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First build help - Focuser

ATM Beginner Reflector
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#1 blundered_bishop

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Posted 25 September 2021 - 04:14 PM

I'm designing my first Newtonian telescope around a 160mm/1300mm f/8.1 primary and a 35 mm secondary.

 

I need help understanding the mechanics and requirements of the focuser. I'm trying to figure out the distance between the primary and secondary: from the focal length I should subtract the outside diameter of the tube plus the height of the focuser.

 

Most references (e.g. The Dobsonian Telescope) recommend a "low profile focuser" that stands at about 50-75mm (2-3 in) when fully racked in. These references rarely go into much detail into the focuser, I imagine because they suppose some prior knowledge of them (which I don't have).

My questions are multiple and some may be simple in nature. I have already played around with some helpful tools (e.g. Newt) as suggested by other users but I haven't found all the answers I was looking for. Thanks to everyone in advance.

  1. How much "travel" (that is, the difference between the fully racked in and out heights) should I aim for? Does it depend on the type of eyepieces I will use?
  2. When calculating the distance between the primary and secondary, should I consider the average travel height of the focuser? Again, does it depend on the eyepieces?
  3. It seems very simple to build a "primitive" focuser that just works (some even use only an appropriate tube of cardboard/brass). Are there any cheap commercially available alternatives? Does it make sense to buy them?
  4. What's the con of using a relatively taller focuser to shorten a bit the distance between the primary and secondary?

Edited by blundered_bishop, 26 September 2021 - 09:27 AM.

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#2 jtsenghas

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Posted 25 September 2021 - 06:45 PM

welcome.gif welcome.gif Welcome to Cloudy Nights, and to the ATM lot.  I think you'll find us a generally amiable and well meaning bunch...... even when there is little consensus on an item.....

 

Anyway,  I'll take your last one first,  since it's a softball.  Taller focusers can vignette at lower powers and on scopes with faster f ratios, which are rapidly becoming the norm.  Your case of f/8.1 has a narrower light cone than most reflectors and may indeed suffer a taller focuser than most without vignetting. 

 

There are other issues with having a taller focuser, however,  they may negate the advantage of a slightly shorter scope. Scope balance,  especially with heavier eyepieces,  can be affected by that center of gravity shift,  etc.

 

Taller focusers must be more sturdily built to maintain alignment of the optics,  too.  A larger low power eyepiece can be quite a cantilevered load.

 

Taller focusers require larger secondary mirrors, reducing light and contrast. 

 

Play around with what works for you,  though.  Look at it optically and mechanically.  Consider any dimensional constraints you may have,  like the length of a car's back seat between armrests. I had to keep my last scope down to 48" overall to fit either of our vehicles....


Edited by jtsenghas, 25 September 2021 - 07:03 PM.

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#3 Pierre Lemay

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Posted 26 September 2021 - 08:28 AM

3- It seems very simple to build a "primitive" focuser that just works (some even use only an appropriate tube of cardboard/brass). Are there any cheap commercially available alternatives? Does it make sense to buy them?

Hello blundered_bishop and welcome to Cloudy Nights. JT did an excellent job answering your last question. I will tackle the third one: build or buy the focuser?

 

Most Amateur Telescope Makers (ATMs) today purchase telescope accessories like focusers, finders, eyepieces and diagonals. Most of these items can be purchased new, but can also be found second hand from websites like CN Classified. Other easy to make items like the primary and diagonal mirror supports might be made or purchased, depending on the ATM's skills. Finally the more adventurous will make the primary mirror but this is becoming rarer.

 

I rate the focuser as fairly easy and inexpensive to make. If you enjoy building things, it's a fun one-evening project to undertake. After all, we are having this discussion on the ATM, Optics and DIY forum!

 

The easiest focuser to build is a wooden base with a sliding tube. Such a simple focuser is compatible with an f/8 Newtonian like you are making. However, to have a finer control over focusing than just pushing and pulling a tube, the second easiest focuser to make is a Helical Crayford like this one:

 

A simple Helical Crayford.jpg

 

You will find a step-by-step description of how to make such a focuser in this thread which also covers a more complex focuser I invented called the Dual Speed Helical Crayford (DHSC). For an f/8 you don't need a dual-speed focuser but you can use the instructions I provide in the thread to make the simpler Helical Crayford focuser shown above.

 

This basic focuser with the simple plywood ring can be made for under 15$ with parts available at most hardware stores, including the bearings. Your biggest challenge will be to find a draw tube able to accommodate either 1.25 inch or 2 inch eyepieces. You might find such tubes in the plumbing section of your local hardware store or ask a local machine shop for a short 3 inch long piece of tubing they might have in their scrap bin. You might even get it for free if you ask nicely.


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#4 javad

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Posted 26 September 2021 - 08:44 AM

You can look into buying a Kineoptics HC-2.
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#5 blundered_bishop

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Posted 26 September 2021 - 09:43 AM

Thanks to jtsenghas and Pierre Lemay for their helpful answers. Since I was planning to start with a simple self-made focuser, I'll have the option of playing around a bit with everything. If that won't satisfy me, I'll give the Helical Crayford a go.

 

I have one more question. Consider these two pictures, taken from bbastrodesigns. If I understand correctly, in order for the image to be in focus, the "circular image plane" in the first picture should coincide with the "field" in the second picture. Hence, the distance between the center of the secondary and the end of the focal length should be the same as the distance between the center of the secondary and the "field" in the eyepiece. Is this right? Thanks again.

 

 

optical%20path%20and%20tube%20assembly%2

eyepiece%20cutaway.jpg



#6 Oberon

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Posted 26 September 2021 - 10:03 AM

Thats right. The field is the focal plane.


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#7 Oberon

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Posted 26 September 2021 - 10:31 AM

More on the focuser.

 

First, Pierre has given great advice, and what I am sharing doesn't take away from it.

 

But this is me 33 years ago having just completed my first telescope. I couldn't afford to buy a focuser (spent the money on better eyepieces) and decided to make my own from 2" PVC plumbing fittings. Focus change was achieved by including the threaded section of a PVC coupling. It was rough but solid and it lasted me many years. I remember having to lap the threads as they were manufactured too rough for a smooth focus, and I also built a bunch of par focal holders customised for each eyepiece to minimise adjustments. Quite a lot of work in the end, but it seemed a good idea at the time.

 

gallery_217007_4886_66733.jpg

 

But years later when I rebuilt the telescope, the first thing I did was buy a commercial focuser from GSO.

Infinitely superior in every way!

 

gallery_217007_4886_3379.jpg

 

Since then I have stuck with commercial focusers.

 

Until now.

My current project requires a bespoke solution where I am forced to make my own focusers, as no suitable focuser is commercially available for my binoscope. I'm sharing this with you because I have exploited what you describe as the very primitive tube sliding in a tube. The key to making this work is having good quality tubes with consistent dimensions, and using layers of slippery tape in between to act as a bearing, with the final sliding layer being teflon coated. You can even use a knob and threaded rod for more precise control. You don't need carbon fibre (as I have done) but a good quality PVC or ABS pipe might work very nicely too.

gallery_217007_4999_35595.jpg

 

The link is here, don't be intimidated as this focuser is far more complex than you require, just go through the thread for ideas. As I said before, Pierre's focuser is probably even better, but if you really want to to keep it simple and try the sliding tube solution then check it out.


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#8 blundered_bishop

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Posted 26 September 2021 - 02:33 PM

I'm extremely thankful to everyone who has helped me in this thread. I've mostly figured things out on paper, now to the actual work. I'm sure very little will actually go as expected, but that's part of the fun. Hopefully you'll see me post again with my completed build!


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