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Why focus with the primary not at the eyepiece?

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#1 UncleMossy

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Posted 21 January 2013 - 01:54 AM

A question,

I'm wondering why the primary mirror is used to achieve focus on my C8 Schmidt-Cassegrain?
Are all Cats & Casses like this?
It seems to me that it would be better to focus farther down the optical path, at the eyepiece, like a Newtonian or refractor.
I would appreciate it if anyone has any information about why this is so.

Cheers
Jim

#2 GlennLeDrew

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Posted 21 January 2013 - 04:45 AM

The notion is to increase the range of focus so as to accommodate a wide variety of accessories. In this way an 8" SCT has a range of about 8-10" in focus position. If the primary mirror was to be permanently fixed in place, the focus would need to be located a good 6" *beyond* the racked-in opening of a focuser's draw tube, and a sufficient draw tube travel and/or extension tubes would be required.

#3 Cotts

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Posted 21 January 2013 - 07:28 AM

Jim, you could spend some $ to purchase an aftermarket focuser such as a Feathertouch crayford (the best brand IMHO). You would then use the SCT's focus for 'rough' focusing and the Feathertouch for critical focusing. You would retain the large focus range that Glenn mentioned too.

So many people do this that it is an off-the-shelf item for Feathertouch. Screws right on to your visual back....

Feathertouch

The other good focuser: Moonlite

Dave

#4 Eddgie

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Posted 21 January 2013 - 09:09 AM

Most commercial SCTs and most Maks use this method of focusing.

It is inexpensive and easy to manufacture and helps the telescope say light and compact.

It also allows the scope to focus over a very wide distance... From infinity to a few dozen yards away. In smaller scopes, this allows them to be used as telephoto lenses, but this is not a benefit for the larger models.

The design is optimized for a given amount of back focus. For the C5, C6, C8, C9, and C11, this is about 100mm. This means that they are working at their stated focal ratio when used with 1.25" diagonals. The C14 is optimized for 150mm of back focus (2" diagonal).

The use of 2" components or aftermarket focusers smaller than the C14 results in the scopes working at between f/10.7 and f/11, depending on how far back the focal plane has been moved.

Excessive back focus (200mm or more) results it some aperture loss to most of the scopes.

Using a 2" diagonal and a focal reducer may also result in aperture loss, and in some cases, the scope may not be able to be focused. So while they have a huge range of focus, the range where the scopes are working at their optimal is usually shorter than about 8 inches (200mm) of back focus. Beyond this and most start loosing aperture. This includes the C14. While it allows for a 2" diagonal, it starts to loose aperture once you go past about 200mm of back focus.

Anyway, the design has some benefits, but mostly they are to the producer because of the simplicity and low cost of manufacture.

#5 rmollise

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Posted 21 January 2013 - 09:52 AM

One other huge benefit is that the camera can be very securely mounted on the rear cell, since a focuser is not involved.

Sigh...2-inch eyepeices work just fine with an f/6.3 r/c...

#6 Steve Cobb

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Posted 21 January 2013 - 12:00 PM

Elementary question perhaps but would be helpful to be sure I'm understanding these comments properly. When talking about "back focus," at least with Celestron Schmidt-Cassegrains" what are you talking about and how far "back" from what? Am I correct to think that you start measuring this distance from the back of the visual back or from where a 2" diagonal touches the back of the scope as you tighten the ring that screws it onto the optical tube after removal of the visual back? Does "back focus" mean the distance from that point to where your eye (or camera sensor) ends up after attaching the diagonal? How far is the eye "back" with a 2 inch diagonal on a C-14 and how far is it back with a 1.25 diagonal? Are those distances the same for the C-8 and C-ll? Is the eye the same distance "back" as with the 2" diagonal on a C-14 as when you remove the visual back on a C-8 and screw on the 2" diagonal. Many thanks in advance.

#7 Eddgie

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Posted 21 January 2013 - 01:31 PM

If you unscrew the visual back, you are left with the port on the smaller SCTs. On the larger SCTs, you measure from the back of the 3.25" to SCT thread adapter.

Back focus is measured from the flat back of the port.

The back focus is the point where the focal plane is located. T

When you turn the focuser, the focal plane moves. Turning the focuser counterclockwise moves the focal plane to the rear.

If for example, you put on a 2" adadapter, 2" diagonal, and eyepice, you have to turn the focuser counterclockwise until the focal plane reaches the field stop inside the eyepiece.

The distance from the flat back of the port to the field stop of the eyepice, or to the CCD chip face if imageing, will be the back focus.

For imageing, the recommended back focus is usually about 100mm.

For example, suppose you use an SCT to 2" adapter. If you measure from the rear of the port when the adapter is screwed on, from the flat rear of the port to the end of the adapter will usually be about 30mm to 40mm.

Since the diagonal nose goes into the adapter, you don't count it, so if you used a 2" diagonal, you would only count from the face of the front of the diagonal housing to the top of the eyepiece holder would normally be about 4", so the total back focus to that point would be about 150mm.

However, some eyepecieces have the field stop near the bottome of the barrle. If the field stop is an inch down in the eyepiece holder, then you would only have about 5" of back focus.

As you change eyepieces to eyepieces with different field stop locations, if you have to move the focuser, the focal lenght changes a bit.

Anyway, a standard visual back and 1.25" diagonal usually give a focal lenght close to the stated focal lenght (usually a bit more... For exmaple, a C5 is supposed to be about 1270mm, but often are a bit more than this... Maybe 1300mm).

A 2" diagonal though will make the focal lenght more like 1400mm.

Again, much depends on the actual eyepeice.

These differences don't matter much, but it should be kept in mind because it does change the focal lenght a bit and narrows the true field a bit, and if you let the back focus get out of hand, you can start loosing a bit of aperture.

#8 Eddgie

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Posted 21 January 2013 - 01:38 PM

Hi, Rod.

Well, it works, but "Fine" implies that there is no damage to the image.

Have you ever measured the apeture you are left with if you use a 2" diagonal behind a focal reducer?

I have not yet encounterd an SCT/Reducer combination that did not loose aperture when used with a 2" eyepiece.

The aperture reduction can be anywhere from 5% to 25%.

If you would be willing to measure your own effective apeture and provide the results, I would be happy to tell you how to do it.

And this doesn't mean at all that people should not do it. That is really their own choice.

But "Fine" to me implies that there is no serious impact done to the scopes performnace, and in some cases, the scope will be operating at a much smaller aperture.

In every case where I have asked people to measure this (Focal reducer and 2" diagonal) there has been apeture loss.

Sometimes, it has been quite severe. It depends on the particular scope and configuration.

Or, if you like it and don't mind that you are probably loosing aperture, then that is fine to. We all get to make our choices.

But everyone that has measured it has confirmed that a focal reducer used with a 2" diagonal on a standard SCT reduces the apeture.

Some choose to do it anyway, and that is fine, but at least they know what the impact is.

#9 Jon Isaacs

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Posted 21 January 2013 - 01:41 PM

Sigh...2-inch eyepeices work just fine with an f/6.3 r/c...



No vignetting with the 41mm Panoptic the F/6.3 RC hooked up to a C-8?

Jon

#10 mclewis1

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Posted 21 January 2013 - 02:07 PM

Sigh ... "Many" 2-inch eyepieces work just fine with an f/6.3 r/c...


... but some don't. There isn't enough focus travel with some 2" eyepiece/2" diagonal combinations on some SCTs when using the r/c.

#11 rmollise

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Posted 21 January 2013 - 02:56 PM

Hi, Rod.

Well, it works, but "Fine" implies that there is no damage to the image.


It does work fine, Ed. I have indeed checked a 2-inch eyepiece r/c vice a 2-incher that yields the same magnification and _I_ can't see a difference--much less DAMAGE. ;)

Naturally if you go long enough in focal length with a 2-inch eyepiece, you'll get _vignetting_, sure.

I, like many other folks, have been using 2-inch eyepieces and 6.3 r/cs for the last twenty plus years and...yes... the combo works just fine.

#12 rmollise

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Posted 21 January 2013 - 02:58 PM

Sigh ... "Many" 2-inch eyepieces work just fine with an f/6.3 r/c...


... but some don't. There isn't enough focus travel with some 2" eyepiece/2" diagonal combinations on some SCTs when using the r/c.


Not really. Not that I've ever found--and I include the 12mm Nagler Type II. What can be a problem _with the Celestron model r/c_ is 1.25-inch eyepieces in a 2-inch diagonal with some 1.25 inch - 2-inch adapters. ;)

#13 rmollise

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Posted 21 January 2013 - 02:59 PM

Sigh...2-inch eyepeices work just fine with an f/6.3 r/c...



No vignetting with the 41mm Panoptic the F/6.3 RC hooked up to a C-8?

Jon


Of course there will be. How could there not be?

#14 Don Taylor

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Posted 21 January 2013 - 07:22 PM

There are quite a few similar scopes that focus at the eyepiece.

Most classical Cassegrains, and things like the Vixen VMC200L have fixed mirrors. You give up the advantage of flexibility with accessories (hugh range of back focus) but gain in optical performance from the optimum mirror spacing. There was an article in ATT a year or two ago that discussed the resulting optical errors away from optimum spacing.

Also, it's easier to incorporate collimation adjustments to each element, unlike most SCTs that adjust the secondary only.

#15 rmollise

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Posted 21 January 2013 - 07:33 PM

There are quite a few similar scopes that focus at the eyepiece.

Most classical Cassegrains, and things like the Vixen VMC200L have fixed mirrors. You give up the advantage of flexibility with accessories (hugh range of back focus) but gain in optical performance from the optimum mirror spacing. There was an article in ATT a year or two ago that discussed the resulting optical errors away from optimum spacing.

Also, it's easier to incorporate collimation adjustments to each element, unlike most SCTs that adjust the secondary only.


True, but since the SCT has a spherical primary, it only NEEDS one adjustment, the secondary. ;)

#16 Jon Isaacs

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Posted 22 January 2013 - 02:55 AM

I, like many other folks, have been using 2-inch eyepieces and 6.3 r/cs for the last twenty plus years and...yes... the combo works just fine.



I believe Eddgie's point is that the scope is not operating at full aperture. I think it's based on ray tracing the actual geometry of the scope including the exact relationship between the mirrors that is required for the position of the focal plane.

An 8 inch scope masked down to 6 or 7 inches works just fine but there is some "damage" to the image...

Myself, I don't know what the answer is but I do know what the question is.

Jon Isaacs






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