How critical is Collimation?

[alan potts]

I had the 190mm Mac Newt out for a spin last night but before it got dark I thought I would have a play with my Hotech SCA Laser, which I got from a site member while in England . I had read the instruction 5 times so I knew them well ( how rare is that for a man ) and after a good look at the Moon , which seemed fine, pluged it in and tighten it up.

I have to say I still find the SW focuser far from as good as I would like but that just means another spending spree and messing with the front element. When I turned it on the laser was showing it was not correct, it was about 5mm from the middle of the mirror, as far as I could see the beam was hitting the centre of the secondry, but some guess work was at play. Anyway to cut the story short I made the correction which were not large, a maximum of a 1/4 turn , it took 4 minutes.

After removing the Laser as far as i could tell it was exactly the same, maybe just a tad brighter, it had not got any darker as far as I could tell but I am guessing there.

Question how critical is collimation on this F5.26 scope, as far as I can see not that much. I would like to here from any same scope or Newtonian owners that have better of worse collimation sessions.

Alan

[barkis]

The Mak/Newt. has a big mass Alan, and takes a while to cool, so

make sure your instrument is in thermal equilibrium before you collimate it .

As to how important is Collimation, the answer is always going to be Very Important, so don't settle for Nearly there, strive for perfection, especially

in fast instruments, where resolution can suffer even if slightly out of true.

The Mak Newt. focuser has proved to be quite satisfactory for some users, Observers and Imagers.

Others have opted to change it for reasons of their own.

Have you got Bobs Knobs fitted to your Primary and Secondary cells?

They make the task of collimating a bit easier.

Good Luck

Ron.

[Tim]

Depends how nice you want your pics to be :-)

To get the best out of the mn190 and to benefit fully from its great flatness, you do need to get very very good collimation, and frankly you won't achieve that with just a laser.

The whole image path needs to be aligned, including the focuser and both mirrors. Once you have the basics in place correctly then its just usually a quick tweak on the primary.

The cats eye system works like magic on this scope :-)

[alan potts]

Tim,

The cats eye system is most likely the best but you have to take the scope apart to put the little stickes on the mirror. If I read the long reply correctly from FLO, James seemed to to think that while not perfect the Hotech laser would do the job. It was only the messing about with the catseye stickers that stopped me taking that path. Looks like I got it wrong again.

Alan

[ollypenrice]

This is the thing with the 190. It is phenomenal when it's right, as good as the best large refractors of comparable focal length. When it's wrong, though, it's likely to be very wrong because of the fast F ratio and large flat field.

Olly

[alan potts]

Olly,

This is what I can't get my head round. If it was wrong to start and now it's right it seems the same. If it was right to start then now it's wrong or on the other hand it was wrong and it's still wrong, but it looks brilliant, but it did anyway.

I guess if it was only a little out then visually maybe it don't notice, it stars tests perfect as far as I can see, but it did anyway, this is why I asked how critical it was, everyone seems to think , quite a lot, but this is not what I am finding.

Alan.

[ollypenrice]

I'm not a collimation guru. Tim gets resolution to die for out of everything he touches!

Olly

[perfrej]

The 190MN simply cannot be collimated with a laser collimator - beleive me. You need a kit like the Catseye Collimation Kit in order to get it right. It also has a "thing" about secondary mirror position in the tube (longitudally), and you find that sweet spot by shooting a flat and observing how symmetrical the flat field is in terms of illumination.

All in all it is a simple process and you do it in about 15 minutes. It then holds collimation well IF you observe the following (all mods easy to perform):

1. Main mirror cell (holder)

You need to carefully adjust the three nylon screws that hold the mirror centered in the cell. The cell needs to be padded with some kind of soft ruber or foam hose that you place around it, between the cell and the scope's bottom assembly. If you do not, it will slide back and forth, ruining collimation slightly and model building severely.

2. Main mirror collimation screws

You need to start with these totally bottomoed in the assembly so that the three o-rings that are around the screws and between the scope and the mirror cell are compressed. You then do collimation by loosening screws, not by tightening (as they are bottomed to start with). This process reduces mirror cell travel laterally in the tube, a problem that will arise if the screws are not tight enough.

3. Main mirror clamps

Loosen these until you can, with some applied force, get an office copier paper between the mirror surface and the clamp's rubber top. If you do not you get triangular stars...

Whatch out for the secondary mirror center screw! It is a tad too short, and if you attempt to lower the secondary towards the primary in order to get even illumination, it may lose the holder! If you have the scope vertical at that time you will damage both mirrors when it falls down.

If you do this and collimate with a Catseye, you are in for a treat. The optics of the 190 is extremely good but the mechanics are a bit too cheap. Easily corrected, though

Good luck!

/per

[Macavity]

I like to have a "numerical handle" on things? The following provided me with some numbers:

http://web.telia.com...olli/kolli.html - The table of "sweet spot" seemed rather interesting tho'?

Ranging from 22mm (f10) to 1.4mm (f4). I believe a plot of this goes as the cube of f-number.

But, whatever the details, illustrates the challenge involved, particularly for faster scopes...

I am also reminded of text book teasing re. students: "suffering from delusions of accuracy"!

For, if we are talking millimetric adjustment of typical (budget) scopes, I sense this does apply.

I don't see the construction of many (albeit fine?) scopes capable of maintaining such tolerances.

Moreover, even if you get all your "dots in a row" (whatever), but give the OTA a few "bumps", as you loft onto the mount? Not to mention tube deformation (temp effects), once off the bench. More seriously, I do wonder about taking huge pains to collimate with a uniquely centered collimator, then using a typical eyepiece (camera), retained by dodgy brass rings etc.

A MAK-Newt is comparatively well constructed though? Frankly, I just "do my best" with my TS 8" F4 Photo-Newt. Heheh. Filing the ends of the secondary adjustment screws ("Bob's knobs", even) has effected significant improvements in stability though. This I believe is reflected (no pun!) in a visible improvement. My (small chip) VIDEO astronomy and "creative" use of focal reducers may render this sufficient...

P.S. If you want a real shock (maybe), check out your budget, short-tube, achromat with a Cheshire eyepiece? But any "focuser droop" and "centrality", is rather easy to fix! Did it make a difference? Or was this swamped by other limitations? As ever, I forgot to check things before I "improved" them.

[memoryman]

"Have you got Bobs Knobs fitted to your Primary and Secondary cells?

They make the task of collimating a bit easier.

Good Luck

Ron".

Where can you get Bobs Knobs from apart from the USA, as I've tried locating them and failed?

Are there any UK firms that sell equivalents?

Thanks in advance

Paul B

East Midlands

[earth titan]

Where can you get Bobs Knobs from apart from the USA, as I've tried locating them and failed?

Are there any UK firms that sell equivalents?

Thanks in advance

Paul B

East Midlands

Just order from the US. Super fast, super slick and very reliable. Ive ordered 3 times with no problems.

Typed by me on my fone, using fumms... Excuse eny speling errurs.

[RikM]

P.S. If you want a real shock (maybe), check out your budget, short-tube, achromat with a Cheshire eyepiece? But any "focuser droop" and "centrality", is rather easy to fix! Did it make a difference? Or was this swamped by other limitations? As ever, I forgot to check things before I "improved" them.

I checked mine, as I was getting little flares on the stars. It was a tiny bit out so I shimmed the whole rear 'black bit' until it was properly centred according to the Cheshire. It has made a very noticeable improvement, to the point that the little scope is now well worth looking through rather than simply bunging a guide camera in

[barkis]

Just order from the US. Super fast, super slick and very reliable. Ive ordered 3 times with no problems.

Typed by me on my fone, using fumms... Excuse eny speling errurs.

+1. Great Service.

[SkyGibbon]

I don't see the construction of many (albeit fine?) scopes capable of maintaining such tolerances.

Moreover, even if you get all your "dots in a row" (whatever), but give the OTA a few "bumps", as you loft onto the mount? Not to mention tube deformation (temp effects), once off the bench. More seriously, I do wonder about taking huge pains to collimate with a uniquely centered collimator, then using a typical eyepiece (camera), retained by dodgy brass rings etc.

I find myself collimating 2 - 3 times during a 3 - 4hr viewing session for reasons you just gave. For my 150mm f/8 and 130mm f/7.7 homemade newts, I just use a hotech laser and get as close as possible on centering because the tolerances are alot wider. I am pretty sure I come within the specs for those speeds. But, on my Orion 250mm f/4.7 I get better results just with a collimation cap than I do with the Hotech. It is extremely close, as I check in the cap after using the laser, but it is about a 1 - 2 mm difference. I then dial it in with the cap and the views are definitely sharper than if I didn't..

[Ouroboros]

I have to confess, Alan, I collimate my 200p from time to time using a laser when it's a bit out and frankly I can never see any difference either. So, either my eyes are too bad to tell good from not so good. Or I don't know what good looks like. Or it's all a little bit over hyped.

[Stargazer_00]

that's my take on it too. for visual close enough is good enough. I collimate my scopes with a hotech sca and a cheshire (both are newts) and am perfectly content with the views.

For AP though I would expect you'd want it absolutely bang on because a camera doesn't hide the flaws your brain can correct when viewed by eye. A camera image always tells the truth of things.

[Macavity]

I sense there is a point, around F5(?), where typical telescope hardware can cope well enough - Visually, at least. I also sense that my budget F4 photo-Newt ("twice as hard" to collimate) may be a little marginal on that one? <G> If I was going to go beyond the realms of video astronomy, I might upgrade the OTA to (now available!) carbon fibre.

In retrospect, I never *really* needed an F4 scope - An F5 would have been fine!

Large secondary [shadow] (no extra visual field), huge back-focus - A salutory warning?

But Hey, it's CUTE - Fits a smaller obsy - And f / 800, gives a not-insignificant gain in FoV...

If you have the dosh (an EQ6 too), indeed an F5 MAK Newt? Or, an <ahem> "Power-Newt" maybe!

http://www.teleskop-...-Corrector.html But most are focal reduced F4s. And there's still collimation...

[ollypenrice]

Now why do I like refractors???

Olly

[John]

I sense there is a point, around F5(?), where typical telescope hardware can cope well enough - Visually, at least. I also sense that my budget F4 photo-Newt ("twice as hard" to collimate) may be a little marginal on that one? <G> If I was going to go beyond the realms of video astronomy, I might upgrade the OTA to (now available!) carbon fibre.

In retrospect, I never *really* needed an F4 scope - An F5 would have been fine!

Large secondary [shadow] (no extra visual field), huge back-focus - A salutory warning?

But Hey, it's CUTE - Fits a smaller obsy - And f / 800, gives a not-insignificant gain in FoV...

If you have the dosh (an EQ6 too), indeed an F5 MAK Newt? Or, an <ahem> "Power-Newt" maybe!

http://www.teleskop-...-Corrector.html But most are focal reduced F4s. And there's still collimation...

The "sweet spot" around the optical axis where a scope is accurately collimated and delivers diffraction limited performance gets smaller as the focal ratio gets faster. With an F/4 scope it's just 1.4 mm, with F/5 2.8 mm, F/6 4.8 mm and F/8 it's a relatively tolerant 11 mm.

[RobH]

Now why do I like refractors???

Olly

Try my Ritchey Chretien, where absolutely everything is adjustable......I'm still trying to get it right!

[RikM]

I'm sticking to f/5 and f/4.7 until I can get a permenant obsy set up and can afford a scope that will cope at a faster ratio. I'm thinking OO AG8 long term rather than budget optics.

[alan potts]

Well I have read and I think I understand what you are all talking about. I think this means I will not buy a CCD for this scope if and when I get one. I have had a play with the knobs on the scope using the laser and the end result before and after to me look exactly the same. There is to my eyes nothing wrong with it but then there wasn't before I started. I am sure everything that has been said takes on a new meaning when a camera is involved and until then I think I will just leave it alone, often good advice.

I would like to extent a special thanks to Tim who took the time to try and explain this black art to me. I do think if I am going to go down the road of either CCD or my Mod 40D on this scope then first it will need a new focuser as I am far from happy with the one it has. This would also seem a good time to do all the other work that our Mak/Newt refit expert from Sweden suggests and buy the Catseye system.

Well it is outside now and I am going to see how my 3-6mm Nagler zoom works on it.

Alan

[ollypenrice]

Try my Ritchey Chretien, where absolutely everything is adjustable......I'm still trying to get it right!

I'm married to a Chrétien.

Ahem.

Olly

[barkis]

I'm married to a Chrétien.

Ahem.

Olly

Olly, I hope your Good Lady doesn't have access to your SGL account .

If she read that, she'd surely make some adjustments to you mate .

Ron

[ollypenrice]

Olly, I hope your Good Lady doesn't have access to your SGL account .

If she read that, she'd surely make some adjustments to you mate .

Ron

You have it, Ron!

Olly