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Astrophotography forum => ATM, Optics and DIY Forum => Topic started by: Michael Postle on December 29, 2017, 06:10:17 PM

Title: Dimensions for 8" f6 reflector
Post by: Michael Postle 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
(https://s14.postimg.org/pq0au5ib1/0_attachment_00.jpg) (https://postimg.org/image/pq0au5ib1/)

(https://s14.postimg.org/qsahcqbf1/0_attachment_01.jpg) (https://postimg.org/image/qsahcqbf1/)
Title: Re: Dimensions for 8" f6 reflector
Post by: Owen Khan 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.
Title: Re: Dimensions for 8" f6 reflector
Post by: Clyde Duke 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.
Title: Re: Dimensions for 8" f6 reflector
Post by: Jacob Cota 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.
Title: Re: Dimensions for 8" f6 reflector
Post by: stancacoho 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
Title: Re: Dimensions for 8" f6 reflector
Post by: Eric Mannasseh 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.
Title: Re: Dimensions for 8" f6 reflector
Post by: mellidonde 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
Title: Re: Dimensions for 8" f6 reflector
Post by: Alex Strouth 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.
Title: Re: Dimensions for 8" f6 reflector
Post by: unexaric on January 22, 2018, 10:17:28 PM
Don

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

Regards ,Ziridava
Title: Re: Dimensions for 8" f6 reflector
Post by: Colin Ramadan on January 23, 2018, 07:01:58 PM
A wire spider is harder to make but can give much better images.
Title: Re: Dimensions for 8" f6 reflector
Post by: Eric Mannasseh 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
Title: Re: Dimensions for 8" f6 reflector
Post by: Roberto Betancourt 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.
Title: Re: Dimensions for 8" f6 reflector
Post by: tingranseattters 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.
Title: Re: Dimensions for 8" f6 reflector
Post by: Ralph Sonberg on January 31, 2018, 10:43:44 AM
Take your pick:
https://www.cloudyni...4746_176849.jpg (https://www.cloudynights.com/uploads/gallery/album_4746/gallery_217007_4746_176849.jpg)
Title: Re: Dimensions for 8" f6 reflector
Post by: housletica 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.
Title: Re: Dimensions for 8" f6 reflector
Post by: bankrybettdog on February 02, 2018, 05:54:59 PM
The spikes get thicker as the % of obstruction of the aperture increases.
Thinner spider vanes (note the spelling) yield thinner spikes.
The spike increases in apparent length because what light is in the spikes is compressed into a smaller area.
The visibility of spikes, because they are fainter than the star, is dependent on the same factors that influence the visibility of DSOs:
exit pupil, contrast with the night sky, etc.

For a smaller scope, the 3-vane spider is a viable alternative to 4 vanes.
Note how the curved spider puts more glow directly around the planet.
http://www.fpi-proto.../img/spikes.gif (http://www.fpi-protostar.com/img/spikes.gif)
Title: Re: Dimensions for 8" f6 reflector
Post by: facwindpsychco on February 08, 2018, 12:01:32 AM
NVR saw a glow with my spider. Maybe I'll have to make a 20min exposure to see it.When theory and experience dont match...
Title: Re: Dimensions for 8" f6 reflector
Post by: jingdilenma on February 08, 2018, 03:27:49 AM
Oh good heavens, I LOVE my 3-vane spider on my 8" f/6 dob. It replaced a 4-vane about 15 years ago.

Is anyone making those anymore? I would probably avoid Protostar even though that's where I bought mine.
Title: Re: Dimensions for 8" f6 reflector
Post by: scenunhadef on February 09, 2018, 12:01:41 AM
And 1800destiny is 3 week lead time.
Title: Re: Dimensions for 8" f6 reflector
Post by: Adam Washington on February 09, 2018, 09:10:36 AM
I made just one attempt or experiment at using curved spider.
One recommendention I remember from ''Gleanigs for ATM'' articles by Cox in ''Sky&Telescope'' is to keep the thickness of the arms at 1/50 of primary mirror diameter.
In my experiment that mean a tickness of 1.44mm so I made the blades 1.5mm thick.
What I obtained is something I don't like : the light from the spikes is spread across the whole visual field like the haze produced by the spherical abberation of a not parabolized mirror.

 bbcodeImage-js (do not remove or edit this tag)
(https://www.cloudynights.com/uploads/gallery/album_5472/sml_gallery_211611_5472_143913.jpg) (https://www.cloudynights.com/gallery/image/42325-spider-t72mm-f810mm/)Before , the secondary was hold by a 4mm thick arm made from a piece of welding electrode.
I'm seriously considering to go back to the one arm : it produced one spike but it was dim and it disturbed me less than the ''haze'' I reported.

Ziridava
(https://s9.postimg.org/ghairxstn/19_attachment_00.jpg) (https://postimg.org/image/ghairxstn/)
Title: Re: Dimensions for 8" f6 reflector
Post by: Drew Bullets on February 09, 2018, 11:33:22 AM
Quote
Oh good heavens, I LOVE my 3-vane spider on my 8" f/6 dob. It replaced a 4-vane about 15 years ago.

Is anyone making those anymore? I would probably avoid Protostar even though that's where I bought mine.

I've seen them in a number of inexpensive scopes. I had a 5" f/6.5 newtonian with a 3-vane spier, and I used to sell a 4.5" with a 3-vane spider.
But I think many of the ones I've seen in the field in scopes >6" have been home-made.
I am also concerned with the ability of the spider to hold collimation as the scope points low, and this seems to be the Achilles foot of the 3-vaned curved spider,
especially as the scope gets larger than 8".