New GSO 10 inch RC - Page 5

Bassnut (Fred) · #81 09-12-2009, 06:26 PM

Well, I was tring to avoid the brand thing, and just talk about focus consistancy (not absolute focus, which granted is tighter on the RCOS).

There well maybe other reasons for the focus change, I dont know, cant see any.

The focuser I use on the Meade has temp compensation (TCF-S), which I never bothered calibrating, dumb I know, call me lazy, would probably solve the problem in one hit.

In the end, the diff bettween aluminium and CF is there and significant IMO, but can be overcome with care or a temp compensating focuser. Whether the cost difference is worth it is another story given focus drift can be overcome, but as Peter says, its the last mile that costs...... And the convienience of no drift CF to start with sure makes imaging more enjoyable.

bmitchell82 (Brendan) · #82 09-12-2009, 06:42 PM

That granted the CF doesn't fluxuate with temperature like ally. but if you think that it only moves in one direction it doesn't it will bloat the OTA as well as increase its length. now theres your problem what is the back of ota made of? and the focuser tube?

there was a really good youtube presentation on a guy with a 32" Relay cassigrain look for the link in the forums. now he shows where you need the Zero shift though careful design. and it didn't stop at the tube (he was using a CF truss setup) and a aluminium primary mirror box, but the link between the mirror box and primary mirror fixing was CF. Zero shift! and carbon fibre in the secondary-primary mirror tubing admittedly there was a series of lenses mounted within. If you want to go that extra mile you should have your odometer calibrated, no use saying ive gone the extra mile just because the odometer says so. whoops i had mini minor wheels on my tractor?!

oh and for a bit O info here is a little link for non belivers.
http://www.engineeringtoolbox.com/th...als-d_859.html

Kal (Andrew) · #83 09-12-2009, 08:28 PM

Quote:

Originally Posted by AlexN

A mate of mine and I were discussing this just last night, the thermal expansion/contraction of an aluminium tube the size that we would guestimate the GSO RC10 has would be, Given a 20°c temperature change, the worst case scenario (assuming very poor quality Aluminium) was a contraction 45 microns.

The F/8 optics of the GSO RC10 give a CFZ (Critical Focus Zone) of 140 microns. Hence, given you focus well at the start of the imaging run, and don't experience more than a 20°c drop in temperature during the imaging run, your focus would not noticeably drift over the course of the run... Its a different story when you're imaging with a fast system, Ie, an F/4 scope with a CFZ of 35 microns.. but with the slower ratio's, your focus would very rarely shift.. not before you hit the meridian and have to flip the mount anyway, at which point, how hard would it be to just touch up the focus before recommencing the run after the flip?

That contracton of 45 microns will be far more pronounced in a two mirror telescope. It is not a simple 45 microns at the focal plane, it is a difference in seperation of the two mirrors by 45 microns, which will shift the focal point by a much larger number (unless your calculations were far more complex than I give you credit for!!!)

bmitchell82 (Brendan) · #84 10-12-2009, 12:18 PM

andrew your still missing the point. you have a CF tube cool. what comes out the back of your telescope is aluminium.... and if im not mistaken RC's inherently have a whole lot of back focus so that you can add in your filter wheels OAG's field flatteners ect ect ect.

You just shot yourself in the foot because what you just took out over there is still over here.!? I smile every time somebody does something and thinks they have eliminated it and then preaches to the world. do the whole lot properly and then you can say you have eliminated it as i stated in my previous post.

As for 45 micron, can you honestly show me what 45 micron looks like? ill tell you now that 45 micron is BLOODY SMALL. and to give you a point to think about when talking about microns, what focuser do you use? if your using a manual focuser you make me laugh as i will 100% know you haven't got a hope of making a sub 500 micron adjustment. then you have to take in atmospheric conditions wow there goes your 45 micron again! I for one can sit there on a dead still night with a ED 80 on a EQ6 Pro mount and watch my FWHM jump from 2.3 to 1.6 and back up without touching the scope! If your using a really high end focuser aka Feather touch or even better with full robotic control, you would only be looking at 100 micron adjustments maybe a little less. but in the same case as i pointed out with my ED 80 setup your chasing seeing to a degree.

IMHO unless your talking about massive optics, super fast F ratios, and getting quality only seen in really high end optics/scopes your flogging the rotting carcass of the horse. sure make things as good as you can but save your sanity as there are alot of other things to worry about before you start worrying about a 45 micron shift in 800-1 meter of tubing!

Bassnut (Fred) · #85 10-12-2009, 12:44 PM

Quote:

Originally Posted by bmitchell82

what comes out the back of your telescope is aluminium.... and if im not mistaken RC's inherently have a whole lot of back focus so that you can add in your filter wheels OAG's field flatteners ect ect ect.

Thats a good point actually, and up to 8" of aluminium spacer tubes, mmmm.

bmitchell82 (Brendan) · #86 10-12-2009, 01:08 PM

for true zero shift telescope you need to get rid of alot of things!. Isolation is the name of the game.

CF in conjunction with Al make for a good combination. CF to join the secondary mirror to the top tube ring, CF for the scope tube to the bottom scope ring, CF to join the bottom scope ring to the primary mirror fixture. Good design will limit the use of tube spacers to get focus. aka design the scope for your end game! that is pick what equipment your going to use and measure it exactly. make the actual scope adjustable to within a poofteenth of where you need to be for focus. that way you can place your sensor in the Critical Focus Zone without having to use 15cm of spacers to get there.

GSO RC scopes are what i call top of the bottom of the range similar to a EQ6pro but they are limited to what they are doing because they need to appeal to the whole community (silly if they didn't) that is, people with DSLRS, DSI's, SBIG's, OSC, Mono, AO, OAGs.

Hence why they make it there setups the way they do so that joe blogs with what ever can make it work! sweet made a sale! and why RCOS and DS instruments and the like are so damn expensive, they go the extra mile, and a basic assumption that your using high end gear on the scope which slots you in a limited range but within saying that gives you Aston Martin proformance!

Head to the Deep sky instruments web page and all their astro photos taken are with Dedicated CCD's, not to discourage would be buyers, but if your coming to buy something like that your normally not about to buy your first scope!

I myself am trying to source some GSO 6" RC mirrors and will be designing a little travel/photography scope. the aim of that game will be as light as possible with quality setup so i can whip it in my suite case and have something to go with! I have access to machines that are accurate to .001 mm yes thats 1 micron but my tolorances will not need to be this tight. and i will assure you that im not worrying about a 20 degree temperature differential (unless im in the sahara desert) so alot of my equipment will be ally with a bit of CF for asthetics

Kal (Andrew) · #87 10-12-2009, 01:30 PM

Quote:

Originally Posted by bmitchell82

andrew your still missing the point.

I am not missing any point. In fact, if you read my post, you will see quite clearly that you missed my point. Mirror spacing has a far greater effect on focus than the tiny focal length that an aluminium focuser has. The focal length of a focuser is maybe 100mm at most, so any contraction that it experiences through a change of temperature is very small. The change of separation between two mirrors in a telescope, however, can have a much larger effect. If the tube is X mm and you get an expansion/contraction that Alex calculated at 45 microns, it will equate to a change in focus at the focal plane of a much higher number. think of a SCT - if you add an external focuser your focus control is very slow compared to how much focus change you get from moving the primary mirror. I don't have the exact math, but a change of seperation of the two mirrors by 45 microns will certainly not equate to a change of 45 microns at the focal plane - because of the secondary magnification it will change the focus point by 5x this amount to 225 microns.

bmitchell82 (Brendan) · #88 10-12-2009, 01:42 PM

225 micron is still insignificant in the quality of our amature setups! and I didn't miss your point one bit. im just saying that your flogging a horse thats already dead because the average human hair on your head is 100 micron rip 2 hairs out sit them together and tell me your still serious that your going to be able to get that kind of accuracy in a mass produced product.
http://wiki.answers.com/Q/What_is_th...air_in_microns
are you seriously telling me that your optical setup is going to be that exact? if it is then i stand aside as you have way more money than i will ever have.

and if i belive it to be correct that expansion is at a 20 odd degree differential. now normally youll get around 10 degrees which is half the 20 so lets half the 45 for a start to 22.5 and times that by your x5 112.5 micron!

It would be like me specifying in the engineering plans for house that all connections and interfaces must be within 1 mm... the contractor will laugh in your face followed by are you serious.!

bratislav (Bratislav) · #89 10-12-2009, 01:46 PM

Quote:

Originally Posted by Kal

think of a SCT - if you add an external focuser your focus control is very slow compared to how much focus change you get from moving the primary mirror. I don't have the exact math, but a change of seperation of the two mirrors by 45 microns will certainly not equate to a change of 45 microns at the focal plane - because of the secondary magnification it will change the focus point by 5x

More like 26x (secondary magnification squared, plus one). That is why SCTs have such a huge focusing range for relatively small movement of primary !

bratislav (Bratislav) · #90 10-12-2009, 01:50 PM

And if anyone missed that, let me rephrase.
Yes, for 45micron spacing change between primary and secondary in a typical SCT (5x secondary magnification) focus will move more than one millimeter !
Now you understand why they use carbon trusses in high end RC designs ... It is not for looks !

bmitchell82 (Brendan) · #91 10-12-2009, 01:55 PM

Thankyou for that Bratislav i am no expert in magnifications hence why i didn't factor that part in. Its great to have somebody with experience and knowledge in these matters weigh in.

It still comes back to my original statement that isolation is the only way you will get rid of the problem, isolation costs $$$ and alot of it for high end tolorances!

Kal (Andrew) · #92 10-12-2009, 02:05 PM

Quote:

Originally Posted by bratislav

More like 26x (secondary magnification squared, plus one). That is why SCTs have such a huge focusing range for relatively small movement of primary !

Thanks mate. I knew the concept, I just didn't know the exact math of how to calculate it.

Kal (Andrew) · #93 10-12-2009, 08:13 PM

OK, I've decided to put the effort in and do the math open to the public so if I get it wrong feel free to correct me. I will have to make a few assumptions, since I don't know the full specifications of the GSO 10" RC

Calculating Expansion of the tube - which effects mirror seperation

Expansion = z * L * T

Where:
z = Coefficient of linear expansion of the material
L = Initial length (metres)
T = Temperature Change (celcius)

For aluminium, z = 0.0000231 per degree
I'll assume a tube length of 40cm (I don't have measurement)
I'll calculate for temperature change of 20 degrees

Therefore expansion (which is a contraction since the temperature change is a negative value) = .0001848 metres = approx 185 microns

Calculating focus shift

Focus shift at focal plane = -Am²/[1-A(m-1)B]

Where:
A = change in mirror separation
m = secondary magnification
B = secondary mirror to primary focus separation, negative in sign for a cassegrain telescope (the RC layout is a cassegrain configuration)

A will be 185 microns
m I will assume to be around 4 (the scope is F8, so maybe F2 primary and 4x magnification secondary?)
B I'll assume to be about the tube length I used earlier, 40cm.

Therefore focus shift works out as 0.002955 metres, or nearly 3mm for a 20 degree drop in temperature !

Bassnut (Fred) · #94 10-12-2009, 09:23 PM

So how much say does 150mm of aluminium spacers at prime on the above example make to focus shift with a 20deg drop in temp ?.

Assume the tube is CF and has no drift at all, ie what affect do the spacers have by themselves. I guess the formula is different as the mirror spacing is constant?.

AlexN · #95 10-12-2009, 09:30 PM

~63 microns expansion for the tube in a 20 degree temp drop Fred..

Expansion is expansion, if your mirror spacing does not change, the only thing you have to calculate is how far the sensor is moving with regards to the focal plane in a given temperature drop.. 63 microns would be well within the CFZ of your scope, hence, as you mentioned earlier, you get 0 focus shift night after night with your scope...

Bassnut (Fred) · #96 10-12-2009, 09:38 PM

Thanks Alex. Then its interesting that RCOS include automatic focus temp compensation in the control system on the tube, makes me wonder why (mines not calibrated, so its not on, never needed it).

AlexN · #97 10-12-2009, 10:03 PM

It snows in some places

I dare say that, combined with wind chill factors would cause more serious expansion than what we commonly experience in Australia...

Its funny that you have TCF-S on your SCT, and TCC on your RCOS, yet in both cases have not calibrated either for temp compensation

My focus doesnt SEEM to shift much in my APO, however in my C11 it would shift almost hourly as the optics and internal tube temp started to equalise.. If I'd had the ability to have temp compensation, I would have!

gregbradley · #98 10-12-2009, 10:36 PM

With the FSQ106 you need to refocus every 2 degrees C change in temp. That is F5.

With my other refractors I see focus shift when temp drops perhaps 4C but 2C is not that noticeable. These are F7 and F7.5

So I take it the faster the scope the more aluminium tube will cause focus differences.

The lens also changes focus with temp and this reduces the amount of focus change on some scopes per Roland Christen. He recommended refocusing with every 1C change in temp. That would be OK if you are using Focus Max or temp compensating Robofocus, FLI PDF and or using CCD commander to automate your runs.

Otherwise I can't see myself hanging around my scope all night in case the temp drops 1C to refocus.

The BRC250 has an aluminum tube but it has temp compensating rods I think in front of the corrector to move it when temp changes. It was not that sensitive to temp changes but still a bit.

CF seems the way to go for compound scopes but not APOs where
cooldown time is important. CF also has more tube currents I am told.
Hence another reason for the fans.

Greg.

AlexN · #99 10-12-2009, 10:51 PM

Greg, Yeah I mentioned fast refractors earlier in the thread.. My TMB 80mm F/6 seems stable at prime, however insert the reducer and run it at F/4.8 and all of a sudden I get shift... I run a JMI motorfocus through a FCUSB controller for auto focus, and while its not temp compensating, I can script in focus confirmation through CCDAP.. There is no way I'd sit there watching a thermometer all night refocusing manually every hour or two...

The reason the faster scopes are more sensitive to thermal expansion is that the faster the optics, the slimmer the Critical Focus Zone.. at F/6 mine is 79 microns where as at F/4.8 its 50.6 microns... I'd hate to be trying to manually focus an F/2 camera lens with a CFZ of just 8 microns...

Hagar (Doug) · #**100** 10-12-2009, 11:14 PM

I have a robofocus unit fitted to my FSQ106Ed and use temp compensation. It is a requirement with this scope.
I also live in an area with probably the biggest shift in temprature or close enough in the country but while daytime tempratures can be as high as 25 during winter and overnight minimums down to -4 it has to be remembered that by the time I start imaging the temp has usually fallen to 0 degrees already so a variation of 4 or 5 degrees is not an awful lot. On most nights I only require 2 or 3 temprature compensation changes during an entire night. if I use the focal reducer I need a few more. f3.6 is just too fast.
The thing which must be remembered with these 10" scopes is that CF or aluminium tubes is probably of no concern. The mirror cell and the front of the tube are connected with huge slabs of Aluminium top and bottom. In reality you are probably better of with the Al tube as there will be less wear and tear on the aluminium to aluminium surfaces with equal expansion coeficients than Al to CF with difering coeficients. Focus changes can be compensated for with hardware and software but wear and tear cannot.