surplus shed refractor

[JC]

Hi;
I was hoping to get some advice on building a refractor..
My thought and question is to what extent if any will the lens be vignetted by the dew shield. is this a matter of extending the light cone from focal point to lens diameter and outward to get the right diameter for the dew shield. This sounds logical at least to me..:help:

[MattT]

Hi JC,
Have a look at this: http://www.istarscopeclub.proboards.com/index.cgi?board=atm&action=display&thread=9 one of the best homemade scopes I've seen. Istar sell objectives in adjustable cells for $500 or so better IMO than surplus shed as its not seconds check them out at istar on my sig below.
Matt

[Shiraz]

Hard to describe in words, but here goes.
Determine the extremities of the focal plane that you want to keep clear of vignetting. For a rectangular detector, these will be the corners of the detector. From one of these extreme points, project a central ray back through the centre of the lens and out into the outside world.
Then, from the front of the lens, draw two rays extending out towards infinity, from each edge of the lens and parallel to the central ray. Do the same for the other side of the focal zone. The shield can be anywhere outside of the cone defined by the external rays.

[ZeroID]

Nope, the light is practically parallel before it enters your scope, the lens then bends (refracts) the light to a focal point to create the image so your dew sheild within reason can be as long as you want.

[JC]

Thanks Matt;
I already took the plunge and bought a 108 X 914 uncoated achro from surplus shed when they had a 30% off special. This is my 1st attempt at building so I wanted to keep costs down and see what I could achieve..Ive a friend who is a fitter who is helping out.
Cheers :thanx:

[JC]

Cheers ZeroID
Great; thats what I wanted to hear,
You put my mind at rest.
:thanx:

[JC]

Thanks Shiraz;
Thats sort of what I had thought/feared, as Ive designed the shield only slightly larger (5mm) in diameter than the objective. Suppose I can do the calculations and reduce the dew shield length if necessary.
Thanks :)

[MattT]

Hi JC,
Love to hear how the build goes. What are you using as a tube, focuser etc
I'm still thinking about doing my own 6" f12 DIY is about half the price of an OTA from Istar but 10X the hassle so interested in yours ....best of luck .
Matt

[Shiraz]

or just put up with a bit of vignetting. The other thing to consider is that anything you put within the input light cone can scatter light into your optics, so if the shield does encroach into your field of view, make it black.

[JC]

Hey Matt;
Yeah I bought a S/H 2 inch Skywatcher focuser here in the classified section. Im using 100mm (97inside) diameter ally pipe, slightly more wall thickness than standard 4 inch irrigation pipe. I also picked up an old aluminium fire extinguisher about 8mm wall thickness X about 118 outside diameter pipe, that a freind is machining up for me. Ive had to include an extra 70mm length @ 103mm to get the most out of the 108mm glass before the light reaches the 97mm tube. Once in the lens cell the glass will be reduced to 103mm.
I operate plant in landfill and work at a couple of different sites so have access to purchase materials at scrap value, which has been helpful here..
So anyway the design Im going with is for a non adjustable lens cell, just applying the KISS principle with this being a first ( and maybe last:rofl:) attempt n all.
Cheers

[brian nordstrom]

:) JC a dew shield that is 1 1/2 longer than the objective's diameter will be as efficiant as it can possibly be , eg . 150mm long for your lense set , good luck and please keep us all informed as to your progress.
Just draw the light cone out full size to get all relivent dimentions .
Brian.

[anj026]

Refractors are fun, good on you for having a go. It makes sense to start with something simple and inexpensive and learn as you go.

As Brian suggested you can draw out the light cone full size to get an idea of baffle apertures etc. If you don't have a piece of paper big enough then you can scale the drawing such that the vertical axis is 1/1 scale and the horizontal axis is 1/2 or 1/4 or 1/6 scale or whatever works for your focal length.

Here is a picture of the plan for my 5" f12 Istar. For the diameter of the focal plane I used 42mm as this is the field stop of the largest eyepiece I expected to use. If you are only using 1.25 inch eyepieces you could use a diameter of 27mm or even a bit less. Once you draw the lines from the focal plane to the lens aperture you can then derive the baffle apertures for any position along the focal length. In my case the baffles were equally spaced as they also served as frames within a plywood tube.

[JC]

Hi Andy;
I read your article on making the timber refractor, the finished product was beautiful.
So glad you raised the topic of internal baffles, I hadnt got that far so Im reading up on a couple of links now.
I get the tube and cell back on Saturday so should be able to put it together for a rough test by Monday, all going well, touch wood etc etc

:) Thanks

[anj026]

That is good to do a rough assembly of everything to check you have the tube length right for the desired focus range. You can use some pillows or cushions to aim the tube at the moon (or some distant terrestial object) if you don't have a useable mount yet.

Here is a link to refractor basics;
http://www.irpoyser.co.uk/makingtelescopes.php

Here is a link to the minimum baffle method;
http://www.berfield.com/baffles.html

If your tube diameter is not much bigger than the clear aperture it may be best to forget about baffles and just flock the inside of the tube with black velvet or similar method. Tele Vue make their refractors without baffles and it has the advantage of making the scope more compact for a given aperture.

About refractive objectives;
http://www.cloudynights.com/item.php?item_id=407

Useful magnification ranges for visual observing;
http://www.cloudynights.com/item.php?item_id=547

Simple telescope testing;
http://www.cloudynights.com/documents/aberrations.pdf

:)