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Considering a 1 metre Optiques Fullum folded newtonian. Please debate pros/cons.

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#26 Ittaku

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Posted 25 January 2021 - 03:41 AM

Have you had the chance to look through a large ladder dob? If not, I would suggest you get in touch with the Astronomical Society of Victoria and ask if you can spend a night on their 40” f/3.3. Then see how you feel about ladder observing after that. A proper observing ladder with closely spaced steps can be quite comfortable. If you definitely don’t like the experience, that narrows down your options.

I'm already a member of the ASV. Not a fan of that ladder arrangement, (nor the scope's astigmatism).


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#27 Allan Wade

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Posted 25 January 2021 - 04:23 AM

Well yes, I wasn’t going to mention the big problem with that scope. Just proves you should choose your optician carefully.


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#28 StellarField20

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Posted 25 January 2021 - 08:49 AM

I'd love to see the advice from Optical engineers on the contrast issue,

 

I think alot of people are splitting hairs with the folded design and possible loss in contrast.

Most of us would never see the difference visually except maybe the most discerning visual observers.

 

If you are looking to be feet on ground with huge aperture, which seems to be a priority with OP, a folded design seems to fit the bill.

 

The folded design has the same central obstruction as an S.C.T design, less than most RC's...

Also for the light gathering issue, since loss in light gathering is measured by the square...Its a non issue...

Calculating rapidly a regular secondary vs a larger one for the folded and then comparaing to the primary's real estate...you are losing peanuts really on effective area.

 

I love my SCT's and don't really think about the central Obs when looking through them. 

I dont see why that would change for a very large newton in order to make it accesible from the ground.

 

As for location, If I was constrained to a bortle 4 sky, then the only way left to get more signal in is more aperture anyway.

OP also said he could still move it later under even darker skies, which would get him the full potential of the instrument.

 

One last thing I'm seeing, the folded design would mean a smaller telescope footprint as well.

This could help to keep observatory costs under control. Don't know if it offsets it by much, should be looked into.

 

In anycase, this seems like an awesome project!


Edited by StellarField20, 25 January 2021 - 09:00 AM.

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#29 Augustus

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Posted 25 January 2021 - 09:16 AM

Well yes, I wasn’t going to mention the big problem with that scope. Just proves you should choose your optician carefully.


I’m pretty sure the stig is a result of putting the cellular mirror in a sling. Tong insists that is the issue

#30 MitchAlsup

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Posted 25 January 2021 - 01:29 PM

I'd love to see the advice from Optical engineers on the contrast issue,

 

I think alot of people are splitting hairs with the folded design and possible loss in contrast.

Most of us would never see the difference visually except maybe the most discerning visual observers.

Most nights the atmosphere will not put up a good enough sky for the difference between 20% CO and 30% CO (in a 20" or above sized instrument) to be vsible--maybe once a year. 

 

2 decades us use of my 20" F/4 with a 18% CO taught me to ignore the rules of thumb of small instruments (smaller than 12"):: 20% and smaller COs are best for planets,... My 13" F/3 with 3.5" secondary produces better contrast than my 20" F/4 with a 3.8" secondary--its the other parts of the scope that mater, too.


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#31 Ittaku

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Posted 25 January 2021 - 04:04 PM


One last thing I'm seeing, the folded design would mean a smaller telescope footprint as well.

This could help to keep observatory costs under control. Don't know if it offsets it by much, should be looked into.

Their engineer Hugo has mentioned this to me many times as it leads to a far smaller observatory. The size of it at zenith is: 127cm x 127cm x 330cm, or 50"x50x80". That's very compact and could fit in a relatively narrow shed when folded down. We worked out that with a recessed base or a small elevated platform of about a foot around the scope, the eyepiece would be at my height standing at zenith since the eyepiece is at 203cm. That wouldn't sacrifice much of the lower altitude settings if I'm careful with its positioning and probably be more limited by the walls of the observatory. Time to do some calculations.


Edited by Ittaku, 25 January 2021 - 04:05 PM.

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#32 Cotts

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Posted 27 January 2021 - 02:08 PM

Random thoughts....

 

32" and 40" performance difference = 20/16 squared.  1.5 factor (rounded) which is only a fraction of a magnitude...

 

Exactly the same as the difference between an 8" and a 10"....... i.e.hardly worth worrying about...  in less than Bortle 1-2 skies perhaps not noticeable at all....

 

The warehouse ladder in a roll off roof - could it be laid down on its side when closing the roof?  How heavy is it?  Could one person lift it?  I'm thinking this sort of a stair weighs hundreds of pounds...probably has to remain vertical...roof options and wall heights will be greatly affected...

 

You seem to have some money to throw at the project - perhaps look into a hydraulic platform which will hold a nice padded chair/stool for seated observing...  Maybe a hydraulic scissor-lift with a motorized base and a chair on top...desk beside it for eyepieces and notebook....red lights.....controlled by a hand paddle/joystick..... use your imagination!!!

 

 

Dave


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#33 Ittaku

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Posted 27 January 2021 - 03:29 PM

You seem to have some money to throw at the project - perhaps look into a hydraulic platform which will hold a nice padded chair/stool for seated observing...  Maybe a hydraulic scissor-lift with a motorized base and a chair on top...desk beside it for eyepieces and notebook....red lights.....controlled by a hand paddle/joystick..... use your imagination!!!

Ooh I like that scissor lift idea, but it still doesn't get me a classic dob design any faster.


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#34 Cotts

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Posted 27 January 2021 - 03:59 PM

Ooh I like that scissor lift idea, but it still doesn't get me a classic dob design any faster.

Did you say that Optiques Fullum can get you your 40" mirror AND a Dob Structure to you in 8-10 months?????  that's fantastic!!

 

The scissor lift idea  would not have any affect on the delivery time of the telescope, of course.  If I were you I'd get the dimensions of the scope from Normand and start making pencil and paper diagrams of how you might achieve seated viewing with the 40"....  Combine hydraulic scissor lift with mobility scooter tech..........

 

It is anecdotally well known that when the observer is in a relaxed, seated position the observer/scope combination sees a bit deeper and shows a bit more detail.....as opposed to standing poised on a ladder......

 

Dave


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#35 Ittaku

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Posted 27 January 2021 - 04:09 PM

Did you say that Optiques Fullum can get you your 40" mirror AND a Dob Structure to you in 8-10 months?????  that's fantastic!!

Yes that's right. I've confirmed multiple times. They have the base materials on a mirror already which shortens the lead time.
 

 

 

The scissor lift idea  would not have any affect on the delivery time of the telescope, of course.  If I were you I'd get the dimensions of the scope from Normand and start making pencil and paper diagrams of how you might achieve seated viewing with the 40"....  Combine hydraulic scissor lift with mobility scooter tech..........

 

It is anecdotally well known that when the observer is in a relaxed, seated position the observer/scope combination sees a bit deeper and shows a bit more detail.....as opposed to standing poised on a ladder......

Sounds good, but it will only be 30cm off the ground to zenith with the NF folded so a scissor lift is serious overkill to get me up just 30cm. Nonetheless something that has me seated may well be worth pursuing.


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#36 MitchAlsup

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Posted 27 January 2021 - 04:45 PM

Random thoughts....

 

32" and 40" performance difference = 20/16 squared.  1.5 factor (rounded) which is only a fraction of a magnitude...

 

Exactly the same as the difference between an 8" and a 10"....... i.e.hardly worth worrying about...  in less than Bortle 1-2 skies perhaps not noticeable at all....

 

This is the same point I was trying to make earlier. Thanks Dave.

 

Two decades ago when F/ratios wer in the F/5 range (Obsession) and even a 20" used a ladder, one could see the advantages of the 40" over the 30" and all you were changing is the cost and the size of the ladder.

 

However, mirror technology has advanced and now F/3 is dé rigueur; the 20" is solidly feet on the floor, 30" is 1 step, and 40" is comercial ladder, may have tipped the balance back twords the 30" for ease of use considerations at the smallish sacrifice to the absolute magnitudes one can reach.


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#37 Tyson M

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Posted 27 January 2021 - 07:48 PM

I would totally get a massive mirrored dob from Normand Fullum.  Has to be a technofusion mirror with fans though. 

 

I like these ideas also. Also a provision to have a south wall fold down half way or something as well, to take advantage of the lower viewing if needed.

 

Did you say that Optiques Fullum can get you your 40" mirror AND a Dob Structure to you in 8-10 months?????  that's fantastic!!

 

The scissor lift idea  would not have any affect on the delivery time of the telescope, of course.  If I were you I'd get the dimensions of the scope from Normand and start making pencil and paper diagrams of how you might achieve seated viewing with the 40"....  Combine hydraulic scissor lift with mobility scooter tech..........

 

It is anecdotally well known that when the observer is in a relaxed, seated position the observer/scope combination sees a bit deeper and shows a bit more detail.....as opposed to standing poised on a ladder......

 

Dave


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#38 Ittaku

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Posted 27 January 2021 - 07:49 PM

I would totally get a massive mirrored dob from Normand Fullum.  Has to be a technofusion mirror with fans though. 

 

I like these ideas also. Also a provision to have a south wall fold down half way or something as well, to take advantage of the lower viewing if needed.

North wall here.


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#39 Tyson M

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Posted 27 January 2021 - 07:57 PM

North wall here.

Right! My mistake everything is backwards down there grin.gif


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#40 eyeoftexas

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Posted 27 January 2021 - 08:21 PM

 it will only be 30cm off the ground to zenith 

Have you considered the "Summit" chair from Cats Eye:  https://www.catseyec...mmitwcush-b.jpg


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#41 Ittaku

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Posted 27 January 2021 - 08:24 PM

Have you considered the "Summit" chair from Cats Eye:  https://www.catseyec...mmitwcush-b.jpg

I like it, I currently have a starbound chair which is a similar but smaller design.
 



#42 eyeoftexas

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Posted 27 January 2021 - 08:31 PM

I like it, I currently have a starbound chair which is a similar but smaller design.
 

The Summit goes from 8" to 52", so much higher than the starbound.


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#43 Tyson M

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Posted 27 January 2021 - 10:43 PM

Have you considered the "Summit" chair from Cats Eye:  https://www.catseyec...mmitwcush-b.jpg

I have been having an issue with eyepiece height with my large refractor with 2x 12" mount extensions.  Looking lower makes me stand at times.  Awesome chair.



#44 John_K

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Posted 28 January 2021 - 12:02 AM

As an active user of the ASV's 40" here In Victoria, I would say that if you can afford that level of aperture as well as going to the expense of a better design and stiffer structure, then totally go for it! Key to success in my mind will be to build a well insulated roll off shed as well as ensuring the telescope has best  in class active cooling to cater for the large temperature changes we experience in South East Australia. 

 

Once collimated, and once it has had time to cool down, the views through the ASV's 40", even for faint objects are simply breathtaking. For most objects they resemble B&W CCD images akin to what I am often shooting just a few meters away through my astrograph scope. Sure, the ladder is not ideal, but hey, it's a 40" scope! The views have made such an impression on me is that I have taken up sketching objects.

 

 

M42 through 40 inch..jpg

 

 

     


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#45 John_K

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Posted 28 January 2021 - 12:09 AM

and one more: 

 

M65andM66Drawing_Web.jpg


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#46 Ittaku

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Posted 28 January 2021 - 01:48 AM

Thanks all for your contributions. The next variable is how much to push that secondary. A few of you have already expressed how much contrast loss there would be from the relatively larger secondary as is, but I also have the option of going for a faster focal ratio. I know this will worsen that equation, but also bring with it a shorter focal length, wider FOV, (and potentially even lower viewing height.) The default focal ratio is 3.5, but they are happy doing down to F/3 by request. Hugo suggested 3.25 as a compromise - that brings it in line with the 3.3 of the ASV's 40" scope. By default, an F/3.5 will give me a rather long focal length of 3000 (I think) which is even longer than my SCT, but the SCT can be reduced whereas this is effectively a hard limit at these fast ratios. We do have some monstrous features in our Southern Hemisphere skies and it would be nice to see more of them. Thoughts?


Edited by Ittaku, 28 January 2021 - 01:49 AM.

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#47 Kunama

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Posted 28 January 2021 - 02:48 AM

Thanks all for your contributions. The next variable is how much to push that secondary. A few of you have already expressed how much contrast loss there would be from the relatively larger secondary as is, but I also have the option of going for a faster focal ratio. I know this will worsen that equation, but also bring with it a shorter focal length, wider FOV, (and potentially even lower viewing height.) The default focal ratio is 3.5, but they are happy doing down to F/3 by request. Hugo suggested 3.25 as a compromise - that brings it in line with the 3.3 of the ASV's 40" scope. By default, an F/3.5 will give me a rather long focal length of 3000 (I think) which is even longer than my SCT, but the SCT can be reduced whereas this is effectively a hard limit at these fast ratios. We do have some monstrous features in our Southern Hemisphere skies and it would be nice to see more of them. Thoughts?

Nice project !!!

 

You also need to factor in the issue of coma correction,  a SIPS or P2 will give you an effective focal length 1.15x the native (or an effective f ratio similarly magnified) this will affect your maximum TFoV and also your exit pupil calculations.

 

A 40" F3.25 @ 3302mm fl effectively becomes a 3797mm focal length @F3.74 which means an Ethos 21mm eyepiece will only serve up a TFoV 0.56 degrees with an exit pupil of 5.7mm.  Depending on your age you might find that the exit pupil will be larger than your dark adapted pupil.

 

Even a Nagler 31mm will only give a TFoV of 0.68 degree but with an exit pupil of 8.4mm so almost certainly leading to a "loss of light".....


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#48 Ittaku

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Posted 28 January 2021 - 03:21 PM

More information. The secondary obstruction is 36.8cm at F3/5, 38.1cm at F/3.25, and 39.4cm at F/3. Each extra change reduces viewing height by another 10cm. That's 7% more and 14.5% more obstruction for each step. No answer is correct in this world, but the F/3.25 option is very enticing, moving the eyepiece height at zenith to 193cm, and the focal length to ~3300.


Edited by Ittaku, 28 January 2021 - 03:31 PM.


#49 Cotts

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Posted 28 January 2021 - 03:45 PM

More information. The secondary obstruction is 36.8cm at F3/5, 38.1cm at F/3.25, and 39.4cm at F/3. Each extra change reduces viewing height by another 10cm. That's 7% more and 14.5% more obstruction for each step. No answer is correct in this world, but the F/3.25 option is very enticing, moving the eyepiece height at zenith to 193cm, and the focal length to ~3300.

So, as a percent-by-diameter  these are all in the 36-38% range which is a wee bit on the high side but not badly so.  Typical f/10 SCT's run around 30-35% and they give nice contrast, not as good as a refractor or purpose-built obstructed scopes, of course. 

 

The real loss of contrast occurs when you are getting into exit pupils around 1.5 or smaller.  At medium and high mags the contrast transfer loss will be noticeable when compared to a 40-inch unobstructed scope....tongue2.gif 

 

With a scope of 40-inches aperture the atmosphere will pretty much be the only limiting factor for resolution and contrast.  You will never actually see the degradation of the diffraction pattern - you would have to use this scope on the moon, in a vacuum for this to occur..

 

Dave.

 

 


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#50 clivemilne

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Posted 29 January 2021 - 12:48 AM

You don't absolutely have to fold the light path at a 45 degree angle...  

 

You could use a smaller round flat and send it out at 22.5 degrees for example.

 

I'm guessing a flat that size is going to be a custom order, so it may not be that big a deal to specify a purpose water jet cut profile anyway.


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