4" f15 Refractor build with questions?

[dmurton]

I stumbled across a home built scope with a 4" f15 Jaegers in the front end, which cleaned up surprisingly well. The tube and focuser were a completely different story, not worth saving.

The plan is to make the tube out of 100mm aluminium tube (3mm wall thickness), and fabricate a collar that attaches to the cell for colomation. A spare feather touch focuser will be used at the other end. I am also thinking of segmenting the tube, for easier storage and transportation purposes. A further thought hit me (dangerous with astronomy!), that I could have the tube in two parts (front with the objective, the rear with the focuser), make 2 rears, one normal empty tube for night or white light viewing, one with the internals of a failed PST in there for H alpha viewing (obviously an ERF will be needed as well).

However, while researching baffles, I encountered an issue that I can not find an answer to. This website http://www.berfield.com/baffles.html seems to have the best description of working out baffle placement, but I can not find a definition for 'desired field size'. I'm planning to use a 2" diagonal for astro or a 2" Herschel wedge for white light viewing, one part of me says that 50mm is right, but another part says no - an eyepiece can not handle 50mm. How do I calculate 'desired field size'?

Any ideas or suggestions are welcomed.

Regards

David

[glend]

Setup a bench ray diagram, it will allow you to work out baffle interval and required diametre. I did this for my 127mm f12 iStar truss tube build and It's worked out perfectly. Get some bright coloured brickies string from Bunnings for the rays. For ep field size most eps have field stop suze info available. At your focal length the cone will be fairly narrow anyway so as long as your focuser has adequate travel you will be ok. Set your focuser for midpoint of travel and run the ray strings through it to a point near the end of travel. That will be close. Final star testing to trim tube length maybe needed. Don't secure everything until you can verify on star testing.

[anj026]

For my refractors I tend to use a 42mm field diameter for 2" eyepieces and 27mm for 1.25" eyepieces. Even if you used 15mm or 20mm you probably wouldn't notice the slight light drop-off at the edges for visual low power. Most Newtonians are designed with a minimum 10-15mm (fully illuminated) field diameter for visual use.

With a refractor there is no reason not to fully illuminate the whole field of your lowest power eyepiece but in practice you won't notice the difference if it is a bit smaller than that.

The Berfield baffle method is fine if you want to minimise the number of baffles. You can also have no baffles and just use flocking like Tele Vue do with their refractors.

My home made scopes are all made of thin ply so I used the baffles as frames to stiffen the tube and put one every 200mm or so.

[anj026]

Here are some pictures that may help. The first one is a drawing for a 127/1500 refractor. The field size is 42mm at the left. The aperture is 127mm at the right.

The horizontal distance of 1500mm is compressed. That is to say the drawing is 1/1 scale in the vertical dimensions but 1/6 scale in the horizontal dimensions.

That enables you to measure the ID of the baffles for any position along the tube.

[Merlin66]

Guys,
I have a 100/1100 TS refractor which has a dual back end - one for the Baader Cool wedge and the other for the PST mod....quick change over.
Re the baffles - setting them for maximum FOV with 2" or 1.25" eyepieces is OK but for solar it would be better to work to the solar diameter + clearances (solar diam. = 1/105 the focal length)

[MattT]

When I built my 6" f12 frac I decided on 38mm as my Illuminated Field size.
Works well and was just a number I chose not wanting to go the max of 42.
There is very little info out there on the subject.
Great views with low power eyepieces.

Used the same baffle method that gave me what is in the photo. I still haven't got around to installing the first baffle, mostly as it is 5mm thick....gonna have to wait for a router to cut it. In the meantime I use flocking to my first baffle and can't complain at all.

Matt

[dmurton]

Thanks for the replies everyone.

Ok dusted off my trig, results in the chart attached. My widest eyepiece has a field stop of 38.7, so I am using a field size of 39mm. The light cone is 39mm across at 585mm from the focal plane.

http://i305.photobucket.com/albums/nn226/dmurton/Scope%20Resto/scopesetupmk1.jpg~original (http://s305.photobucket.com/user/dmurton/media/Scope%20Resto/scopesetupmk1.jpg.html)

Feel free to check or correct, I haven't done trig since I left school 20 years ago. Assuming I have the maths correct, I can use the 'berfield' method for placing the baffles?

Regards

David

PS excuse the crappy diagram, I now know why paint come for free with windows!!!!!

[anj026]

Hi David,

The field stop should be 39mm at the focal plane. On your diagram you want the 39mm to be where your eyepiece will be. Any baffles will be somewhere between 102mm in diameter and 39mm in diameter depending on the position along the tube.

[dmurton]

Ok. If the tube has a maximum possible length of the light path (1530mm), then with a 39mm illuminated field the tube length would have to be shorter I assume. Using the diagram from last night, the tube length should be 1530-585(the point at which 39mm illumination occurs)-150(focuser)-100(2"diagonal)=695mm (or 945mm at the point where the light enters he eyepiece - counting the focuser + diagonal as part of the tube). That seems very short for a 1530mm fl scope, or have I completely missed the point?

Regards

David

[MattT]

What I did was do a full ray trace on paper. Step one was mark on the cone my Desired Field Size....mine is actually 37mm not 38 but no difference really. Then I followed the instructions with lines going all over the paper, this part was confusing to read, but easy in practice :thumbsup: Then I had where to place baffles. On my rather messy drawing I had already guessed where to put baffles, then found the Berfield method, which gave me less baffles and put them in the right place. The only one I didn't do was no 1. I found from Refractor builders on Cloudy Nights that flocking is as good as a baffle so I did that. Looking down my tube in the Daylight all I see is ...nothing..... just Black.

How long to make the tube is variable. I wanted to use WO Bino's without a barlow/OCS so I took the 110mm of those + 2" diagonal and a GSO Linear Bearing Focuser into account. My tube lenth is 1430mm roughly. In the pic no2 it says 1530 but that was my pre Berfield guess... miles out, 100mm to be exact. Easier to draw than cut an aluminium tube in the wrong place.

It helped to put the objective on a length of wood and have an eyepiece in the focuser on a sliding wooden job, and measuring that. I forgot about close objects focusing further out than infinity objects, about 10mm in it.

I also asked Ales the owner of Istar what they do for tube length and the answer was take 300mm off the objective FL....mine is a bit shorter for binoviewing native.

Your tube might end up at 1200mm :question:

Some photos 'might' help....

[anj026]

Hi David

Hopefully it is making sense for you. I agree with Mathew, your tube will probably end up around 1500-300 (1200mm) give or take.

I think you might be confused about the 39mm illumination occurring at 1500 -585mm. The 39mm illumination should be at the focal plane, 1500mm. If there is a baffle required at 1500 - 585 then it will need an ID much larger than 39mm.

Drawing it out full scale is a good idea but the large drawing can be unwieldy and you'll need a 1500mm straight edge. The drawing can be just as accurate if you scale it sown to a more manageable size.

[dmurton]

Thanks for the help guys, it is definitely making more sense now. I now understand the idea of the field size and it will be easier to calculate everything now. But it will make the design a bit more complicated.

As I mentioned previously, I want to segment the tube for 2 reasons. To make it easier to store and transport AND have a second back end with the etalon of a PST in it for solar viewing. I was planning to have a nice big illuminated field for using the scope at night, but a much smaller field is better for solar work as merlin suggested in an earlier post. There may have to be a compromise made in baffle arrangement t get the best of both worlds. I'll need to spend some more time with the pen & paper, and see what I can work out.

Some pictures of the scope as it came to me.
http://i305.photobucket.com/albums/nn226/dmurton/Scope%20Resto/IMG_20151125_210927.jpg~original (http://s305.photobucket.com/user/dmurton/media/Scope%20Resto/IMG_20151125_210927.jpg.html)
http://i305.photobucket.com/albums/nn226/dmurton/Scope%20Resto/IMG_20151125_210943.jpg~original (http://s305.photobucket.com/user/dmurton/media/Scope%20Resto/IMG_20151125_210943.jpg.html)
Tube is made of very thin wall steel, and is very flimsy. The focuser is a cheap 1 1/4 R&P job that you find on many older scopes. The history of it was that the owner put it together after sourcing the objective from the surplus shed some 30 odd years ago. He put it together himself, with a minimum of cost. However, the optics (aside from being filthy and very dusty) are still in first rate nick.
http://i305.photobucket.com/albums/nn226/dmurton/Scope%20Resto/IMG_20151124_192734.jpg~original (http://s305.photobucket.com/user/dmurton/media/Scope%20Resto/IMG_20151124_192734.jpg.html)
http://i305.photobucket.com/albums/nn226/dmurton/Scope%20Resto/IMG_20151124_192702.jpg~original (http://s305.photobucket.com/user/dmurton/media/Scope%20Resto/IMG_20151124_192702.jpg.html)
The finder is a little gem, a 26mm x6 with circle T optics.
http://i305.photobucket.com/albums/nn226/dmurton/Scope%20Resto/IMG_20151124_192553.jpg~original (http://s305.photobucket.com/user/dmurton/media/Scope%20Resto/IMG_20151124_192553.jpg.html)
And despite the general 'Jerry Built' appearance, it is very functional, and gives a wonderful image. The colomation system for the cell is very effective.
http://i305.photobucket.com/albums/nn226/dmurton/Scope%20Resto/IMG_20151124_192456.jpg~original (http://s305.photobucket.com/user/dmurton/media/Scope%20Resto/IMG_20151124_192456.jpg.html)

I'm looking forward to getting it up and looking much better. I'm uming and ahing about investigating AR coatings, but I'm not sure if it will be worth the cost or effort - especially if it can't be done in Australia.

Regards

David

[Merlin66]

David,
Re the white light/ PST changeover... remember the PST etalon must be positioned at -200mm from the prime focus.
This may affect your thoughts on the length of the OTA.

[dmurton]

It certainly will adjust my thoughts on the tube length Ken. I've decided to leave the design of the PST back end for later, I need to think about it more. However I've crunched some numbers, and played with sketchup for the first time tonight - a cool little program. Very useful for working out baffle placement and technical drawing in general. This is to scale.

http://i305.photobucket.com/albums/nn226/dmurton/Scope%20Resto/103X1575mk4.jpg~original (http://s305.photobucket.com/user/dmurton/media/Scope%20Resto/103X1575mk4.jpg.html)

I've planned to segment the tube at the second baffle, for easier storage and transport. Each section will be about the same length if a dew shield (not shown) is factored in. At this stage, I'm focusing on night astronomy or solar viewing with a Herschel wedge. Due to the requirements of the placement of the etalon of a PST, I will most probably make another rear half with the etalon on there already, as the tube may need to be a different length to fit it in (and all the blocking filters are only 1 1/4", I've allowed for a 2" diagonal).

Any ideas or suggestions welcome.

Regards

David