21 replies to this topic

[N3p]

I am not hiding that during the last days, one of my favorite subject of pondering was a 12" telescope, which is different then my previous project.

 

(It was a 11" F5.5 solid tube, too difficult to build and too cumbersome to carry in the Honda Civic, or other small cars)

 

Now it's clear, it's going to be a standard truss design DIY, the solid tube idea for larger aperture, it's not working, I accepted the fact that I should build and collimate the instrument after a car trip, this is the price to pay to use a larger instrument, just arriving at the spot 30 minutes sooner should be fine.

 

I wish to have very good primary mirror from a mirror maker and a nice telescope that I will take my time to build, out of wood. (thanks Augustus for the inspiration lately)

 

The mirror, I'll ask Mr. Fullum for that, it's going to be either F5.5 or F6, this will be decided after some time spent on AutoCAD, seeing about the eyepiece height, power, exit pupil, and the choice of the secondary mirror. According to my numbers from my previous 11" project, 12" f6 is very tempting for planetary observation, it would be a general purpose telescope for lunar and planetary observation from home (a lot of light pollution) but I want it to be good for DSOs as well, not necessarily a planet killer but very good for planets and moon. For wider field of view, lower power, I keep my 8" F5 of course.

 

I'll start the design soon and try to improve my listening skills according to your suggestions this time, I'll need help with the design... if you want to help me.

 

--> You can already give me your thoughts right now.

[MeridianStarGazer]

11-12" sound like nice sizes. Very portable, but satisfying views.

[N3p]

11-12" sound like nice sizes. Very portable, but satisfying views.

I am very satisfied with the views in my 8" already! But I am ready for a larger option, I know the difference between 6" and 8" and the difference between 8" and 10". 12" will hit the spot. Just another option of telescope along with my smaller ones. 

 

For the eyepiece height, roughly F6 it looks good also, to observe while sitting. However I am not 100% sure with an equatorial platform under the telescope.

Edited by N3p, 13 June 2022 - 10:46 AM.

[MeridianStarGazer]

I am very satisfied with the views in my 8" already!...

I was somewhat happy with my 8". But the trasportability detracted from the fun. Get a good ratio views to hassle, and you'll be happier. Others will argue that once set up, it needs to be solid and free of slewing hassles. So there is that too.

[MeridianStarGazer]

Regardless, 12" is the biggest size that is easy to construt stiffly. Go up to 16", and you have to really think about spider stiffness and secondary holding.

[N3p]

Regardless, 12" is the biggest size that is easy to construt stiffly. Go up to 16", and you have to really think about spider stiffness and secondary holding.

 

Oh ok I'll keep that in mind, the stiffness, I want it to be good the first time and it's my first experience, I'll keep the stiffness in mind while choosing the spider.

[N3p]

I was somewhat happy with my 8". But the trasportability detracted from the fun. Get a good ratio views to hassle, and you'll be happier. Others will argue that once set up, it needs to be solid and free of slewing hassles. So there is that too.

I agree 100% with the hassle..it appears like 12" is the biggest I am willing to handle.. the hardest part is the idea of building and collimating at each trip, that's about the most difficult thing in my mind.

 

But I don't have a choice really.

 

it really depends on the day,  on a certain night I take the 6" instead of the 8", to save me some pain, on other nights, I would have the energy to handle a 12". I feel the pain of not owning a bigger mirror right now, I know what I am missing.

 

 

[MeridianStarGazer]

I agree 100% with the hassle..it appears like 12" is the biggest I am willing to handle.. the hardest part is the idea of building and collimating at each trip, that's about the most difficult thing in my mind.

But I don't have a choice really.

it really depends on the day, on a certain night I take the 6" instead of the 8", to save me some pain, on other nights, I would have the energy to handle a 12". I feel the pain of not owning a bigger mirror right now, I know what I am missing.

You can design a 12" that weighs lesd than the 8" and sets up fast. Only issue is Tublugs are pricey. But once you have them, precision collimation is easy, I'm told.

[MeridianStarGazer]

If you are willing to go faster, you can make a 12" lighter than a GSO 6". But I understand the desire for f6.

[N3p]

You can design a 12" that weighs lesd than the 8" and sets up fast. Only issue is Tublugs are pricey. But once you have them, precision collimation is easy, I'm told.
 

I want to put more emphasis on the regidity of the instrument and the mirror before the weight, ideally I would prefer to have a mirror with a thickness of 1/6 of the diameter, that would be 50mm in the case of a 12". It will depend on the price and the available thicknesses, somewhere between 38mm and 50mm of thickness for the primary to prevent the distortion. I suspect that the primary cell will be easier to build also with a thicker mirror.

 

The mirror box will certainly be between 30 and 40 lbs, ideally bellow 40lbs (and hopefully not more then that)

 

F6 would be gentle on the eyepieces and good on the moon!

Edited by N3p, 13 June 2022 - 01:16 PM.

[Don H]

I think your worries about set up and collimation are more than they need to be. I have a 12.5" f/5.9 and it only takes me about 10-15 minutes to set it up, and even faster to take it down. And the collimation is often right on after setting it up. Even my finder often remains very close after each assembly. It helps if you number your blocks and poles, so they are in the same spot every time. 

 

If you like to keep your feet on the ground, f/5.5 is probably the way to go. When I made my 12.5" mirror and scope, my goals were very similar to yours. To view planets and DSOs from my suburban patio, but to also have a capable scope for dark skies. I was shooting for f/6 during grinding, but when I overshot to f/5.9, or 73.75", I figured that would be fine to keep me off a step a wee bit more. It also did not require making my diagonal larger. I chose a 2.1", but at f/5.5 maybe a 2.6" might be best. It could depend on your UTA diameter and focuser height.

 

My mirror is only 1" thick, so I had to make a tall mirror box to balance without adding counterweights. But that made the poles shorter and the structure more rigid. The 1" mirror and Novak cell allow the mirror to passively cool most nights. The 12.5" is a great size, although I kept smaller scopes, too. I also made 14.3 and 16" truss scopes, but favored the lightweight 12.5" as time progressed. I think you will be very happy when you are done.

 

Here are my 8, 10 and 12.5" set up together to give you an idea of their comparative sizes. I also included a photo of my 13.1" f/5.3. It had a 69.5" FL and was still feet on the ground all the time. If you are getting your glass custom made, you might consider getting it made to f/5.75, or a 69" FL, depending upon your height. I am 5'11" or so. This might allow you to use the 2.1" flat, too.

 

 

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[N3p]

I think your worries about set up and collimation are more than they need to be. I have a 12.5" f/5.9 and it only takes me about 10-15 minutes to set it up, and even faster to take it down. And the collimation is often right on after setting it up. Even my finder often remains very close after each assembly. It helps if you number your blocks and poles, so they are in the same spot every time. 

 

If you like to keep your feet on the ground, f/5.5 is probably the way to go. When I made my 12.5" mirror and scope, my goals were very similar to yours. To view planets and DSOs from my suburban patio, but to also have a capable scope for dark skies. I was shooting for f/6 during grinding, but when I overshot to f/5.9, or 73.75", I figured that would be fine to keep me off a step a wee bit more. It also did not require making my diagonal larger. I chose a 2.1", but at f/5.5 maybe a 2.6" might be best. It could depend on your UTA diameter and focuser height.

 

My mirror is only 1" thick, so I had to make a tall mirror box to balance without adding counterweights. But that made the poles shorter and the structure more rigid. The 1" mirror and Novak cell allow the mirror to passively cool most nights. The 12.5" is a great size, although I kept smaller scopes, too. I also made 14.3 and 16" truss scopes, but favored the lightweight 12.5" as time progressed. I think you will be very happy when you are done.

 

Here are my 8, 10 and 12.5" set up together to give you an idea of their comparative sizes. I also included a photo of my 13.1" f/5.3. It had a 69.5" FL and was still feet on the ground all the time. If you are getting your glass custom made, you might consider getting it made to f/5.75, or a 69" FL, depending upon your height. I am 5'11" or so. This might allow you to use the 2.1" flat, too.

Your telescopes are really nice really, great craftsmanship, It must be a great design you have if you can almost keep your settings, I really wish for the same thing. Finding the right parts is not a done deal at all, this will take some time.

 

Your 12.5 is way too high, mine need to be smaller! my height is 5'-10.75" because it will have an equatorial platform under the telescope eventually.

 

Thanks for your support!

Edited by N3p, 13 June 2022 - 04:35 PM.

[N3p]

I think your worries about set up and collimation are more than they need to be. I have a 12.5" f/5.9 and it only takes me about 10-15 minutes to set it up, and even faster to take it down. And the collimation is often right on after setting it up. Even my finder often remains very close after each assembly. It helps if you number your blocks and poles, so they are in the same spot every time. 

 

If you like to keep your feet on the ground, f/5.5 is probably the way to go. When I made my 12.5" mirror and scope, my goals were very similar to yours. To view planets and DSOs from my suburban patio, but to also have a capable scope for dark skies. I was shooting for f/6 during grinding, but when I overshot to f/5.9, or 73.75", I figured that would be fine to keep me off a step a wee bit more. It also did not require making my diagonal larger. I chose a 2.1", but at f/5.5 maybe a 2.6" might be best. It could depend on your UTA diameter and focuser height.

 

My mirror is only 1" thick, so I had to make a tall mirror box to balance without adding counterweights. But that made the poles shorter and the structure more rigid. The 1" mirror and Novak cell allow the mirror to passively cool most nights. The 12.5" is a great size, although I kept smaller scopes, too. I also made 14.3 and 16" truss scopes, but favored the lightweight 12.5" as time progressed. I think you will be very happy when you are done.

 

Here are my 8, 10 and 12.5" set up together to give you an idea of their comparative sizes. I also included a photo of my 13.1" f/5.3. It had a 69.5" FL and was still feet on the ground all the time. If you are getting your glass custom made, you might consider getting it made to f/5.75, or a 69" FL, depending upon your height. I am 5'11" or so. This might allow you to use the 2.1" flat, too.

I will work to determine that in the next days, I did the exercise previously, but I don't fully remember how now.
 

[Don H]

I will work to determine that in the next days, I did the exercise previously, but I don't fully remember how now.
 

This link to Mel's stuff might help. Click on the Diagonal button to try different sizes, focal lengths, etc. His whole website is fun to check out...

 

https://www.bbastrod...wtDesigner.html

Edited by Don H, 13 June 2022 - 10:12 PM.

[N3p]

This link to Mel's stuff might help. Click on the Diagonal button to try different sizes, focal lengths, etc. His whole website is fun to check out...

 

https://www.bbastrod...wtDesigner.html

Thank you, I didn't know his website, looks very interesting.

 

I think it would be useful to know which focuser I am going to use also for my project, there does not seem to be a large number of models available right now. In canada I can get a Antares dual speed such as this one, but the height specifications, probably not available.

 

https://www.ontariot...ings_p_770.html

 

(Oh it's listed in Mel's calculator. I'll check the website first.)

Edited by N3p, 14 June 2022 - 09:14 AM.

[MeridianStarGazer]

This link to Mel's stuff might help. Click on the Diagonal button to try different sizes, focal lengths, etc. His whole website is fun to check out...

https://www.bbastrod...wtDesigner.html

You can set the focuser height to whatever you want, but his default is 3 inches. Very low. Likely because he uses f3, and CC intrusion is better than an even bigger secondary.

[Piero DP]

Nothing wrong with the F6, but F5.5 will be more manageable as you might not need a step to observe near the zenith.

Regarding planetary observing, I wouldn't worry about the focal ratio. My F4 dob with PC2 delivers super views of the planets.

[N3p]

You can set the focuser height to whatever you want, but his default is 3 inches. Very low. Likely because he uses f3, and CC intrusion is better than an even bigger secondary.

Yeah it looks like a complete tool, I really like the website.

 

 

Nothing wrong with the F6, but F5.5 will be more manageable as you might not need a step to observe near the zenith.

Regarding planetary observing, I wouldn't worry about the focal ratio. My F4 dob with PC2 delivers super views of the planets.

Oki! My F5 too, I consider it good on the planets, judging from the things I can see, it's impressive.

[N3p]

A bit of evolution today, I tried the calculator to find the height of my eyepiece with a quick simulation, my pupil is at 1650mm from the ground. My obvious path is a 12" with F5.5.  With a 12.5 Mirror, the eyepiece will be too high at F5.5 and F6, like in Don's picture.

 

With a 12" F.5.5, the rocker box would need to be as short as possible and hopefully a primary cell not thicker then 7". With a 5" thick equatorial platform, a step will be required, looks pretty definitive, with 12" F5.5 it's already above limits.

 

 

***

SECONDARY MIRROR:

 

I get these results for the choice of the secondary mirror, I don't know which one would be the best for my need, for general purpose telescope these are the numbers from the calculator. @Don H you suggested a 2.6 (66mm) previously.

 

Can I assume directly that the 66mm would be optimal for a general purpose telescope? I would like to use a GSO focuser if they are available again in 2022.

 

FL = 1650
FR - F5.5
Aperture = 300

Focal Plane to diagonal distance 263.6
Max Field diameter 40.64

Acceptable magnitude loss = 0.4

 

Results:  Smallest possible diagonal = 47.93 (mm)
Smallest user defined diagonal = 63 (mm)
Diagonal size maximizing illumination integrated across the field = 78 (mm)
Diagonal size for most even illumination across the field = 82 (mm)
    illumination integrated across the field:  63: 87.5%; 64: 88.3%; 66: 89.8%; 75: 93%; 78: 93.1%; 82: 92.5%;
Equivalent aperture ranges from edge of field to center of field for each diagonal size:
    63: 260.85 to 293.31 (mm); 64: 263.14 to 293.09 (mm); 66: 267.51 to 292.65 (mm); 75: 283.38 to 290.47 (mm); 78: 286.82 to 289.68 (mm); 82: 288.58 to 288.58 (mm);

Central obstructions are 63: 21%; 64: 21.3%; 66: 22%; 75: 25%; 78: 26%; 82: 27.3%;
RMS wave deformations due to central obstruction are 63: 1/22; 64: 1/22; 66: 1/21; 75: 1/19; 78: 1/18; 82: 1/17;

Diagonal offset on fully illuminated field (towards primary mirror and away from focuser) = 2.65 (mm)
Diagonal offset (along diagonal face) = 3.74 (mm)
Diagonal offset on focal point (towards primary mirror and away from focuser) = 2.69 (mm)
Diagonal offset above focal point (towards primary mirror and away from focuser) (76.2 (mm) above) = 2.92 (mm)
Diagonal offset field edge (towards primary mirror and away from focuser) = 1.40 (mm)

Edited by N3p, 16 June 2022 - 02:09 PM.

[Don H]

Quote:

"With a 12" F.5.5, the rocker box would need to be as short as possible and hopefully a primary cell not thicker then 7". With a 5" thick equatorial platform, a step will be required, looks pretty definitive, with 12" F5.5 it's already above limits."

 

"I get these results for the choice of the secondary mirror, I don't know which one would be the best for my need, for general purpose telescope these are the numbers from the calculator. @Don H you suggested a 2.6 (66mm) previously.

 

Can I assume directly that the 66mm would be optimal for a general purpose telescope?"

 

 

The height of the rocker box only depends upon the height of the mirror box. The height of the mirror box usually depends upon the weight of your cell and mirror, assuming you do not want to add weights to balance. Then it is just the length of the truss poles that varies, or where the bearings or cradle go if it's a tube. That is why the mirror and rocker box for my 12.5" is so tall. My mirror is only one inch thick. The whole LTA, including mirror, cell, box, bearings, blocks, etc is 35lbs. The bearings balance at the top of the box. One can move the bearings up and use a shorter box, but they will begin to flex or need to be built more sturdy as the box support becomes less.

 

You might also consider that if you are using your scope for planetary views on the eq platform, it will almost never be pointed at the zenith. You will probably be able to sit for that more often than not.

 

For your diagonal, a 2.1" may still be good for 12" instead of 12.5". Others may have better supported opinions to go one way or another. If you start a thread just for that, those who know most might reply. Here is a screenshot form Mel's calculator showing both. The 9.5" distance from diagonal to focal plane might be the deciding factor.

 

 

 

 

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[N3p]

Quote:

"With a 12" F.5.5, the rocker box would need to be as short as possible and hopefully a primary cell not thicker then 7". With a 5" thick equatorial platform, a step will be required, looks pretty definitive, with 12" F5.5 it's already above limits."

 

"I get these results for the choice of the secondary mirror, I don't know which one would be the best for my need, for general purpose telescope these are the numbers from the calculator. @Don H you suggested a 2.6 (66mm) previously.

 

Can I assume directly that the 66mm would be optimal for a general purpose telescope?"

 

 

The height of the rocker box only depends upon the height of the mirror box. The height of the mirror box usually depends upon the weight of your cell and mirror, assuming you do not want to add weights to balance. Then it is just the length of the truss poles that varies, or where the bearings or cradle go if it's a tube. That is why the mirror and rocker box for my 12.5" is so tall. My mirror is only one inch thick. The whole LTA, including mirror, cell, box, bearings, blocks, etc is 35lbs. The bearings balance at the top of the box. One can move the bearings up and use a shorter box, but they will begin to flex or need to be built more sturdy as the box support becomes less.

 

You might also consider that if you are using your scope for planetary views on the eq platform, it will almost never be pointed at the zenith. You will probably be able to sit for that more often than not.

 

For your diagonal, a 2.1" may still be good for 12" instead of 12.5". Others may have better supported opinions to go one way or another. If you start a thread just for that, those who know most might reply. Here is a screenshot form Mel's calculator showing both. The 9.5" distance from diagonal to focal plane might be the deciding factor.

 

I started a new thread on the subject there:

https://www.cloudyni...ndary-question/

 

I rarely use my telescope at the zenith even for DSOs, it's true!

 

 

 

 

 

 

[N3p]

Update here:

 

The project shifted into another direction then expected, I ordered a 12" F5 GSO solid tube with the 10:1 focuser, and a pair of ring, no base just the OTA. I'll build a strong thick plywood base myself and now I have no choice, telescope will require a good equatorial platform for me to be able to sketch the moon. The eyepiece height should be comfortable while sitting @ F5, the finder will be high enough to search the sky.

 

I am prepared to start over the EQ project a couple of time to make it very good. (I doubt I'll have success the first time... but we shall see)

 

  Wow after 6 years using the 8" and knowing it's limits, I had an amazing observation 2 days ago again with it. I can't believe that I'll finally have 4 more inches of aperture ,  it's truly a dream coming true.