Collimation tools and secondary offset

[Shimonu]

I'm trying to convince myself that I wouldn't be too bothered with collimation of an f/4 newtonian even if it would be my first newtonian. I'm doing my best to read guides and understand the tools that are used but I'm getting somewhat confused. I've seen collimation cap, collimation eyepiece and cheshire. These all seem to be either a long or short tube with a hole in the center and an opening in the side(except for the collimation cap). Could someone please clarify what the differences between these are and how they are used?

I'm also seeing some confusing discussion on if the secondary should appear in the center or if it should have an offset, or maybe some shadow should be offset and this appears to be different depending on if it's a "fast" or "slow" scope. Is there a clear guide for how to align the secondary and which references to use? Does the manufacturer provide any collimation information(likely TS Optics in my case)?

How good of a collimation can you do at home that will then give you good stars? Some seem to feel that you always need to do a final star test.

[johninderby]

Best tool for setting up the secondary is a Concentre.

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p5506_TS-Optics-Concenter-2--Collimation-Eyepiece-for-Newtonian-Telescopes.html

Allows you to get the secondary perfectly centred and takes care of offset. Once the secondary is perfect collimating the primary is much easier.

BTW before adjusting the secondary it’s a good idea to check to see that the focuser is square to the tube.

Edited December 28, 2021 by johninderby

[bottletopburly]

+1 for the concentre initially I wasn’t too sure until I found my focuser not 90degrees to OTA , I’m impressed now 👍.

[Spile]

Try my guide.
https://astro.catshill.com/collimation-guide/

As you can see, I use letters to identify correct alignment (it works for me) and the diagrams help me far more than just words.

The cap is perfect for alignment of the primary and is just a small hole surrounded by a reflective surface. You can make one with lens cap and a shiny washer.

The Cheshire eyepiece and sight tube combination tool is great for alignment of the secondary. Like the cap it is a small hole surrounded by reflective material. Unlike the cap, it  lets in light so it’s reflection is easier to see and it has cross hairs at the end of a long tube.

Even though these tools are great, it is worth getting to grips with alignment by simply looking down the focus tube…

https://astro.catshill.com/through-the-eyepiece-tube/

[Shimonu]

Fantastic help, I feel like I understand the steps of collimation better now and it certainly doesn't feel as daunting.

I always put my equipment in the car and go just 5-10 minutes away. Is it crazy to think the collimation would hold if done at home and then going out for a night?

[alacant]

11 hours ago, Shimonu said:

crazy to think the collimation would hold

If you have the primary fixed with silicone to prevent sideways and up and down motion, then no. This also has the advantage of the coillimation holding at all angles the telescope assumes during imaging. 

HTH

[Shimonu]

1 hour ago, alacant said:

If you have the primary fixed with silicone to prevent sideways and up and down motion, then no. This also has the advantage of the coillimation holding at all angles the telescope assumes during imaging. 

HTH

Is this possible to do with the scope assembled or will it need to be taken apart?

[Ricochet]

13 hours ago, Shimonu said:

Fantastic help, I feel like I understand the steps of collimation better now and it certainly doesn't feel as daunting.

I always put my equipment in the car and go just 5-10 minutes away. Is it crazy to think the collimation would hold if done at home and then going out for a night?

If you do the collimation/locking screws up tightly enough then it might be fine. In any event you can easily check once you get to your site using either the barlowed laser method or by shining a red light torch into the side of a cheshire. Once on site I would only ever adjust the primary.

[alacant]

1 hour ago, Shimonu said:

will it need to be taken apart?

Don't know which telescope you have (all f4s are nice), but it will almost certainly need to be dismantled. Whilst it's apart, replace the springs with ones which are up to the task. 8", we use 1.6mm wire. 6", 1.4mm etc. Include passive springs over the locking screws leaving the latter loose.

[Shimonu]

5 hours ago, alacant said:

Don't know which telescope you have (all f4s are nice), but it will almost certainly need to be dismantled. Whilst it's apart, replace the springs with ones which are up to the task. 8", we use 1.6mm wire. 6", 1.4mm etc. Include passive springs over the locking screws leaving the latter loose.

I'm currently looking at the TS Optics Photon 150/600. I'll likely ask more about disassembly if I get it, I'm sure it's easier once you have something in front of you to look at.

Regarding a carbon fiber tube, are the benefits the reduced weight and not having to refocus with temperature change? I'm wondering if it's really worth essentially double the price.

[alacant]

18 hours ago, Shimonu said:

TS Optics Photon 150/600

Hi

Look carefully at which model it is. AFAIK, there is only one type available at sensible prices. The one we tried had a metal tube and of good quality, but the spider for the secondary was buried quite a way from the open end of the tube, was un-reinforced and the tube dented as you tightened the spider thumb screws. It had an unnecessarily large secondary throwing the focus position to a position way out from the tube, even with a dslr. It really was unworkable and un-balancable, as half of the light path came after the secondary. Here's an example with an eyepiece at focus. A camera is not going to be much better. We came to the conclusion that if you wanted a sensible 150 f4, you'd have to specify an ONTC yourself with the secondary further up the tube, as there seem to be no other versions other than the GSO/TS reasonably priced.

If you find otherwise, please post, as the idea of something as light and as fast as this appeals:)

 

[Shimonu]

12 minutes ago, alacant said:

Hi

Look carefully at which model it is. AFAIK, there is only one type available at sensible prices. The one we tried had a metal tube and of good quality, but the spider for the secondary was buried quite a way from the open end of the tube, was un-reinforced and the tube dented as you tightened the spider thumb screws. It had an unnecessarily large secondary throwing the focus position to a position way out from the tube, even with a dslr. It really was unworkable and un-balancable, as half of the light path came after the secondary. Here's an example with an eyepiece at focus. A camera is not going to be much better. We came to the conclusion that if you wanted a sensible 150 f4, you'd have to specify an ONTC yourself with the secondary further up the tube, as there seem to be no other versions other than the GSO/TS reasonably priced.

If you find otherwise, please post, as the idea of something as light and as fast as this appeals:)

 

Wow! I had never considered the position of the secondary and now that you point it out it looks really far back and strange.

This UNC would be much better then? https://www.teleskop-express.de/shop/product_info.php/info/p5881_TS-Optics-6--f-4-UNC-Newtonian-Telescope---Carbon-Tube---made-in-Germany.html
It's certainly more expensive but that's less important in my mind if it works well and is more pleasant to use.

[alacant]

46 minutes ago, Shimonu said:

This UNC

Yes. That's where the focus podition should be, around 6cm above the top of the focuser. But why the massive 7cm secondary? If you don't need to cover full frame, specify 58mm or 63mm.

I believe though that even the UNC retains the toy springs. And still only 3 of them. Easy to fix of course, but if you're paying €high...

 

[Shimonu]

10 minutes ago, alacant said:

Yes. That's where the focus podition should be, around 6cm above the top of the focuser. But why the massive 7cm secondary? If you don't need to cover full frame, specify 58mm or 63mm.

I believe though that even the UNC retains the toy springs. And still only 3 of them. Easy to fix of course, but if you're paying €high...

 

So stepping back and instead of looking at my requirements. Is there a recommended scope if I want something around 600-700 mm focal length and 130-150 mm aperture that is well built, pleasant to use and gives good results out of the box? And let's say the budget goes up to around €1000

[alacant]

I know of no Newtonian which works for astrophotography (well) out of the box. 

Maybe TS can help within your budget?

Or how about the SW 130pds? 650mm. It can be made skyworthy -i.e. made to retain collimation- in around an hour.

Don't forget the extra for the coma corrector.

Cheers

[Shimonu]

1 hour ago, alacant said:

I know of no Newtonian which works for astrophotography (well) out of the box. 

Maybe TS can help within your budget?

Or how about the SW 130pds? 650mm. It can be made skyworthy -i.e. made to retain collimation- in around an hour.

Don't forget the extra for the coma corrector.

Cheers

Maybe I'll do more research on the UNC, if the springs are what I should fix first then that seems like a minor thing I could live with. I just want to feel that I can at least attach my equipment for a few sessions and get a good baseline before I start doing bigger modifications and taking it apart. I'm sure I'm handy enough and capable but I like getting comfortable and knowing the stuff first.

Moving up to 100mm+ aperture refractors just feels too expensive at the moment.