27 replies to this topic

[mark379]

Hello,

A friend of mine gave me the Idea to use my spare 8" f6 Newtonian as an inexpensive double star/ planetary scope. Was thinking of changing the secondary to something smaller and putting the curve vane spider to diffuse the diffraction spikes to find close doubles.

Any recommendations???

Thanks,
Mark

[Max Power]

Get a quality main mirror, you know with papers. Or get yours refigured.

 

If the scope is not very good, optimizing only gains a few %, not hardly worth doing cause you mite not be able to see any difference.

 

My 8" f/5 was optimized for photos, refigured main mirror to 1/14 wave, moved mirror up tube so focal point would reach camera, bigger second mirror,  just about everything you shouldn't do for visual.  And IT'S FANTASTIC for visual for planets.  I have to use a 35mm extension tube so eyepieces will focus, but it's still the best 8" I've ever had for planetary.

 

Beats the old 8"f/7 optical mechanics 1/8 waver easily.

[macdonjh]

Conventional wisdom, like Max alluded to above, is to install the smallest secondary mirror practical, extend the OTA a few inches past the secondary to block stray light, blacken the OTA, perhaps a boundary layer fan, make sure your primary is as good as possible.

 

I don't notice the diffraction spikes from my straight spider on planets, except Venus.

[jgraham]

Pull the mirror back (add a short tube extension if you have to), reduce the size of the secondary, curved vane spider, and add a low profile focuser. A good ray-trace diagram would be helpful in optimizing the size and spacing of the mirrors.

[CHASLX200]

A F/6 will do great if the optics are top notch.

[mark379]

Yes Optics are good in this scope, I'd like to improve it for double star use and possibly planetary too.

[Pinbout]

A 20% 2ndry thats not a stock 2ndry. For planetary. For dbls a larger 2ndry helps with higher frequencies.

 

I dont care about curved spiders. I have one and dont see that big of a difference on my 8inf6.

 

What kind of mount are you going to use?

 

A metal tube needs insulation inside as well as flocking opposite the focuser.

 

I like 1in all around the primary, are imported tubes sized smaller? Get a hasting tube if you want alum.

[mark379]

Danny,

I do plan on flocking around the primary and the secondary / focuser. So you say keep the stock size secondary, and not to do curved vane? I was told that the curved vane may help with tight doubles and keep the diffraction spikes from interfering.

Thoughts?

[Pinbout]

A 1.52" is that stock? Antares optics. Unless you test the 2ndry. I've  seen  a lot of messed up ones.

 

Never really heard that about the vanes, cause the curved ones spread the diffraction all over everything.

 

But a large central obstruction can make the the mft curve better than a 100% in the high frequencies. So just make a 33% or 50% mask and see how the difference is.

[mark379]

I think My Orion scope has something around 1.83-2" on the secondary. I was told to try to go down to 1.5"

 

So for splitting doubles which is better, large or small secondary? I thought smaller would have been better to split the tight ones.

Thoughts?

[Pinbout]

My 8in has a 1.52 and its awesome on jupiter, and i tested the primary at delmarva and i know its about 1/4 wave overcorrected.

[mark379]

1.52 was what I was considering... Thoughts on that mirror on doubles? Was thinking of going protostar...

My 8in has a 1.52 and its awesome on jupiter, and i tested the primary at delmarva and i know its about 1/4 wave overcorrected.

[CHASLX200]

I think My Orion scope has something around 1.83-2" on the secondary. I was told to try to go down to 1.5"

 

So for splitting doubles which is better, large or small secondary? I thought smaller would have been better to split the tight ones.

Thoughts?

Smaller is better. Going from a 1.83 to a 1.52 won't make a huge diff.  A 1.30 would be better, but then you would need a low profile focuser and using low power 2" eyepieces could be effected.

 

Chas

[mark379]

at 1.3 it would cause vinyetting with 2" ep's?

I think My Orion scope has something around 1.83-2" on the secondary. I was told to try to go down to 1.5"

 

So for splitting doubles which is better, large or small secondary? I thought smaller would have been better to split the tight ones.

Thoughts?

Smaller is better. Going from a 1.83 to a 1.52 won't make a huge diff.  A 1.30 would be better, but then you would need a low profile focuser and using low power 2" eyepieces could be effected.

 

Chas

 

[Pinbout]

I wouldn't go that small, too much trouble.

Binoviewers would be great for high mag planetary views.

[eyepiecedropper]

This is a heavily modified 8"f/6 GSO Low Expansion Newt. Phenolic tube. Skywatcher mounting rings.

 

1,3inch secondary. Shortened Kineoptics helical focuser. Gives 1/2" of 100% illuminated FOV. 16% Obstruction. 9,5 kgs total weight.

 

Would I do it again: no. I would choose bigger aperture instead.

Martin

 

 

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[KaStern]

Hello,

A friend of mine gave me the Idea to use my spare 8" f6 Newtonian as an inexpensive double star/ planetary scope. Was thinking of changing the secondary to something smaller and putting the curve vane spider to diffuse the diffraction spikes to find close doubles.

Any recommendations???

Thanks,
Mark

 

Hello Mark,

 

your friend has a good ideas  

 

First of all I like to say that perfect collimation is vital.
Although not very fast an 8" f/6 must be near-to-perfection collimated to give best views.

 

The next point is thermal behavior of the telescope.

Depending on the glass-type of the mirror the mirror has to cool down

longer or shorter to re-gain it perfect parabolic curve.

But even if the mirror is made from ultra-low-expansion material

the mirror needs cool-down.

Otherways the mirror warms up the air surrounding the mirror,

the air gets lighter than the rest of the air in the tube and so it rises up

and causes density differences of the air, disturbing the incoming wavefront.

Therefore you need to control the thermal behavour of the scope.

I recommend to add a fan behind the mirror wich sucs the air out of the tube.

The fan should not cause vibrations!

 

The tube should not consist of thin metal. This has disadvantages.

During a clear night a tube wich is directed to an object wich is 45° above the horizon

cools down on the upper side due to infrared radiation.

The metal cools down the air at the inner tube wall and this begins to fall down.

Again you will get density differences inside the tube an this disturbes the wavefront,

lowering the contrast and if it is bad, even the resolution.

Non-metal tube material plus flocking inside can reduce these problems a lot.

Here in Europe the Gerd Neuman HP Tubes are well known.

In the USA there are these from fpi-protostar:

http://www.fpi-protostar.com/blite.htm

 

The tube should be longer than the original one. I would make it 20cm to 30cm longer.

Or add an additional 20 to 30cm long removeable dew-cap.

All this preventy the observer's breath from crosing the light path and prevent straylight.

 

Having done all this it is time to know about the optical quality of the primary mirror.

If it is good enough (I would consider a 0.95 Strehl as "good enough", but better is welcome)

this is fine, but if it is only 0.85 or worse one should get a better mirror, or refigure it.

The secondary should be very good. It will be a replacement,

because in many cases the original secondary is not good enough and it is unnecessarily big.

 

Take a low focusser. A helical or one with 1:10 reduction for precise focussing.

 

You can tak a curved spider like this one

 

http://www.fpi-proto...com/crvmnts.htm

 

to avoid spikes. Spikes can be bothersome if you want

to split a double in wich the second star is much less bright.

The spike emerging from the brighter star can hide the second star.

 

Ed Turco has decided to make an optical window, another solotion,

wich you can read in his paper there:

 

http://www.cloudynig...reflector-r2983

 

 

... my window gets rid of the problem of spider diffraction ...

 

While close doubles and planets do only need a small field,

the moon is so big that you might find the edge correction of a newtonian a bit too bad.

You can largely increase the diameter of the diffraction limited field of a newt

by using a coma-compensating device.

This can be a coma-corrector (there are some of good quality and some less-good),

a coma-compensating eyepiece (in former times there was a "Pretoria Eyepiece"

designed by H.Klee and distributed by by University Optics,

and a Brandon Comacompensating Eyepiece, unfortunately all were discontinued)

or a coma-compensating barlow (2,8x or 2,2x University Optic Klee Barlow, made in Japan,

discontinued, and today there is a APM comacompensating barlow, made in China).

 

And you should use eyepieces that do not show light scatter.

The cheapest solution is a good upper-mid-class Orthoscopic

like the U.O. HB Abbe Orthos:

 

http://www.universit...125inch.html#HD

 

Behind a good barlow they perform very well in my own 8"f/6 ATM newtonian. 

 

I hope I could give you some useful hints.

 

Cheers, Karsten

[mark379]

SO I am looking at going with portostar for the secondary ( 1.52") and curved U spider...

Should make the 8" f6 even better at doubles and planets....

Thoughts?

 

 

http://www.fpi-proto...com/crvmnts.htm

 

http://www.fpi-proto....com/quartz.htm

[gnowellsct]

SO I am looking at going with portostar for the secondary ( 1.52") and curved U spider...

Should make the 8" f6 even better at doubles and planets....

Thoughts?

 

 

http://www.fpi-proto...com/crvmnts.htm

 

http://www.fpi-proto....com/quartz.htm

 

Protostar makes first rate stuff (I have two) but they haven't shipped secondaries for a couple of years and have ticked off a large number of people.  So I would get something else with the idea of getting a protostar at a later date, if they ever start shipping inventory on a regular, predictable basis.  Greg N

[mark379]

Greg,

Any good second choices? Do they still ship the spiders?

Best,

Mark

 

SO I am looking at going with portostar for the secondary ( 1.52") and curved U spider...

Should make the 8" f6 even better at doubles and planets....

Thoughts?

 

 

http://www.fpi-proto...com/crvmnts.htm

 

http://www.fpi-proto....com/quartz.htm

 

Protostar makes first rate stuff (I have two) but they haven't shipped secondaries for a couple of years and have ticked off a large number of people.  So I would get something else with the idea of getting a protostar at a later date, if they ever start shipping inventory on a regular, predictable basis.  Greg N

 

[gnowellsct]

does royce sell curved spiders?  His designs are attractive

[gnowellsct]

http://www.rfroyce.c..._10_spider3.jpg  for the spider

 

write to him here

 

http://www.rfroyce.com/

 

These guys look very good I would consider this design.  The protostar is simpler (blocks less light) but these should be great

 

http://www.destinyco... store menu.htm

 

I've never done business with them however.

 

Greg N

[gnowellsct]

I wouldn't mess with the secondary size.  Anything you shrink in that department comes back at you in terms of field illumination.  You might say NOW that you don't want the scope for looking at wider field objects but once you are OUT and the planet has set or the seeing goes to hell do you really want to haul out another scope?

 

A scope without two inch wide field eyepiece capability today is a crippled scope IMO.  

 

When I optimized my 4.25" f/10 Newt it had a single stalk spider and a 3/4 inch secondary giving a 17% central obstruction.  That was the 1960s version of "optimized" for planets.

 

In my rebuild I decided to optimize *collimation*.  I got a three vane spider (protostar, they were shipping in those days) and tossed the crummy focuser and put on a good one.  If the focuser doesn't wobble you get better collimation.  The protostar had many more degrees of movement than the stupid single stalk diagonal.  I also got a better mirror cell, more easily adjustable.

 

I put on (horrors!) a one-inch secondary giving a 24% central obstruction.  

 

The result of these changes was small Newt that performs as well on Jupiter as my 4" refractor, near as I can see.

 

Getting a curved secondary does get rid of the unwanted issue of spikes at higher powers so I would do that but I would pay more attention to the other hardware (mirror cell, focuser) more than to the central obstruction.  And with the larger secondary mirror you'll have a scope that can go from very nice low level magnifications to sharp high level magnifications.  

 

My 2c.  A good planetary scope, in my view, is the one in which fewer compromises have been made in terms of the quality of the components, as opposed to the largely theoretical issues of optical theory with regard to central.

 

 =>You might also want to consider a boundary layer fan.

 

With good hardware and good collimation--and we hope an excellent mirror optic--you can have your planet apo eater *and* your wide field views.

 

With remarkable frequency, people tend to focus on single-shot solutions (such as obsessing about the secondary size) rather than looking at their scope as a total system.  

 

Greg N

Edited by gnowellsct, 06 August 2015 - 03:52 PM.

[gnowellsct]

I read the article on optical window with great interest.  In NJ you would get dew on the plate, same as an SCT, so you would have a dewed up Newt and none of the mechanical advantages of a short stubby SCT.

 

I think a curved secondary is better.  To be sure all it is doing is smooshing out the energy that would otherwise go into a spike but I can assure you that a curved secondary makes the eyepiece experience much nicer.  And it is much harder to dew up a solid tube Newt than it is to dew up something with a corrector plate or flat glass to hold the secondary.

 

The author objects to the idea of an inch or two of aperture to beat out an apo and he has a point, but Newtonian aperture is cheap, so it's an easy thing to do.  And the extra inch or two will yield deep sky benefits.  It's not wasted. 

 

Greg N

Edited by gnowellsct, 06 August 2015 - 04:06 PM.

[mark379]

Well,

Ordered the diagonal from Protostar ( 1.52") He said he would ship out next Monday which is nice, and went with a Destiny 3 vane Curved spyder...

Next some flocking paper, and then the project will begin in a couple of weeks ( I hope)

For Planetary Ep's , Ive currently got 8, 10, 12 mm delos,

Parks GS Ep's and some Zhumel planetaries which have been in my case for awhile...

How do you find Ortho's vs the parks GS? anyone do a side by side comparison?

Thoughts?

Edited by mark379, 11 August 2015 - 04:43 PM.