Author Topic: Building my own Newtonian for the first time, but need help.  (Read 595 times)

Adam Martin

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Re: Building my own Newtonian for the first time, but need help.
« Reply #15 on: January 18, 2018, 09:21:18 AM »
if its for imaging you could do a double truss

https://3dwarehouse....ravel-Telescopeflatten the tube ends keeps costs down so you don't have to buy 16 $30 connectors

https://www.youtube....h?v=D97ie6Kiccs

fronenfiten

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Re: Building my own Newtonian for the first time, but need help.
« Reply #16 on: January 23, 2018, 06:45:18 PM »
Quote
if its for imaging you could do a double truss

https://3dwarehouse....ravel-Telescopeflatten the tube ends keeps costs down so you don't have to buy 16 $30 connectors

https://www.youtube....h?v=D97ie6Kiccs

Thanks Pinbout

I saved this bookmark in my browser the other day. First time I had seen a double truss design. I was originally considering building it, and it looks very tempting. Regarding the mirror being exposed the elements (dust, moisture, particulate, etc) with a truss design, do you know anyone who has talked about having to have mirrors recoated? Is that something I have to think about?

http://www.rfroyce.c...ng Projects.htm

Ethan Gechem

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Re: Building my own Newtonian for the first time, but need help.
« Reply #17 on: January 25, 2018, 08:56:31 PM »
Quote
if the wall thickness was .064, the tube would weight about 11lbs. Maybe that's why both ends were rolled.

Go grab the tube and step on a scale. It may help you make decisions.

Will do

So I was using the Newt builder program from Stellafanehttps://stellafane.o...ota/design.html

Do you know if this is a reliable program for planning an OTA design?

Would you or someone else be able to explain what it means to have 75% illuminated area and 100% illuminated area? I don't quite understand that part.
Also, no matter what tube design and numbers I put in, I can never get that yellow beam in the picture to land the same spot as the green and red beams. Don't I want all 3 colored beams to be in the exact same spot? Otherwise does that mean i'll have color fringing and abberations?

That is my screenshot, so I made it. It is nobody else's work.




Michael Litvack

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Re: Building my own Newtonian for the first time, but need help.
« Reply #18 on: January 28, 2018, 11:00:25 PM »
Quote
do you know anyone who has talked about having to have mirrors recoated? Is that something I have to think about?
No

You can always sew some shrouds out of spandex or lycra, not hard at all.

ciomasbure

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Re: Building my own Newtonian for the first time, but need help.
« Reply #19 on: January 31, 2018, 02:56:33 AM »
"Do you know if this is a reliable program for planning an OTA design?"

I believe that "Newt for the web" is good. I have used the program just to see if I've done everything right. I have also used the program to place baffles in the tube.

I think you found out that it asked for some parameters that you had to figure out first. Like how far was the mirror surface from the end of the tube; how far the focuser was from the front of the tube; the size of the diagonal; and it's position.

If the secondary is too small, you could get a reduced illumination area at the edges, as opposed to a secondary that will give you 100% across the entire fov.

The colors shown are for ray tracing, they do not represent the color of light coming in. They are not suppose to be in the same spot. A mirror does not suffer from chromatic aberrations like a lens does, so you will not get any "color fringing".

So in the program, how far inside the focus did you place the secondary?
Is your 1.3" secondary going to be large enough? or are you getting reduced illumination at the edges?
For photography, and low magnifications, I believe you don't want any reduced illumination at the edges.
For high magnifications your FOV is smaller, so you could get away with a secondary that doesn't fully illuminate the edge.

halubicom

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Re: Building my own Newtonian for the first time, but need help.
« Reply #20 on: February 02, 2018, 05:04:23 PM »
Quote
<p class="citation">Quote

do you know anyone who has talked about having to have mirrors recoated? Is that something I have to think about?
No

You can always sew some shrouds out of spandex or lycra, not hard at all.[/quote]
That's a good idea, thank you. I am starting to think maybe a truss would be nice, especially for optimal cooling without a fan

adpotabza

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Re: Building my own Newtonian for the first time, but need help.
« Reply #21 on: February 03, 2018, 03:17:15 AM »
Quote
Quote

Quote
<p class="citation">Quote

do you know anyone who has talked about having to have mirrors recoated? Is that something I have to think about?
No

You can always sew some shrouds out of spandex or lycra, not hard at all.
That's a good idea, thank you. I am starting to think maybe a truss would be nice, especially for optimal cooling without a fan
IMO, fans are a result of dobs being made with the mirror end of the tube being sealed off. Air cannot circulate around the mirror naturally. Even some commercial newts made for photography, have fans, also because the mirror end is almost sealed off.

If you make a newt with the mirror end open, where air can easily get to the mirror, you should have no problems. (generally)

erafquacor

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Re: Building my own Newtonian for the first time, but need help.
« Reply #22 on: February 03, 2018, 08:13:02 AM »
Quote
"Do you know if this is a reliable program for planning an OTA design?"

I believe that "Newt for the web" is good. I have used the program just to see if I've done everything right. I have also used the program to place baffles in the tube.

I think you found out that it asked for some parameters that you had to figure out first. Like how far was the mirror surface from the end of the tube; how far the focuser was from the front of the tube; the size of the diagonal; and it's position.

If the secondary is too small, you could get a reduced illumination area at the edges, as opposed to a secondary that will give you 100% across the entire fov.

The colors shown are for ray tracing, they do not represent the color of light coming in. They are not suppose to be in the same spot. A mirror does not suffer from chromatic aberrations like a lens does, so you will not get any "color fringing".

So in the program, how far inside the focus did you place the secondary?
Is your 1.3" secondary going to be large enough? or are you getting reduced illumination at the edges?
For photography, and low magnifications, I believe you don't want any reduced illumination at the edges.
For high magnifications your FOV is smaller, so you could get away with a secondary that doesn't fully illuminate the edge.

Yes, it did ask for some of those measurements regarding how far the mirror was to the back of the tube, focuser to the front of tube, etc. At this point, I have no idea what my mirror cell will look like, or any idea of how far the mirror will be from the back of the tube. I think I have to design the mirror cell first, right? What is the minimum-point mirror cell you would recommend? The gentleman selling the mirror said he intended to make a 6-point cell, but I think for absolute assurance i should do more than 6, especially because I want maximum grip on the mirror.
I did receive a mirror cell with my order, but it has a lot of old cork stuck to it, and I don't like the fact that it has clamps.
Thank you for clearing up the ray tracing. If I am correct, does the 75% illumination area refer to the outer portion of a frame when taking an image or viewing through the eyepiece? The area where 75% of the light hits versus 100% illumination area being where all of the light hits? The reason I chose F7 was because I wanted to do DSO, but I never liked the short FLs of all other Newts. I wanted closer views of Planetary Nebulae and 1' galaxies. I think as a consequence of wanting closer views of these small targets, my field of view has to become smaller. So, does it matter at all about the 75% illuminated area? I do want to hang my pictures up on the wall, but I think that can be accomplished because I would crop out any of the outer frame anyway that would have abberations, coma, etc. I would just keep the center of the image with my subject of choice. Can I then afford to have a smaller secondary mirror and minimize the central obstruction, thus maximizing contrast of what I really care about in the center of the frame?

tradunjuwa

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Re: Building my own Newtonian for the first time, but need help.
« Reply #23 on: February 09, 2018, 03:06:58 AM »
For an 8" mirror, 20mm is on the thin side. That is good/bad. It is good because it's half the weight of a full size mirror. Being lighter may help in the way that it's mounted. You may be able to use that Meade cell placing some cork at the 70% zone.
Then on the other hand, being thin it may need a flotation system. 6 pt, 9pt, I honestly don't know, but I am thinking, no.
I personally do not use adhesive to secure my mirrors. I use clips. The clips are not to be thought of as "clamps", and if done properly, will not pinch the mirror. They keep the mirror from falling out. I really don't think you need a fancy mirror cell.

Since you want to do photography, I will go on the line and say that the 1.3" secondary is not large enough. I would go with a minimum of 1.5". There is a trade off between the obstruction and maximizing contrast. With these sizes I don't think the difference will be noticeable. From a 1.3" secondary to a 1.5" secondary the difference in obstruction is 3%.

In your scale drawing, and using "newt for web", have the surface of the primary 4" from the end of the tube.
Your tube is 60". Your f.l. is 56"
Have the center secondary 9" from the front of the tube. That also means that your eyepiece center is also at 9".
Use a 1.5" secondary. (maybe even go a bit larger)

Here's the parameters that I plugged into Newt for Web:


belohalcu

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Re: Building my own Newtonian for the first time, but need help.
« Reply #24 on: February 09, 2018, 10:45:59 AM »
I don't know if you've done any looking but here is a good explanation:

http://garyseronik.c...nian-secondary/