Orion Optics VX12 focuser upgrade

[Trentend]

Looking to upgrade the focuser on my scope to either a moonlite or Baader diamond steeltrack. I use the scope with a denkmeier binoviewer and 24mm panoptics - have crisp views at low power but can’t focus on the medium and high. Previously thought this was poor collimation (and it may still be) but when I had a look through it the other night I noticed a significant amount of sag with the medium and higher settings as the focuser travel out is much greater. Wonder if this explains the poor views?

I need to replace the focuser as it’s a bit battered and the fine tuning mechanism doesn’t really work.

Any thoughts on the best one to get and what accessories such as fitting plates etc would I need (I have previously used the Baader clicklock mechanism so leaning towards that). Really hope I can get this scope to work as it’s so easy to set up and transport.

Edited August 22, 2019 by Trentend

[Space Hopper]

I'm still a bit puzzled as to why you can't get focus on mid or high powers ??

May be worth asking on the CN forum (bino-viewers). 'Eddgie' who is the expert on there may be able to help.

If you could post a pic of this alledged 'sag', that would be useful.

 

I went for the Moonlite CR2 from FLO, but with the non standard extra long drawtube. Mine is a 60mm, and they used to do a 70mm as well.

As these are non standard there may a wait to get one shipped in from the US (a month or so)

However, the 'standard' 50mm will probably be ok : If you use the large 1" spacer or both the 1" and 1/2" that they supply.

Another possible option is to raise the primary up to its higher setting (OO have 2 different settings for this ; mine does)

 

Its a bit of a trial and error thing with a bino / powerswitch / newt setup. A bit of a juggling act.

What you want to look for is being able to get the low power switch to focus as 'close in as you can' by either raising the primary or trying different focuser spacer options etc.

You can of course simply adjust the OCS position to do this, but remember, the further out you set it the higher the base power will be, ie it won't be the advertised 1.2 or 1.3x

but more like 1.4x or 1.5x. And we want our low power to be as low as we can for those wide field, rich field views. Its quite a feat to be able to get 60x with a 12" Newt with a binoviewer.

Post some pics and keep us posted with how you get on.

 

https://www.firstlightoptics.com/moonlite-focusers/moonlite-cr-dual-rate-tri-knob-crayford-focuser-for-newtonians.html

[John]

I put a Moonlite on my Orion Optics 12" F/5.3. The original was a single speed Chinese unit probably made by GSO. It didn't sag but I felt that the dual speed was desirable with this scope. I'm very pleased with the Moonlite. I've used both the spacers in the Moonlite fitting kit. I found that the 10 inch one fitted the Orion Optics tube - it's slimmer than the Chinese 12 inch dobs use.

 

 

[Piero]

I would suggest the Baader one. The bearings play on a steel surface and the focuser doesn't use teflon inside. 

The drawtube length is 40mm though.

If your telescope is f4, you need a coma corrector - CC. With a paracorr type 1, the focuser alignment error tolerance is much smaller (0.005xD mm) than without (0.03xD mm). 

Edited August 29, 2019 by Piero

[Trentend]

Thanks both. Potential school boy error. Have removed the slack/play (there was at least 5mm) by tightening the two adjustments screws (yellow arrows). The fine focuser (red arrow) doesn’t work and never has so still want to replace the unit. Have a free evening so going to reacquaint myself with the astrobaby collimation guide... One thing I always notice when using my Hotech to align the secondary is I have to apply a lot of pressure to get the laser in the donut ring on the primary. Suggests the secondary still isn’t in the right position perhaps.

[Trentend]

4 minutes ago, Piero said:

I would suggest the Baader one. The bearings play on a steel surface and the focuser doesn't use teflon inside. 

The drawtube length is 40mm though.

If your telescope is f4, you need a coma corrector. With this, the focuser alignment error tolerance is much smaller than without CC. 

I have a denkmeier OCS A45 which I believe should address this?

[Piero]

1 minute ago, Trentend said:

I have a denkmeier OCS A45 which I believe should address this?

I never used it so, cannot answer.

[John]

I didn't realise that your scope was F/4. It needs somewhat more precise collimation than my F/5.3. The collimation "sweet spot" will be a lot smaller.

 

[Piero]

19 minutes ago, John said:

I didn't realise that your scope was F/4. It needs somewhat more precise collimation than my F/5.3. The collimation "sweet spot" will be a lot smaller.

That's my concern too, but the OP didn't say whether it's and F4 or not. 🙄 

[Trentend]

Apologies yes an F4 so appreciate smaller margin for error. Presume this is more critical the higher the magnification (fine on low power, stars shine nice, bright and pin sharp but not if medium or high).

[Piero]

On 22/08/2019 at 19:47, Trentend said:

Apologies yes an F4 so appreciate smaller margin for error. Presume this is more critical the higher the magnification (fine on low power, stars shine nice, bright and pin sharp but not if medium or high).

 

The coma free area is about 16 degrees FOV in a F4. Assuming good optics, that's the area better than what is called diffraction limited optics. Of course, this changes if you use a coma corrector.

If the telescope is not properly collimated, you will see coma on axis (primary mirror axial misalignment), out of focus off axis stars (focuser axial misalignment), or uneven field brightness (secondary mirror misalignment).

Stars are tighter at low power, but the above issues are only solved with careful collimation.

At F4 the error tolerance for the primary mirror axial alignment is only 0.32mm. As the laser beam is barlowed, the error is magnified 2 times. Therefore, the silhouette of the primary mirror that is projected on the 45 degrees surface of your laser collimator should not shift more than +/-0.64mm. 

If you do use a paracorr type 1, the error tolerance for the focuser axial alignment is 1.5mm. I don't know your laser, but with my Glatter laser I use the 1mm aperture stop to reduce the size of the beam. Instead of checking the beam on the primary mirror, I check the beam on the silhouette projection on the 1mm aperture stop. That's more comfortable.

Edited August 29, 2019 by Piero

[John]

The info that I've seen suggests the collimation "sweet spot" of an F/4 mirror is 1.4mm compared to 3mm for an F/5.3. Outside of this the scope still works of course but it's not giving diffraction limited performance, apparently.

An F/8 mirror has a wacking great 11mm sweet spot !

Edit: Piero's post contains much more detail !

 

[Piero]

On 22/08/2019 at 22:11, John said:

The info that I've seen suggests the collimation "sweet spot" of an F/4 mirror is 1.4mm compared to 3mm for an F/5.3. Outside of this the scope still works of course but it's not giving diffraction limited performance, apparently.

An F/8 mirror has a wacking great 11mm sweet spot !

Edit: Piero's post contains much more detail !

 

John, the formula that you referred to is the linear coma free region. That is 0.022*F^3  where F is the F-ratio. As you said, in an F4 that is ~1.4mm, in a F5.3 that is ~3.3mm. This is evaluated with the eyepiece field stop.

I find that formula rather difficult to use though as coma does not increase with magnification. While magnifying, the FOV decreases (=>less coma), but the Airy disk increases (=>more coma), so coma remains the same basically. I tend to prefer a formula that describes the angular coma free region, instead. There are a few, but an easy one to remember is N*F^2, where N=1. This results: 16 deg for F4, 25 deg for F5, 36 deg for F6, 49 deg for F7, etc. Basically, for an F7 telescope, the coma free region is like a Plossl eyepiece. This is independent of magnification. N is a factor depending on how much one is tolerant to coma. N=0.5 means detection smaller than 3 arcmin.

 

For collimating primary and focuser axial alignments, the numbers in my previous post are about error tolerances. Roughly speaking how much the collimation error can be large before we lose the "sweet spot" completely (=miscollimated telescope). The formulas are 0.005 * F^3 for the primary mirror axial alignment, and 0.03 * D for the focuser axial alignment (without paracorr type 1). A paracorr type 1 reduces the tolerance to 0.005 * D. Type 2 reduces the tolerance furthermore. These are evaluated with a collimator. 

 

---------

As you said, outside the coma free region (of a collimated telescope), the telescope still works and can provide good views. The best ones are on-axis though.

Edited August 29, 2019 by Piero

[Space Hopper]

A Coma Corrector is not needed with the OPs binoviewer set up.

The OCS will remove it sufficiently for it not to be as issue. Baader do a similar item for use in their own systems.

 

Regarding collimation, i've never had a problem with my own F4 which i find just as easy to do as when i had my old 10" F6.3

(i also use the excellent Glatter / Tublug combo)

 

Edited August 22, 2019 by Tubby Bear

[John]

9 minutes ago, Piero said:

 

John, the formula that you referred to is the linear coma free region. That is 0.022*F^3  where F is the F-ratio. As you said, in an F4 that is ~1.4mm, in a F5.3 that is ~3.3mm. This is evaluated with the eyepiece field stop.

I find that formula rather difficult to use though as coma does not increase with magnification. While magnifying, the FOV decreases (=>less coma), but the Airy disk increases (=>more coma), so coma remains the same basically. I tend to prefer a formula that describes the angular coma free region, instead. There are a few, but an easy one to remember is 1*F^2, which is 16 deg for F4, 25 deg for F5, 36 deg for F6, 49 deg for F7, etc. Basically, for an F7 telescope, the coma free region is like a Plossl eyepiece. This is independent of magnification.

 

For collimating primary and focuser axial alignments, the numbers in my previous post are about error tolerances. Roughly speaking how much the collimation error can be large before we lose the "sweet spot" completely (=miscollimated telescope). The formulas are 0.005 * F^3 for the primary mirror axial alignment, and 0.03 * D for the focuser axial alignment (without parrcor). A coma corrector reduces the tolerance to 0.005 * D. These are evaluated with a collimator. 

 

---------

As you said, outside the coma free region (of a collimated telescope), the telescope still works and can provide good views. The best ones are on-axis though.

Thanks Piero - I'm afraid that formulae and maths leave me rather cold but I'm sure this is all excellent stuff

 

[Piero]

2 minutes ago, Tubby Bear said:

Regarding collimation, i've never had a problem with my own F4 which i find just as easy to do as when i had my old 10" F6.3

(i also use the excellent Glatter / Tublug combo)

I don't think there is a problem (in terms of difficulty) in collimating an F4 dobson. It only requires a little bit more attention. 

Of course, given the stricter tolerances, there are other factors that can play a role in miscollimating the telescope. For instance secondary holder and spider, pole trusses, primary mirror cell. Often faster dobson are also larger in terms of aperture. This means a larger and heavier secondary mirror which can lead to focuser axial miscollimation when moving the telescope at different altitudes. I am just saying that the faster one goes, the more imperfections in optics and mechanics can become critical. 

[Piero]

On 22/08/2019 at 22:50, John said:

Thanks Piero - I'm afraid that formulae and maths leave me rather cold but I'm sure this is all excellent stuff

 

Personally, I enjoy both observing and how these things work. I find the latter helpful in order to understand whether I should or should not purchase an accessory, but also whether I'm using my instrument at its highest potential. 

We tend to spend a lot of time (and money) about the best optics. True, these reveal their best on a night of great seeing. Said this, if the actual area giving the "best view" is too limited, maybe a coma corrector is a good investment. On the other hand, if collimation is not done properly, the whole point of having great optics falls. 

I don't think that collimating a telescope is a difficult task, but just a matter of understanding how to do it well. I actually enjoy using my Glatter laser + 1mm stop aperture. With it, I can check both axial alignments in one go.   

Edited August 29, 2019 by Piero

[Trentend]

I left a 3cm extension tube in between the ocs and power switch (see photo) as this seemed to do the trick with the 8inch skywatcher. Will try removing it for the vx212 to see if it makes any difference (Rob, not sure if you have this on yours?).

[Space Hopper]

I'm not sure you should 'remove it'.

Its designed to 'extend' to enable you to focus. Note how mine is extended a touch.

Nice system you have there. In fact its a beauty. I love the 45mm OCS. Mine is a 40mm as its an Earthwin. It works well enough, but with an F4 the 45 OCS is a good move.

Edited August 23, 2019 by Tubby Bear

[Trentend]

Eddgie suspects the weight of the binoviewer is causing the focuser to sag a little when fully extended (it is very heavy with a filter switch, two eps, ocs and power switch) and has suggested I try placing the laser first in the focuser on its own and then repeat with the laser in the binoviewer when fully extended to see if the dot on the primary moves. Will try tonight and report back. 

[Space Hopper]

Good idea. It will be interesting to see the results.

Eddgie is a top man on the CN site. I've learnt alot from him.

It does indeed sound like you have a touch of focuser sag.

When i first purchased my old OO 10" i could see noticable sag when using my old N31 eyepiece (1 kg load)

OO focusers didn't ever have enough out focus with a 2" eyepiece so you were always forced to use an extension.

The bottom line : the older type OO focusers are not up to the job a lot of the time. Certainly not for a heavy binoviewer setup. Mine weighs 1.85kg fully loaded

and cheap, crappy focusers are simply not designed to carry that sort of weight.

The Moonlite is a good starting point, but even that could eventually start to fail as Eddgie points out and it happened to him.

But he's a prolific user and observer, and to 'wear out' a focusers bearings takes a bit of doing.

 

The extravagant solution would be to spend £600- £700 on a Feathertouch which are bomb proof and last a lifetime,

or invest in the cheaper Moonlite (£300 ish with kit) which is a lovely focuser and should do the job ok with normal usage.

 

Mine certainly works well enough. 

I cant do the binoviewer laser test with mine as i only have a 2" laser, but i'd be very interested to try it out with an 1.25" given the chance.

I've no reportable issues with the thin rolled tube 'flexing' either.

It may flex a tiny tiny amount, but i've not seen any degradation in performance.

I've thought about doing a tube upgrade, as another chap on here has done recently to a carbon fibre tube, but its not on the top of my list of priorities at the moment.

I don't get too concerned with collimation at all with my F4 either : it seems to hold it pretty well with only minute adjustments needed with my Glatters.

All 3 power settings with my 24P's yield good views and star shapes. Ok : the outer 10% of the fov may not be perfect but its not too noticable for me and my 52yr old eyes.

I get 60x 90x and 120x magnifications which is ideal for most deep sky work and other stuff as well, and i often do a whole session with just the 24 eyepieces alone.

 

The photo below shows my old OO scope and its rather poor focuser which had the in built extender in it.

Even fully racked out i had to unseat the eyepiece by a centimetre to reach focus.

Although the image doesn't show it that well there was an obvious sag with that 1Kg eyepiece in that position. No surprise really.  

The focuser worked well enough otherwise with 1.25" glass, but fell well short of what i personally required.

 

 

 

 

 

Edited August 23, 2019 by Tubby Bear

[Trentend]

Too excited to wait for a test this evening (I so want this to work as have £££ ready for related kit that I’ve been holding off for months) so tried in my lunch break.

Interesting results.

The laser centred nicely in the donut on the primary with the hotec only. When I put the binoviewer in place and swapped one of the eyepieces for the hotec with the focuser fully in, the dot was about 1-1.5 inches away from the donut 😳😳😳 . When I then moved the focuser fully out there was definitely some drift, not much (around 2-3mm I’d say) but you could feel a little play, even with the various screws tightened up. So definitely need to try a new focuser. Guess what I don’t understand is how the dot can be so far out in the first place??

 

Edited August 23, 2019 by Trentend

[Trentend]

I will try a repeat of this tonight in my smaller 8 inch skywatcher (that works a treat on all settings and has never been collimated).

[Space Hopper]

Wow..! I thought you'd see a few mm, but 1.5" ?? 😵

Thats a big difference. Little wonder you're seeing comets for stars.

[jetstream]

On 22/08/2019 at 10:11, Trentend said:

Wonder if this explains the poor views?

I tested my Moonlight with a weight simulating the Binotron 27's and 2 Docter 12.5mm eyepieces. This allowed my to watch the Catseye cheshire and Glatters laser as I racked the focuser in and out. After about 1/2 travel miscollimation began, enough to damage views.

What I did then is adjust the extension tube to allow 1.3x and 2.3x (Powerswitch) focus in the first third of focuser travel. I will then pull the bino out to focus for 3x with the focuser at half travel or less.

I've heard new Feathertouch's might sag more than the Moonlight and would like confirmation (or not) actually. I use the 45mm OCS with the bino's.

Edit: Oops forgot you had an aluminum OOUK tube- I tested my VX10 and it was the tube that flexed, lots. I have a different stock OOUK focuser than yours though. @faulksy had to reinforce his VX14 if I remember, to help with the issue. Mine flexes so much I don't binoview with it.

Edited August 25, 2019 by jetstream