Focal length of GSO 10" RC

Backfocus of 55mm?

Shouldn’t it be 233mm backfocus?

Edit: I mentiond 233mm without the reducer. Regardless, did you check the right backfocus requirement?

Backfocus of 55mm?

Shouldn’t it be 233mm backfocus?

Edit: I mentiond 233mm without the reducer. Regardless, did you check the right backfocus requirement?

Unfortunatelly there are two "definitions" of backfocus out there. One is the distance from the back of the telescope to the focal point. I just roughly measured it to be between 210 and 220mm without a corrector. So it clearly is too low as well.

The other is the distance from the back of the corrector/reducer to the sensor. It is a property of the corrector/reducer and it is 55mm from the M48 thread of the reducer for my case and it is achieved by putting the ASI EFL and OAG in front of the ASI 6200 camera.

I am pretty sure it is supposed to be 2000mm instead of 2032mm.

First of all, according to GSO webstie, it is 2000mm.

 http://www.gs-telescope.com/content.asp?id=151

When I was cleaning the mirror of my 10" RC, I measured actual size of the mirror. It came out to be 250mm instead of 254mm.

I know some stores that are selling re-brand of GSO 10" RC states 2032mm.

I believe it is derived from converting 250mm into 10 inch ( which is actually 254mm).

As you mentioned, you can adjust focal length by adjusting center screw on the secondary mirror.

When I received mine, it was at 1987mm. As I recently re-assemblied after celaning the mirrors, I set mine at 1995mm. However, I ended up getting 1990mm after all the collimation is done.

Matthew Proulx:
You can adjust the secondary distance, towards or away from the primary mirror. In this case you need the secondary to move closer to the primary. There is a large nut around the secondary housing, turn the secondary housing clockwise when facing the scope from the front, then lock the large nut against it. Very easy task. 

The focal length is 2000 or 2032 depending on whether the company selling it uses metric or imperial. Ive measured the mirrors of 2 10" to be 250mm not 254mm so 2000 is my estimate.
Mine is 1998 and I did not care to further adjust it. The focus is sharp and the performance is not reduced. It takes a lot to actually degrade the performance, around 100mm.

Thank you, I will try and test this asap.

John Hayes:
The proper way to set the mirror spacing in any Cassegrain type system is to first make sure that the focal plane located is at the correct back working distance (also called the back focal distance).  This is how the mirrors are figured to produce the best image quality.  Small variations in the radii of the primary and secondary will cause each telescope to wind up with slightly different effective focal lengths.  The BWD is the critical number and it’s the first thing to check.  The actual focal length is very sensitive to component radii and to mirror spacing and it’s an irrelevant factor in determining the correct mirror spacing.  Once you have the spacing properly set, then you can do a plate solve to see what focal length you’ve got.

John

Question, If the primary mirror changes position (collimating in one direction) then do I not affect the BWD?

Götz Golla:

Álvaro Méndez:
Hi,

Focal length always differs a little. I have two RCs and they are both slightly under the official FL. 

The important thing here is: how many extension threads are you using? They come with three: two smaller ones and the other twice the size. You need to use all of them for use without any reducer. But if you’re reducing you need to decrease the back distance. I think only with the two smaller ones or the equivalent which is just the big one you’d reach focus.

I am using no extension threads, even without the reducer. So there is no chance to reduce the back distance.

Hi,

how long is the R&P focuser from the telescope wall to the M48 end of the reducer?

Matthew Proulx:

John Hayes:
The proper way to set the mirror spacing in any Cassegrain type system is to first make sure that the focal plane located is at the correct back working distance (also called the back focal distance).  This is how the mirrors are figured to produce the best image quality.  Small variations in the radii of the primary and secondary will cause each telescope to wind up with slightly different effective focal lengths.  The BWD is the critical number and it’s the first thing to check.  The actual focal length is very sensitive to component radii and to mirror spacing and it’s an irrelevant factor in determining the correct mirror spacing.  Once you have the spacing properly set, then you can do a plate solve to see what focal length you’ve got.

John

Question, If the primary mirror changes position (collimating in one direction) then do I not affect the BWD?

Sorry for the slow reply.  The BWD is most precisely defined from the front surface on the primary mirror, which is normally fixed to the backplane.  Therefore in nearly all cases, manufacturers specify a BWD number measured from fixed fiducial surface on the backplane.  For example, Celestron specifies all of their BWD values from a reference plane at the rear surface of the baffle nut.  I'm not familiar with your particular telescope but the manufacturer should specify a fiducial location from which the BWD is measured.  Most RCs adjust the mirror spacing by moving the secondary; not the primary--and that's how I am assuming that your telescope works.  If the spacing adjustment is done by moving the primary, that makes measuring the BWD difficult, which is why it's not commonly done.  In either case, it is important to understand that the mirrors are properly spaced when the image plane is at the correct BWD, which ultimately comes from the optical design.  

Once the mirror spacing is properly set, it becomes a task of correctly tilting the two mirrors so that their optical axis are coincident.  In a RC, you first use geometry to roughly point the secondary at the center of the image plane and then tilt the primary to remove on-axis coma.  Then you have to evaluate the field balance with respect to astigmatism.  If you have to adjust the tilt of the secondary to balance the field, you'll also have to iteratively dial out the on-axis coma using the primary tilt screws.  Being super careful to get the initial geometrical alignment really close when you start helps to minimize how much you might have to adjust the secondary mirror.

John


PS. I went and looked up your telescope and here's the specification for the BWD that I found:  210mm from the male M117x1 thread on the tube.

[ref:  https://www.teleskop-express.de/en/telescopes-4/rc-ritchey-chretien-telescopes-75/ts-optics-gso-10-f-8-rc-astrograph-with-carbon-truss-tube-and-focuser-tilting-10815 ]

John Hayes:

Götz Golla:
BWD was measured to about 210mm without collimator, it should be 233mm, which also explains why I did not get into focus with the collimator.

In other forum posts at cloudy night it is stated that the mirror has a certain design focal length, which is close to 2000mm. Of course it may vary a bit, but I dont think by 53mm.

Given the hard fact that I did not get into focus with this setup with the collimator,, I did move the secondary in by about 4mm. Now the focal length is 1987mm, the BWD is close to 233mm and I do get the focus with and without the collimator.

Only little nuisance is that I have to collimate all over again.

If it turns out that I have been wrong with my assumptions and the way to solve this, I am happy to let you know.  If I was right you will hopefully see a beautiful  first-light image later this month.

BTW.: I did send an EMail to GSO about the problem a few days ago, but I havent got a reply (yet?)

Maybe you already know this but I want to make sure.  Do you realize that moving the secondary by 'x' distance, causes the position of the focal plane to shift by x*m^2, where m is the optical magnification of the system?  The optical magnification is the ratio of the system F/# divided by the F/# of the primary mirror.  If for example, you have an F/8 system with a say, F/2.8 primary, the optical magnification would be 2.857 and changing the spacing by 1mm will move the image plane by 8.2 mm.  Changing the spacing by 4 mm will move the image plane by 32.7 mm, which is a lot.  Changing the mirror spacing moves the image plane and it also changes the optical correction of the system so be careful about moving things around too much.  It is easy to totally screw up the optical alignment to the point where it's hard to dial it back in without going back the the fundamentals of the raw design requirements.

John

Thank you John.  I was aware of a factor, but its very good to know the formula.

I have collimated the telescope last night on stars, and its looking pretty good, even at the edges. See http://www.deepskyinstruments.com/truerc/docs/DSI_Collimation_Procedure_Ver_1.0.pdf for my procedure. The new focal length according to plate solving is 1985mm, which +/- 5mm is what most users seem to have.

The coming night I will make some guided long exposures to see how the stars look at the edges when in focus.

I will also see what GSO and TS will say about the situation. TS has sold the scope with the 0.8x reducer and their  3" focuser pre-configured as a package, so I am sure they will also look into it and advice some day.

For me the situation is a chance to learn and get some in-depth hands-on experience, just as it was with my TAK Epsilon (which was way more difficult to handle). Sometimes its worth to take risks for it

That's ultimately what matters

This was an interesting read. I am also close to pulling the trigger on this model.

I have read many threads where it is considered sacrosanct to even think about moving that secondary as it will torch the proper factory configuration of the scope. I have my doubts to the validity of this because I still find it hard to believe that the folks in assembly at the factory are as invested in perfection as we assume them to be.

Please do follow through with closing this thread once you hear back from the manufacturer and/or TSO. I would really value understanding what position they take on moving that secondary.

So now here is some news from GSO. I asked about the right focal length and BWD and also about the need for a baffle extension.

They say that the right focal length is 2000mm, and the right BWD is 239.8mm. If you have the right values, there is no need for a baffle extension for newer RCs build from three years ago.

However, if the mirror distance is too large, the focal length will be much shorter (1947mm  in my case) which means that you need to remove the extension threads to get into focus, which in turn means that the build-in baffle is not long enough to keep stray light away and you need a baffle extension.

But the solution in that case is not to do all that, but rather to correct the mirror spacing to get the right effective focal length of 2000mm and BWD of 239.8mm.

Here is the 2D Layout of the RC10A GSO sent me:

RC10A-2DL_Model.pdf