96 replies to this topic

[cduston44]

Our observatory has a 20" RC from OGS, which we recently sent back to the factory to be realigned. Upon return, we noticed that not all of our eyepieces were able to achieve focus, as the focal plane had been moved closer to the optics (shortened). Our usual optical path is optics - diagonal - 2" eyepiece, and in the current configuration, eyepieces under ~35 mm cannot come into focus. To be fair, the company did this because apparently the placement of the primary-secondary was incorrect, and the result is a dramatic improvement in optical quality...but still, 20-35 mm eyepieces are our workhorses.

 

So, I'm thinking of ways to solve this problem, so that we can use 2" 20-35mm eyepieces again. A few ideas I have, and I'd love comments about any / all of them:

 

1) Get a shorter focuser. This is not simple, as I think the back of our instrument is non-standard. Anyone know custom focuser-makers? We have a Van Slyke right now, but we know he's out of business.

 

2) Low profile diagonal, possibly paired with "shorter" eyepieces. For example, Tele Vue has a nice spec sheet (https://www.televue....page.asp?id=214), I could purchase the shortest barrel eyepieces if I could find a 2" diagonal to match those small barrel sizes (looks like 0.94" is the shortest). How to find a "low profile diagonal"? Is that what these are intended to be? https://www.baader-p...2-part-01b.html

 

3) Switch to 1.25" diagonal and eyepieces since those will have less distance. I've verified this works for our 'scope, but we really like the large size of the 2" eyepieces for public viewing - if you're a 7 year old kid, 2" eyepieces can be the difference between seeing Saturn and not. This is "eye relief", yes? Is there a specific set of 1.25" eyepieces that have similar eye relief to a typical 2"? Put another way - that Tele Vue sheet says that some of their 2" and 1.25" eyepieces have the same ER - would that really be the same viewing experience?

 

4) One suggestion from the company was "get a computer controlled focuser", but this does not help, right? Unless the focuser allowed the draw tube to go further inside the optical path - maybe some of them do that? Any clarification there would be helpful.

 

It's not exactly that "money is no object", but I am very motivated to return to our previous experience, so I will also take ideas that involved purchasing i.e. an entirely new set of eyepieces, a new focuser, etc etc. Appreciate it!

[glittledale]

in the past when i've had a similar problem with some of my short fl 1.25" orthos, i've used astrosystems  2" - 1.25" adapters with deep recesses that allow the eyepiece to sit closer. 

 

here is the astrosystems adapter to show what i mean

https://www.astrosys...ieceadapter.htm

 

if you happen to have a 2.7" draw tube for instance, having recessed 2.7" to 2" adapters made with the same idea  might help.  

 

edit 11/11

if you felt like it, you could even make adapters for all of the eyepieces you use to make them parfocal - which might be nice for public viewings.  

Edited by glittledale, 11 November 2024 - 11:24 AM.

[John Berger]

why not just remove the diagonal? you wouldn't have to do gymnastics to view without a diagonal; if your object is higher in the sky, then you can just sit lower, right?

Edited by John Berger, 10 November 2024 - 07:26 PM.

[luxo II]

I would take a close look at the primary mirror support cell, and the secondary mirror cell.

 

To put the focal plane a little further out you need to bring the mirrors closer together and if you have the typical 3-point collimation screws this could be accomplished by turning all three screws 1 or 2 turns. As little as 2-3mm may be sufficient, because the focal plane shifts by the delta multipled by the secondary magnification squared.

 

You will have to re-collimate the scope after doing this.

[Avgvstvs]

I would want my money back after all the problems you have had.

[TOMDEY]

I would take a close look at the primary mirror support cell, and the secondary mirror cell.

 

To put the focal plane a little further out you need to bring the mirrors closer together and if you have the typical 3-point collimation screws this could be accomplished by turning all three screws 1 or 2 turns. As little as 2-3mm may be sufficient, because the focal plane shifts by the delta multiplied by the secondary magnification squared.

 

You will have to re-collimate the scope after doing this.

cduston44 says of the factory alignment "the result is a dramatic improvement in optical quality"; they adjusted the PM-SM spacing to correct the spherical aberration. Fiddling with the alignment of the mirrors at this point would probably meaning sending this 20-inch RC right back to the factory!

 

I measured a bunch of commercial Star Diagonals. The 2-inch Lumicon LumiBrite Star diagonal is the shortest added path of all 2-inchers (95.5mm). I don't know if this will bring all of your eyepieces to focus, but it's certainly the best possible (both in theory and in practice) in the 2-inch format.     Tom

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[luxo II]

Ya I realize, Tom, if they don't have the means to collimate it. 

 

I had the same problem on one of my Rumaks - very limited backfocus - and found the lowest possible low-profile 2" focuser from TS in Germany, but still had to have a custom adapter made to attach it to the OTA and adjust the mirror separation a little for visual. And collimation took a while to get it spot-on again.

Edited by luxo II, 10 November 2024 - 09:05 PM.

[Jon Isaacs]

The focal plane position depends on the individual eyepiece and not it's focal length. 

 

Some shorter focal length eyepieces should focus.  What eyepieces do not come to focus?

 

What diagonal are you using? The optical path length of various diagonals varies quite a bit.

 

Barlows can move the location of the focal plane...

 

Jon

[ihf]

Maybe build a Baader 32mm T-2 prism diagonal (page 9) with the right kind of minimum length adapter for your scope (possibly Baader Reducing-Ring 2"i / T-2a #2958244, page 16). Maybe there will be enough space for a 2 inch clicklock for the eyepiece (page 17)? If you have enough backfocus step up to the 36mm prism which should support the 30mm UFF.

[SeattleScott]

There are a few lines of great 1.25” eyepieces with long ER for easy viewing. Delos, Delite, Pentax XW, Nikon SW. The Televues typically focus further out than other brands, which could be helpful, although it sounds like you can reach focus with a 1.25” diagonal. If nothing else, you might be able to keep using the 2” diagonal and still use Televue 1.25” eyepieces. However, these eyepieces only go up to 18.2mm so might not hit your sweet spot.

My ES 24mm 82 focuses pretty far out compared to 1.25” eyepieces, also my 42LVW. I mean you want eyepieces that focus farther out, right? What 2” eyepieces are you currently using that you are unable to reach focus with.

[luxo II]

There is one eyepiece that focuses so far out it needs a 50mm extension tube in my scope - the Vernonscope 48mm Brandon. Should be very nice in a 20" RC.

[eblanken]

Hi All,

 

From a diagnostic approach, I suggest OP provides us with photos and/or a sketch. Also it would be helpful to have a tabular list of eyepieces and measurements of focuser settings from full-in-travel to full-out-travel (and a "go/no-go" on achieve focus). I'm a measurements guy, so details like mm and cm can sometimes uncover issues & solutions (like Jon was asking). Also a tabular list of the eyepiece specifications with barrel size (2 inch vs 1.25 inch) would be helpful. We could from eyepiece model number cross reference to Coma Corrector II Settings and get a better sense of what is happening for/to the OP. How about a 1.1x Barlow to "bump out" the focal plane ?

 

Best,

 

Ed

 

P.S. Another thing to try (as a diagnostic) is to point the RC to "an object closer than infinity" which should shift the focal plane so more eyepieces achieve focus for the tabular data sets I'm asking for . . . just saying ?

Edited by eblanken, 10 November 2024 - 11:20 PM.

[eblanken]

Hi Again,

 

Two Three additional thoughts:

 

(1) Investigate a "T2" diagonal from sources such as Baader that can have an intermediate light path length between 1.25 inch and 2 inch (ie. I am agreeing with ihf) which can effectively give you some more in-travel and can be configured with 2 inch male & female fittings

 

and

 

(2) Investigate eyepieces that are really 1.25 inch, but also have a 2 inch option, such that the field lens is farther into the optical path. Examples of these are Baader Hyperion and Vixen LVW which both have a large eye-lense and "look like" 2 inch eyepieces to the viewer. One caution is that some of these insert far enough that eyepiece hitting the mirror may be problematic

 

and 

 

(3) I know lots about viewing with eyeglasses and the need for Eye Relief (ER). Yes, it is part of the "success/comfort factor" in a public observatory setting with a wide variety of experience "at the eyepiece" and such. I have a friend who has discussed this with me at some length. Perhaps I can help ?

 

I have some additional suggestions, but perhaps a Private Message (PM) may be better for these.

 

Best,

 

Ed

Edited by eblanken, 11 November 2024 - 12:53 AM.

[SeattleScott]

Hi Again,

Two Three additional thoughts:

(1) Investigate a "T2" diagonal from sources such as Baader that can have an intermediate light path length between 1.25 inch and 2 inch (ie. I am agreeing with ihf) which can effectively give you some more in-travel and can be configured with 2 inch male & female fittings

and

(2) Investigate eyepieces that are really 1.25 inch, but also have a 2 inch option, such that the field lens is farther into the optical path. Examples of these are Baader Hyperion and Vixen LVW which both have a large eye-lense and "look like" 2 inch eyepieces to the viewer. One caution is that some of these insert far enough that eyepiece hitting the mirror may be problematic

and

(3) I know lots about viewing with eyeglasses and the need for Eye Relief (ER). Yes, it is part of the "success/comfort factor" in a public observatory setting with a wide variety of experience "at the eyepiece" and such. I have a friend who has discussed this with me at some length. Perhaps I can help ?

I have some additional suggestions, but perhaps a Private Message (PM) may be better for these.

Best,

Ed

Good tips, just don’t combine #1 and #2. Either or, not both.

[eblanken]

What Scott said (#1 XOR #2) not (#1 AND #2).

 

Best,

 

Ed

Edited by eblanken, 11 November 2024 - 03:13 AM.

[ausastronomer]

To be fair, the company did this because apparently the placement of the primary-secondary was incorrect, and the result is a dramatic improvement in optical quality...but still, 20-35 mm eyepieces are our workhorses.

 

 

I'm not sure why you're being so fair to the company.  You paid a very large amount of money for something that wasn't right in the first place, when it was delivered, and it should have been!

 

When you buy a new telescope, particularly something that costs as much as they do, it should be absolutely perfect when it's delivered, and the vast majority of eyepieces should reach focus.

 

Cheers

[RichD]

Is this scope primarily intended for imaging and not optimised for visual use from the manuf?

Seems strange that eyepieces worked fine with incorrect mirror spacing and now don't with the correct mirror spacing. Is this scope just intended to have a camera/sensor at the rear and no diagonal?

I too can't see what difference a computer controlled focuser would achieve. It wouldn't change the focal plane position.

Otherwise agree with ausastronomer above.

[Jon Isaacs]

Optical Guidance Systems manufactures research-grade observatory systems for professional and discriminating amateur astronomers. OGS can also build custom instruments tailored to the intricate and demanding requirements of our customers.

 

 

It seems to me, these are not meant for visual.  In any event, if it is being used visually, there are a number of solutions.  Shorter path length 2 inch diagonals, 1.25 inch diagonals, eyepieces that will come to focus.  One could also use a Barlow as an Optical Path corrector. 

 

We are shooting in the dark since we don't know what eyepieces, it any, do come to focus.  The Baader Morpheus might work as 2 inch eyepieces.

 

Regarding the need for collimation:  It is possible that someone at the club moved the mirrors in order to get it to focus with eyepieces. It's an RC so mirror spacing is critical.  

 

Jon

[chemisted]

I have used my OGS RC-10 visually for 24 years with all makes of eyepieces without problems. The OP has plenty of options, but, as others have said, he needs to provide some specifics.  My current favorite diagonal is the Baader BBHS T-2.  It has a very short 43mm pathlength and a clear aperture of 33mm.

Edited by chemisted, 11 November 2024 - 10:03 AM.

[TOMDEY]

This may sound drastic... but considering it's a precious 20-inch RC >>> You could take your longer focal length eyepiece and literally "cut the end off" --- which may well include its field stop going in the scrap bin. Might also need to then cut down the receiver end of a 2-inch star diagonal so that eyepiece can nestle in farther than usual. Permanently mate that dedicated pair as a "custom 90^o eyepiece". It would come to focus nicely and become part of the dedicated eyepiece kit for the 20" RC, without modifying the valuable telescope at all. The edge of the visible field will be defined by the star diagonal... which is often the case anyway with a very long wide field eyepiece. As long as you are removing only metal structure, and not glass... this should work nicely!    Tom

 

PS: At work (Kodak Research Labs) we often butchered stock hardware to custom fit our concoctions. We'd take the parts to the opto-mechanical machine shop and the tool & die makers would perform the surgeries with operating room skill.

Edited by TOMDEY, 11 November 2024 - 09:03 AM.

[chemisted]

Here is another thought.  When I purchased my OGS they were all supplied with Astro-Physics focusers.  If the OP has such a focuser all that may be required is an alternative adapter that provides more in-travel.  Specifically, this is one that I use when I have a focal reducer in the train:  https://www.astro-physics.com/ada20132

[Starman1]

A diagonal with a shorter internal light path might help, as would drop in adapters for 1.25" eyepieces.

Ultimately, though, the answer is a shorter focuser. You might be able to get one from Starlight Instruments, Planewave, or Clement.

Edited by Starman1, 11 November 2024 - 01:52 PM.

[ihf]

Cutting the end off only works with eyepieces that have a focal plane far away from the glass. Say Ploessl and I think some 5 element eyepieces like IIRC Masayama or UFF 10mm come to mind. All fancier ones won't work after that surgery, or cutting has no benefit as the shoulder doesn't move. (I have played with that idea while trying to mod a small spotting scope.)

[SeattleScott]

Using Morpheus eyepieces as 2" won't solve the issue because that doesn't change the position of the focal plane in the eyepiece.
A diagonal with a shorter internal light path might help, as would drop in adapters for 1.25" eyepieces.

Ultimately, though, the answer is a shorter focuser. You might be able to get one from Starlight Instruments, Planewave, or Clement.

It doesn’t change the focus position but it changes the focus travel.

[luxo II]

Ok so the options are:

 

1. Alter the mirror spacing to shift the focus back out.

2. Swap the focuser for a shorter one. 

3. Find a diagonal with shorter light path.

4. Hack the eyepieces to sit further in.

 

#1. Is feasible but means recollimating. No hardware mods.

#2. The VSI focuser isn’t the lowest profile possible which suggests a swap. Requires an adapter to be made and might yield  just a few mm.

#3. Might be possible, might not.

#4. Personally I would not start chopping eyepieces. They’ll never be the same afterwards.

 

My money is on #1, you’ll have to learn to collimate it.

Edited by luxo II, 11 November 2024 - 07:31 PM.