obstruction .
Mark

There is no mystery about this. As far as I know those figures are for primary and secondary combined, as a functioning instrument. The Star Instruments web site , who make RC Optical instruments clearlys state that these wavefront figures are Root Mean Square, not P-V .

So assuming the surface figure is smooth, 1/24 RMS would equate to 1/6 wave PV wavefront. In the situation where they are measuring points all over the surface it makes sense to stick to talking in RMS as simply picking the highest and lowest point on the wavfront to define its quality simply makes no sense.

A heavily obstructed instrument like this has its Strehl ratio knocked down into 60's regardless of how good the mirrors are because of the large obstruction, so any wavefront better than 1/4 wave , 1/20 RMS is fine for the job of prime focus.

I did ray traces once that showed perfected optics with a 45% obstruction producced an EER ( Strehl taking into account the obstruction ) of 0.72 and system with 1/4 wave spherical aberration of 0.8 ended up at EER of 0.68 obstructed.

Its the guys doing planetary photography at 4X-5X their native focal length that will pick the difference between 0.95 and 0.8 Strehl wavefronts.

Clearly the lesson is to keep the obstruction to a minimum which generally visual observers, and planatary photographers have the luxury of doing.

Mark

[toryglen-boy (Duncan)]

Originally Posted by Satchmo

There is no mystery about this. As far as I know those figures are for primary and secondary combined, as a functioning instrument. The Star Instruments web site , who make RC Optical instruments clearlys state that these wavefront figures are Root Mean Square, not P-V .

So assuming the surface figure is smooth, 1/24 RMS would equate to 1/6 wave PV wavefront. In the situation where they are measuring points all over the surface it makes sense to stick to talking in RMS as simply picking the highest and lowest point on the wavfront to define its quality simply makes no sense.

A heavily obstructed instrument like this has its Strehl ratio knocked down into 60's regardless of how good the mirrors are because of the large obstruction, so any wavefront better than 1/4 wave , 1/20 RMS is fine for the job of prime focus.

I did ray traces once that showed perfected optics with a 45% obstruction producced an EER ( Strehl taking into account the obstruction ) of 0.72 and system with 1/4 wave spherical aberration of 0.8 ended up at EER of 0.68 obstructed.

Its the guys doing planetary photography at 4X-5X their native focal length that will pick the difference between 0.95 and 0.8 Strehl wavefronts.

Clearly the lesson is to keep the obstruction to a minimum which generally visual observers, and planatary photographers have the luxury of doing.

Mark

i didnt understand any of that ....

but now i feel strangely aroused

[Analog6 (Odille)]

Ditto on the not understanding, toryglen boy. It's all double dutch to me, some study ahead obviously.

[toryglen-boy (Duncan)]

Originally Posted by Analog6

Ditto on the not understanding, toryglen boy. It's all double dutch to me, some study ahead obviously.

LOL

i did understand it really ....


it was more the passion that was embued by the poster in explaining it

[TrevorW]

Simplified I think it means the're good mirrors for DSO and OK for planetary at high mag but not as good as some.

IMO

[Satchmo]

Originally Posted by Satchmo

I did ray traces once that showed perfected optics with a 45% obstruction producced an EER ( Strehl taking into account the obstruction ) of 0.72 and system with 1/4 wave spherical aberration of 0.8 ended up at EER of 0.68 obstructed.

To calarify :

The Strehl ratio is the proportion of possible light into the the Airy disc that
optics can deliver. In broad terms an unobstructed system can theoretically deliver 1.0 and a system with 1/4 wave P-V of pure spherical aberration delivers 80% into the disc and 20 % scattered into the rings.

My example of the 45% obstructed instrument was to show that a system with `perfect' optics and a Strehl ratio indicating a full 20% more light intensity into the centre of the airy disc ( compared to the the 1/4 wave system ) only achieves a real gain of 5% when the large obstruction is taken into account. In other words it's really a case of diminishing returns seeking perfection in a heavily obstructed instrument..

Hope this is not too arousing

Mark

[marki]

Originally Posted by Satchmo

There is no mystery about this. As far as I know those figures are for primary and secondary combined, as a functioning instrument. The Star Instruments web site , who make RC Optical instruments clearlys state that these wavefront figures are Root Mean Square, not P-V .


Clearly the lesson is to keep the obstruction to a minimum which generally visual observers, and planatary photographers have the luxury of doing.

Mark

Thanks for explaining that again Mark. I couldn't find the thread last night to link to.

Ciao Mark

[Paddy (Patrick)]

Originally Posted by toryglen-boy

i didnt understand any of that ....

but now i feel strangely aroused

:rofl ::rof l::ro fl::r ofl:: rofl:

[gregbradley]

That is a good point. Its the strehl ratio at the eyepiece that should be quoted.

Another reason a good refractor can outperform its aperture compared to compound scopes.

However at a certain size compound scopes take the lead as a 10 inch APO is too big, heavy, long and expensive compared to say a 14 inch compound scope.

Where they meet is a point of debate but I estimate from my experience a good 7 inch APO will match or even surpass a 12.5 inch compound scope even an RC. The RC will get a bit more detail but not necessarily a more pleasing image and will take longer exposure to get there.

A 14.25 inch RC though I would say has a definite advantage over a 7 inch APO but is less versatile as reducers do not work on RC scopes with big chip cameras which nearly all new modern cameras are.

So with that in mind are you better off getting an ED120 or EON120 or the APO127 which I think are a similar proce tag compared to an 8 inch RC with unproven and most lilkely poor optics.

Are these RCs made in Taiwan? If they are I would have more confidence in them. Astrotech make really nice refractors.

Greg.

[toryglen-boy (Duncan)]

Originally Posted by Paddy

:rofl ::rof l::ro fl::r ofl:: rofl:

thank you !!

try the fish, i am here all week

[AlexN]

I think I'll be investing in one of these in the not too distant future.

Alex.

[KenGee (Kenith Gee)]

I think Greg is right, The Chinese are not dumb. If they could make RC as good or better then say Star instruments why would you sell them for less than a 1/5 of the price? If you have ever been to china you would know, the Chinese know more about capitalism then the yanks. In the end you get what you pay for. A good example The cameras that Theo imports are good, but they are not as good as say Sbig. I’m sure that the guy who makes could learn to make cameras as good, but I bet you the price would be about the same.

[Tilt (Michael)]

So you already have one in the mail heading your way Alex?

Michael