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Old 19-05-2017, 12:01 PM
Herbie (Peter)
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Collimating

Hi just wondering if anyone can help
Looking through the eyepiece focuser I can see the three tags that hold the primary mirror in but the secondary mirror looks like it out to me, u line up the cross hairs to the collimating dot on the primary mirror and there's a part black ring to one side of the of the view on the secondary mirror and not sure if it's ok or out of alinement.
I have attached a photo but not sure if it's going to work
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Old 19-05-2017, 12:43 PM
glend (Glen)
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What sort of scope is it? What f #, that influences secondaey offset. Looking at your photo, its hard to tell if the photo is centred properly. If i assume it is, then rhe secondary is tilted, you can tell this because you can see the secondary vane mount blocks on the upper left of the photo. Do you have a Cheshire tube to check secondary placement? Or even a focuser plug with a centre hole can be used for this. Suggest you read Astrobaby's goid article on Collimation, which should be in the Artickes section of this forum.
Good luck.
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Old 19-05-2017, 12:56 PM
Herbie (Peter)
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Sorry it's a f5 Skywatcher 150/750
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Old 19-05-2017, 04:37 PM
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Nebulous (Chris)
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Hi Herbie,

I also have a Skywatcher 150/750 and found getting that secondary mirror right could be a bit of a pain. If you wind the focus tube out, that black rectangle on the right of your picture should disappear from view, which may have an effect on the blob in the middle. Not sure though.

I found that when I reached in and checked the small mirror that reflects the image into the viewfinder that it was quite wobbly and that getting it exactly balanced and lined up, and then tightened properly, using the three allen key screws in the centre mounting block was quite awkward. The required adjustments were pretty fine.

My only tool was a small pinhole drilled in the centre of a plastic cap that fitted where the eyepiece normally goes. But it was enough. My only precaution was to do the adjusting with the tube facing down so that if I dropped the allen key it would fall out of the tube and not down onto the big mirror.

Sorry that I can't be of more help, but the secondary mirror is held in place by a simple but pretty crude system. Initially I made it a lot worse before I found the right touch and adjustments to make it better.

If in doubt, it might be better to get a more experienced person to assist the first time you do it.

Best of luck with it.
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Old 19-05-2017, 05:05 PM
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Jason D (Jason)
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Your secondary mirror is little too high in the tube. You will need to lower it by loosening the central secondary mirror bolt then re-collimate.
BE CAREFUL!!!! If you loosen it too much then the secondary mirror could break loose and drop on the primary mirror. Hold the secondary mirror stalk while loosening it to avoid any accidental drops. In addition, have the tube in a horizontal position to keep the primary mirror safe from any accidental drops.
That dark crescent you see at the right edge of the secondary mirror as shown in your photo is actually the reflection of the inner wall of the tube in the area right above the focuser opening towards the tube opening.
Jason
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Old 19-05-2017, 06:11 PM
Herbie (Peter)
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Cheers guys I've read the post from Astrobaby on collimation very helpful looks fiddle to, so practice makes perfect I guess
Cheers
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Old 05-07-2017, 04:26 PM
cletrac1922 (John)
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Hi Guys

The quickest way is to use a laser collimator

Use on lowest power setting

Adjust the three outer screws of the secondary mirror until red dot of collimator is centre of dot primary mirror

Have scope horizontal, and target of collimator facing down the tube, slightly loosen the three locking screws holding primary mirror

Adjust primary mirror, until no image on the target

Secure locking screws primary mirror, ensuring primary mirror stays aligned with target of the collimator

Takes only a few minutes to do

I use my 10" collapsible dob away from home a lot with school and scouts programs, and hardly never needs adjusting
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Old 06-07-2017, 01:51 AM
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Steffen
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Peter,
listen to Jason, he's forgotten more about Newtonian collimation than most of us will ever know.
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Old 06-07-2017, 08:27 AM
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When you're getting closer to having the mirrors sorted, another thing to consider is if the vanes are all perpendicular. One of yours seems a bit twisted close to the OTA wall.
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Old 07-07-2017, 02:37 PM
ausastronomer (John Bambury)
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Quote:
Originally Posted by cletrac1922 View Post
Hi Guys

The quickest way is to use a laser collimator
Not in this situation it isn't. It's a good way to end up with a poorly collimated telescope.

As Jason D mentioned the secondary is a little too high in the tube. When the secondary is incorrectly positioned in relation to the primary / focuser it is possible to adjust the tilt of the primary and secondary mirrors to have the laser striking back on itself at its exit point from the collimator, making the telescope appear collimated when in fact it is not.

Once the position of the secondary is verified correct either by using a sight tube, or similar device, or your eye if you know what you're doing, a laser collimator is then a handy tool to adjust the tilt of the primary and secondary mirrors.

ie. Once the telescope is properly set up the laser is then a good time saving collimation device.

Cheers,
John B
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Old 08-07-2017, 11:46 AM
ausastronomer (John Bambury)
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When you're getting closer to having the mirrors sorted, another thing to consider is if the vanes are all perpendicular. One of yours seems a bit twisted close to the OTA wall.
Hi Rob,

Most definitely. One of the vanes certainly has a twist in it. With me having CDO (that's OCD but I need to put the letters in order as well) I really like to have everything straight, square and concentric. It definitely doesn't hurt doing this in the optical world, notwithstanding that in some cases it doesn't offer any gain.

However, looking at how the focuser draw tube impinges on the light path in the photograph, if the telescope reaches focus with the eyepieces at anywhere near this amount of focuser in travel, the focuser drawtube will have a significantly greater effect on image quality (due to diffraction) than will the slightly twisted spider vane. In any case it won't be a huge effect. One only has to consider the minimal effects of a central obstruction (secondary mirror) when the obstruction is < 20%.

Cheers
John B
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Old 08-07-2017, 12:14 PM
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Totally agree John. I'm probably well into OCD territory having used my Newt for astrophotography for many years some time ago. The effects of the impinging drawtube and a twisted vane are then visible as imperfections in star roundness and tightness of diffraction spikes respectively.


Was interested to read your comment on lasers - always found a cheshire to be a dependable guide. For mass-produced Newts, movement in the focuser, tube flex, or mirror movement (along with the challenge of most cheaper lasers themselves not being properly collimated) is a problem. Ironically, you only seem to get maximum time saving benefit from a laser after having invested a lot of time beforehand learning basic collimation skills. For a beginner with no assistance nearby, a laser can be frustrating experience.

How do you go about setting up and checking the "big newts", out of interest. With a decent laser, or something like catseye tools?
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Old 08-07-2017, 08:10 PM
ausastronomer (John Bambury)
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How do you go about setting up and checking the "big newts", out of interest. With a decent laser, or something like catseye tools?
I knew someone would ask me this Unfortunately it's not as easy to put on paper as it is to show someone standing beside you at the telescope

The purpose of this forum is an information exchange so here we go.

Ok. Before you worry about collimating the telescope you need to understand the concept of "secondary offset". This basically involves pulling the secondary slightly out of position with regard to perfect mechanical alignment in the telescope tube, to align the optical axis and to center the fully illuminated field (FIF) in the eyepiece. When I refer to "telescope tube" the same applies to the Upper Cage Assembly (UCA) on a truss dob. Here is a link to a really simple article about offset by Nils Olif Carlin which appeared in Sky and Tel in 2006

There are basically 3 offset scenarios.

No offset - secondary in the center of the tube and centered with the focuser axis

Partial Offset - This is where the secondary is left centered in the tube but moved down the tube towards the primary

Full offset - This is where the secondary is moved away from the focuser and towards the primary.

The amount of offset is calculated using the formula:-

Offset = (secondary size)/(4*focal ratio). For full offset it's the same amount of offset both toward the primary and away from the focuser.

If you use a sight tube, or Cheshire combination tool to position the secondary mirror, this will automatically give you a "partial offset" towards the primary. A lot of people use one of these tools and aren't even aware that it has automatically given them a "partial offset" towards the primary. That's a good thing.

As the telescope gets larger and the F-ratio faster the secondary offset becomes more important and also a larger amount of offset is required. In addition it becomes more important if your secondary mirror is marginally sized, or slightly undersized for planetary optimisation. Generally with telescopes under 10" aperture you don't need to worry about offset. With a typical 10"/F5~ using a 46.5 mm secondary the amount of offset required is only about 2mm. With my 14"/F4.5 (2.6" secondary) the offset is about 3.5mm and with my 18" Obsession (3.1" secondary) the offset is about 4.5mm. With the 25"/F5 3RF Obsessions which I also set up regularly the offset if about 5mm. With all of these scopes I only ever worry about using a partial offset which is achieved by positioning the secondary mirror with a sight tube in the first instance. Experience has shown me that there is nothing to be gained in these apertures and F-ratios by using a "full offset" as opposed to a partial offset. I have over the past 12 years set my 18"/F4.5 Obsession up using a full offset on several occasions, but I haven't noticed any gain over collimating it with a "partial offset". With big fast scopes using large secondary mirrors I would definitely set them up using a full offset. For instance a 28"/F3.3 uses an 8" secondary mirror. The offset required in this scenario is about 16mm. You can undertake a full offset by mechanical measurement, or by marking the offset on the secondary mirror with a fine black marker pen and hitting it with a laser from the focuser. You might need a mirror on a stalk to see where it's hitting the secondary with a tubed scope. With a truss scope you can see it from underneath the UCA with no shroud.

I would recommend that with 95% of Newtonians a partial offset will do just fine. The underlying factors that may change this are large aperture and fast f-ratio and / or a marginally sized secondary.

Lets assume we are doing a partial offset on a medium aperture scope, or a full offset and the secondary has been moved away from the focuser by mechanical measurement. Just use a sight tube or combination Cheshire/sight tube to correctly position the secondary mirror in terms of its rotation and distance from the primary. I like to adjust the rotation and get it properly facing the focuser before adjusting the position relative to the primary, but that's just personal preference. Once the secondary mirror is correctly positioned it's pretty easy from there and you can basically do things "any old how". Choose your own poison. Laser, Barlowed Laser from whoever, Cheshire, Combination Cheshire/sight tube, a pair of highly skilled eyeballs

The secondary mirror once correctly positioned shouldn't need to be repositioned unless you have removed it or adjusted the secondary assembly or spider vanes in some way. Even with truss dobs which are continually assembled and disassembled they invariably go back together with a very regular consistency, if they are high quality and well made.

My preferred method is to use a sight tube to set the secondary position with a partial offset on my 3 truss Newtonians. I own a set of Tectron Tools which I acquired in about 1991 or 1992. These were the predecessor to the Catseye system. These were US made by Tom Clark. They include a separate sight tube, a separate Cheshire and an autocollimator for final fine adjustments. Generally once I have the secondary positioned I use an Astrosystems 2" laser to adjust the secondary tilt and then use it with the barlow attachment to adjust the primary tilt. I often double check this with the Tectron autocollimator and invariably it is "spot on". I also always do a final check on a defocused star. Despite what some people will tell you, you don't need a high power eyepiece. A 3mm exit pupil is good. You are checking the collimation not star testing the telescope. Defocus slightly inside focus until you get a diffraction pattern where the diffraction rings are clear. make sure the diffraction rings are nice and round and the secondary mirror shadow is nice and round and in the centre of the diffraction rings. Take note of how far inside focus you rolled the focuser. Then go back to focus and then go outside focus by the same distance. The inside and outside diffraction patterns should be similar and again the diffraction rings should be round, the secondary shadow should be round and centred in the diffraction rings.

It's important to note that if you use a laser collimator, the laser collimator itself must be collimated to achieve good collimation. In other words as you rotate the laser the beam point remains fixed in its position. Most good quality lasers have grub screws to allow adjustment of the lasers collimation. The best way to check this is with the laser in a "V-block". You point the laser at a wall about 5 meters away resting in a V-block. If the laser is out of collimation the beam will move through a circle as you rotate the laser, whereas it should remain as a fixed point on the wall. You adjust the grub screws until the laser pint remains fixed through it's full 360 degree rotation. This might take 30 minutes, or more, to get right but once its done it shouldn't move. You can also check this by rotating the laser in the focuser and see if it remains a fixed point on the primary, or transcribes a circle as it is rotated. A V-block is more accurate than using the focuser due to side play when the laser is in the focuser.

When you know what you're doing you can actually "eyeball" it really close. If you stand about a metre back from the focuser, align your head properly to the focuser then look through the focuser to the primary, your eye and your eye's pupil should appear dead centre in the reflection. When you got this right you will know what I mean.

I think that covers most of it.

Cheers,
John B
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Old 08-07-2017, 09:32 PM
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Thanks so much - gold mine of info for Newt owners.
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