Stellalyra 8 inch dob collimation question

[greenery]

OP here 😀. Haven't got round to adjusting collimation yet but unexpected first light opportunity last night. The moon and Jupiter were looking great. Orion nebula and andromeda v faint. Managed with just my RACI as not fitted Rigel Quickfinder yet.

Quite pleased w how it all went. I think dew was maybe a bigger issue than anything else. Cue googling hairdryers and batteries.

Was surprised by how fast objects moved out of view at 130x but quickly got used to nudging the scope.

[bosun21]

You should try a standard dew shield first as I have found this sufficient for dobsonians. I experience bad dew formation in my area but I have never had a problem with the primary mirror and the dew shield protects the secondary.

[Starfazed]

Hmm, when scope shopping I looked at all the collimation videos I could find, which only confused me all the more. For now I'm increasingly glad that I went for a reflector that claims not to need collimation and doesn't have an adjustable primary, though if I bite the bullet later for anything bigger than 150mm I'm going to have to figure it out some day!

[Mandy D]

28 minutes ago, Starfazed said:

Hmm, when scope shopping I looked at all the collimation videos I could find, which only confused me all the more. For now I'm increasingly glad that I went for a reflector that claims not to need collimation and doesn't have an adjustable primary, though if I bite the bullet later for anything bigger than 150mm I'm going to have to figure it out some day!

Yeah, most of the collimation tutorials are very confusing and contain stuff that the beginner does not want or need. So, getting a collimation-free scope as your starting point was a good idea even though basic collimation is so easy to perform once you are doing it. The first time I attempted it on a Skywatcher 200P it took me literally no more than ten minutes with a primary mirror that was so far out I couldn't see anything. Just get a good Cheshire eyepiece and forget all the laser doodads. The first time you collimate, I would not bother with the secondary, which is supposed to be the first step and simply focus on the primary, which is where you will get most benefit for the least effort. Don't let fear of collimation put you off upgrading when you are ready to.

[Mr Spock]

Collimation with a laser takes 30 seconds... follow this guide 

For those just starting, do not attempt to adjust the position of the secondary. It's not needed and will only get you in a tangle. Nearly every guide elsewhere and here starts with that and it only complicates things beyond the abilities of most. Just leave it alone! All you need is the optical train to be aligned for best performance - that's it! Moving the secondary around is best left to those with a lot of experience.

[DAT]

On 25/12/2023 at 13:09, bosun21 said:

You should try a standard dew shield first as I have found this sufficient for dobsonians. I experience bad dew formation in my area but I have never had a problem with the primary mirror and the dew shield protects the secondary.

Well done, first sessions are pretty exciting. 

[Spile]

If you are buying new I can see the reasoning about not adjusting the secondary but otherwise I cannot agree with that advice. There may be a number of reasons why a secondary mirror might be poorly positioned and it will be obvious when looking through a Cheshire eyepiece and sight tube combination tool or even a simple cap. If that is the case then it should be corrected. 

[bosun21]

31 minutes ago, Spile said:

If you are buying new I can see the reasoning about not adjusting the secondary but otherwise I cannot agree with that advice. There may be a number of reasons why a secondary mirror might be poorly positioned and it will be obvious when looking through a Cheshire eyepiece and sight tube combination tool or even a simple cap. If that is the case then it should be corrected. 

The OP has just bought a new scope. He has never collimated before so he should try it as is before messing with the collimation. They generally arrive reasonably collimated.

[Spile]

23 hours ago, bosun21 said:

The OP has just bought a new scope. He has never collimated before so he should try it as is before messing with the collimation. They generally arrive reasonably collimated.

Indeed. I was reacting to the comment that those just starting should not attempt to move the secondary and of course not everyone starting will have a new reflector. To leave a badly aligned mirror as that comment suggests makes no sense.

[Chandra]

The main advice for anyone who receives a reflector, whether new or used, and before starting to loosen any screws or move any mirror, is to check the alignment status of the optics. I also believe that this verification is best done with Cheshire and a collimation cap, since the laser, even when collimated, will not give all the information about the optimal performance of the tube. Once the check is done, it will then be decided what needs to be touched. If the figure of the secondary mirror is circular and concentric with respect to the focusing tube, and the contour of the reflection of the primary mirror in it is also concentric, it will not be necessary to touch the secondary mirror and we will save ourselves a lot of work. All these phases of the alignment are the most complicated but, fortunately, also the most durable...

On the other hand, the final phase, the alignment of the primary mirror, is the simplest, but also the one that can most easily be lost when transporting or moving the tube. This is where I think using a well collimated laser can be most useful, although the Cheshire or even the collimation cap also serves the purpose. I agree with the view that delivery to the customer of a new tube with the correct secondary settings should be guaranteed.

[Spile]

12 hours ago, Chandra said:

On the other hand, the final phase, the alignment of the primary mirror, is the simplest, but also the one that can most easily be lost when transporting or moving the tube.

Interesting that you find that. After transposting in my car, I find the primary mirror rarely moves. So much so, that I have kept the original adjustment and lock screws on the primary as I rarely need to adjust it. 

Very tight spider vanes reduced the number of times I needed to adjust the secondary.

[Chandra]

10 hours ago, Spile said:

Interesting that you find that. After transposting in my car, I find the primary mirror rarely moves. So much so, that I have kept the original adjustment and lock screws on the primary as I rarely need to adjust it. 

Very tight spider vanes reduced the number of times I needed to adjust the secondary.

I suppose it may depend on the build quality of the cell holding the primary mirror and also how tight the locking knobs are, but I think it's better not to tight them too much in order to avoid putting stress on the mirror (which may lead to introducing additional aberrations), especially when it is subjected to large thermal gradients.

[greenery]

OP here. I finally got round to tweaking collimation. First just did the primary. Should have stopped there. Then decided to try to improve the already pretty good concentricity. Slightly loosened the 3 secondary screws, slightly tightened the centre screw. Only 1/4 turn on former. Result: massively worse. An hour of fiddling and it's now maybe similar to how it was. A bit off-putting but hopefully on path to knowing what I'm doing.

[Mr Spock]

1 hour ago, greenery said:

First just did the primary. Should have stopped there. Then decided to try to improve the already pretty good concentricity. Slightly loosened the 3 secondary screws, slightly tightened the centre screw. Only 1/4 turn on former. Result: massively worse. An hour of fiddling and it's now maybe similar to how it was. A bit off-putting but hopefully on path to knowing what I'm doing.

That's what I was saying earlier. Adjusting the secondary is a daunting task even for those experienced with Newtonians. The position (not collimation) is best left alone unless you are really confident in making the necessary adjustments. As long as your spider is dead centre the position of the secondary (which is a flat) isn't going to affect collimation; it will affect the circle of illumination if it is too far forward or back but that's minor compared to actual alignment of the optics.

It's always worth getting it right, but not at the expense of pulling your hair out. If you have perfect alignment of your optical train then you are good to go.

[Spile]

On 05/01/2024 at 21:29, greenery said:

OP here. I finally got round to tweaking collimation. First just did the primary. Should have stopped there. Then decided to try to improve the already pretty good concentricity. Slightly loosened the 3 secondary screws, slightly tightened the centre screw. Only 1/4 turn on former. Result: massively worse. An hour of fiddling and it's now maybe similar to how it was. A bit off-putting but hopefully on path to knowing what I'm doing.

It will be because after adjusting the offset with the centre screw which is a coarse adjustment you will always need to change the tilt and probably rotation with the three outer screws. The good news is once a=b you don’t need to touch the centre screw again. Once you get “your eye in” it becomes second nature and far less daunting. Enjoy the views.

[bottletopburly]

 Here’s my take on reflectors buying a reflector will require collimation at some point so if your thinking of buying a reflector and collimating scares you buy a refractor, if you do buy a reflector at the point of having to collimate don’t worry dive in and make a pigs ear of it you’ll get your head around it eventually and there is plenty of peeps on forums to help , YouTube videos etc ,yes it can be frustrating but believe me once you get your head around how a reflector works it does become easier , for me two things that I have upgraded  to make my life easier but Not necessary was cnc spider off of Aliexpress for my 200pds no need to fret and centralise spider and Steeltrack focuser (but only because I’m imaging) and I picked up an Ocal electronic collimator which I personally found very effective for checking the focuser was aligned to the OTA  which having the Steeltrack you can align the drawtube easier than the stock focuser on a 200pds but a Cheshire collimator and a colli cap are the very basics you need until collimating OCD takes over 🤣 

 

DAVE