Author Topic: Dimensions for 8" f6 reflector  (Read 702 times)

Michael Postle

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Dimensions for 8" f6 reflector
« on: December 29, 2017, 06:10:17 PM »
I plan to build a Newtonian Reflector with an 8" f6 mirror and 1 1/2" diagonal.
I would appreciate any advice as to optomizing the design.
I have attached the results from the "Newt" design tool.
Can this design be improved?
Attached Thumbnails






Owen Khan

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Re: Dimensions for 8" f6 reflector
« Reply #1 on: December 31, 2017, 08:37:00 PM »
I would use a larger secondary and/or a low-profile focuser, otherwise you're going to get some field illumination loss.

Clyde Duke

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Re: Dimensions for 8" f6 reflector
« Reply #2 on: January 03, 2018, 07:55:01 PM »
Quote
I plan to build a Newtonian Reflector with an 8" f6 mirror and 1 1/2" diagonal.
I would appreciate any advice as to optomizing the design.
I have attached the results from the "Newt" design tool.
Can this design be improved?

Yes, the very minimum size secondary for an 8" f/6 with conventional 2" focuser is 2.14".
If you plan to use 1.25" eyepieces only and use an extremely low-profile 1.25" focuser, like a Kine-Optic, you can possibly get by with a 1.83".
But if you plan to use large 2" eyepieces with large field stops, go for 2.14".
For normal use in general viewing, 100% illuminate a 0.4-0.5" field and illuminate to the 75% level a field 1.5" wide.

Jacob Cota

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Re: Dimensions for 8" f6 reflector
« Reply #3 on: January 11, 2018, 01:51:22 AM »
I would use a 1.83in 2ndry

moonlite focuserif you want to minimize the 2ndry to focal plane reduce tube dia.

.25 wall thk is sturdy but a 10in tube will be heavy.

if you use a fan you can get away with a 9 something tube.

also make the tube longer or build and attachment so the spider won't be right at the edge of the tube.

stancacoho

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Re: Dimensions for 8" f6 reflector
« Reply #4 on: January 12, 2018, 02:38:14 AM »
I use a Kineoptics HC-2 focuser on my 200mm F/6.16 Dobsonian.
It allow me a 15mm fully illuminated field for a 46mm or 1.8'' secondary and focal plane at 182mm or seven inch from optical axis.
End of tube is at 182mm from focuser hole axis.
My tube is 250mm diameter , I'm not using and not plan to use any fans.

Your tube is too short, diameter is good.
As suggested , use a bigger secondary , 0.15'' fully illuminated field is way too small,it should be at least 7mm or 1/3''.
0.5'' for extra travel is too much.
Keep in mind ''Newt for the Web'' don't add the extra travel to ''focuser minimum height''.
I suggest to you to use a focuser baffle and don't disregard ''front aperture diameter''.
In the two weeks since I'm using this telescope I saw NGC 6196 galaxy of 13mv and split 16 Vulpeculae double star of 0.735''.

Good luck ,Ziridava

Eric Mannasseh

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Re: Dimensions for 8" f6 reflector
« Reply #5 on: January 12, 2018, 08:40:51 AM »
Quote
0.5'' for extra travel is too much.
Keep in mind ''Newt for the Web'' don't add the extra travel to ''focuser minimum height''.Good luck ,Ziridava

0.5" (12.7mm) of extra out-travel to the focal plane is not too much.
Many Barlow lenses and focal extenders require this much additional in-travel.
And it's entirely possible some eyepieces will too.
In my scope, for example, eyepieces focus over a 1.2" (30.5mm) range and that doesn't even take the extra in-travel, required by many Barlows, into account.
I would suggest, for safety (you may also have near-sighted friends) 3/4" of focal plane lift, i.e. the focal plane should be a minimum of 19mm above the racked-in focuser's top.

mellidonde

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Re: Dimensions for 8" f6 reflector
« Reply #6 on: January 13, 2018, 07:37:05 AM »
Don

I understand what you are saying and fully trust.
But please be pacient with me , I don't understand why Barlow lenses need extra-travel.
The barrel of my Barlow lenses are smooth , I have quite a wide range to locate them along the focuser axis.
This is why I don't understand why this is an issue for others.
If I lock my focuser at lower position , my Barlow with the eyepiece on top is towering above...
The Barlow lenses used by everybody else in this world have some shoulders or are they all that kind of short Barlow lenses ?

I provided 11mm in-travel just to be sure the focuser tube is not protruding in the optical pass when using short focus oculars.
Above this I added 18mm for my one and only two inch ocular LV 30mm.
This way I used 29mm of the 32mm travel of the HC-2.
This is how my focal plane ended at 182mm from optical axis.

I hope this details will be of some use to ''chasm3158''.

Ziridava

Alex Strouth

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Re: Dimensions for 8" f6 reflector
« Reply #7 on: January 15, 2018, 02:58:20 PM »
When an eyepiece is inserted into a barlow, the eyepiece is lifted out of the focuser by the length of its barrel.
If the Barlow were a simple extension tube, the eyepiece would need to move in the full length of its barrel to come to focus again.
But most Barlow lenses extend the focal plane of the scope backwards some.
Often, the backward extension is still not as much as the eyepiece has been lifted out of the focuser.
Ergo, additional in-travel of the Eyepiece + Barlow combination is required much of the time.
It varies a lot, but 1/2" of additional in-travel is often common.

And a near-sighted person may need a few millimeters more in-travel to focus for his eyes.

So it seems safe to allow for the focuser to move in by ~3/4" more than the eyepieces by themselves to plan for every contingency.
If you never use barlows, then you need allow only a few millimeters of additional in-travel to accommodate near-sighted friends.

unexaric

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Re: Dimensions for 8" f6 reflector
« Reply #8 on: January 22, 2018, 10:17:28 PM »
Don

Thank you for the explanation,now is clear to me.

Regards ,Ziridava

Colin Ramadan

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Re: Dimensions for 8" f6 reflector
« Reply #9 on: January 23, 2018, 07:01:58 PM »
A wire spider is harder to make but can give much better images.

Eric Mannasseh

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Re: Dimensions for 8" f6 reflector
« Reply #10 on: January 29, 2018, 10:15:06 PM »
Quote
A wire spider is harder to make but can give much better images.

that's a subjective statement.

I prefer curved spiders

Roberto Betancourt

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Re: Dimensions for 8" f6 reflector
« Reply #11 on: January 30, 2018, 03:36:55 PM »
Quote
Quote

A wire spider is harder to make but can give much better images.

that's a subjective statement.

I prefer curved spiders
As is yours. I prefer straight.

tingranseattters

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Re: Dimensions for 8" f6 reflector
« Reply #12 on: January 31, 2018, 09:17:28 AM »
Quote
Quote

Quote

A wire spider is harder to make but can give much better images.

that's a subjective statement.

I prefer curved spiders
As is yours. I prefer straight.
Duh! But my statement isn't about you.
From what I've in photos, wire spiders are skinnier but brighter , longer spikes.

Ralph Sonberg

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Re: Dimensions for 8" f6 reflector
« Reply #13 on: January 31, 2018, 10:43:44 AM »

housletica

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Re: Dimensions for 8" f6 reflector
« Reply #14 on: January 31, 2018, 11:58:40 AM »
Thank you for the input. I like that chart, too.

Well, some curved ones are a lot worse than others, but the best curved one looks better than straight ones, to my eye. It has a lot to do with the degrees of arc. Someone posted how many degrees are needed. Voodoo.As for straight veins, off axis they are supposedly going to show more. But I've not seen it in the eyepiece.

And for the person who told me to put a ruler in front of the aperture: I put my binocular strap across it. Is that close enough? No difference in brightness, at low power. I just say another spike of the same brightness.

From that chart, I'd think wire spiders are not the answer. However, I read some love their wires, and that maybe super thin ones are the key. Well, at least they block less light.

The post shows a thick spike in that picture. However, I never saw any spikes in my 114mm, not even on Arcturus, at any power. Also not in my 130mm, which had thick veins. So, forgive me if I doubt that picture and all the theory that goes with it. The 130mm was only at low power, but the 114mm was at very high power on a bright star.