Ultra-wide eyepiece limitations

[Roo]

Hi,
I'm a newbie to this forum and planning to build a Newtonian reflecting telescope (Dobsonian).
I'm interested in the ultra wide eyepieces available that have apparent fields of view around 82 degrees. I have read that sometimes they can be limited by the telescope internal geometry. Can someone tell me what the limitations might be and how to calculate them for a Dob?
Cheers,
Andrew.

[iceman (Mike)]

Hi Andrew

to the forum!

Are you planning on grinding your own mirror or buying a premium or GSO one?

There's a few telescope makers here so I'm sure they'll answer your questions.

[Roo]

Hi Iceman,
I've bought a 10" f/5 mirror from Anttler Optics in the US. I have a mate there this week picking it up for me. I plan to buy the rest of the bits from Aussie suppliers.

[AstroJunk (Jonathan)]

http://www.dalekeller.net/atm/newton...t/newtsoft.htm

Check out the program on this page - It will help you get your design right.

Field of view is limited by "clipping" the light available to the eyepiece. This can be done by:

1 Having too narrow a tube,
2 Having too small a diagonal or putting it in the wrong place,
3 Using too small a focusser (avoid old 1.25" units).

It's a ballancing act, but easy enough when you follow the rules.

[Roo]

Thanks Astrojunk. The program looks pretty good.
Cheers,
Andrew.

[Roo]

The Newt software has calculated a small "Diagonal Offset" dimension defined as: The distance from the centre of the tube toward the tube wall opposite the focuser that the diagonal mirror should be positioned."

OK, I can understand what this is telling me, position the diagonal mirror off centre a little bit but not why.

Can anybody tell me what the basis of diagonal offset is and why it is important?

[bird (Anthony Wesley)]

Andrew, the offset happens because the light cone from the primary mirror is getting smaller as it comes up the tube, and the top and bottom ends of the diagonal mirror intercept it at different points.

So, if you can imagine it, the light cone is larger where it hits the bottom of the diagonal than where it hits the top of the diagonal.

The offset moves the diagonal across and down a bit to correct for this, so that it intercepts the light cone in the right place to catch all of it evenly.

Draw it on a piece of graph paper and it'll make sense.

cheers, Bird

[AstroJunk (Jonathan)]

Getting the Diagonal in the right place is the hardest part and quite crucial - its easiest to do without the offset, and that's the way to start off. Once you are quite happy, then you can move it down and out.

From my experience most scopes don't have offset diagonals, and if your secondary is large enough, then it doesn't matter anyway. My advice is - don't bother!

[Merlin66 (Ken)]

The intersection of a line at 45 deg to a cone gives an ellipse ( by definition), however in the real world to cut an accurate ellipse from a lump or sheet of glass is very difficult, so they normally use a biscuit cutter ( ie a rotating cylinder) to cut the secondary mirror at 45 degrees, this DOESN'T give an ellipse.
To make this cylinderical section better fit the cone of light in a fast reflector ( say < f5) it must be offset away from the eyepiece to maximise the amount of beam picked up without causing excessive shadowing of the primary.
You can see the above for yourself if you do a scale drawing of a cone with a line at 45 degress; one side of the line relative to the centre line is longer than the other, whereas if you do it for a cylinder both sides are the same length!!.

[Roo]

Thanks for the explanations on diagonal offset. Very clearly understood now.

Along a similar path, the Newt program is telling me I can have a 50mm diagonal mirror with my 10" f\5 primary. Another formula I have for calculating diagonal size tells me a 50mm diagonal is too small for this primary. The next size diagonal (GSO) is 63mm. Jumping up to a 63mm diagonal will reduce the light gathering ability a little. Can anyone advise which way to go (50 or 63mm diagonal) from practical experience?

Also, going back to the title of this post, I gather that the wide angle eyepieces will only be effected by the telescope design if there was a problem with the design in the first place (ie they amplify problems)? Is that a correct assumption?

[skies2clear]

Hi Andrew
the difference between a 50mm and a 63mm diagonal mirror is irrelevant regarding difference in light gathering. It would be such a small loss you would never notice it at all. More relevant is the difference the central obstruction (diagonal) makes to contrast performance. The 50mm gives about 20% central obstruction and the 63mm gives about 25% central obstruction. To work it out yourself, divide the diagonal minor axis size by the main/primary mirror diameter.

So you may ask next, what is the difference between a central obstruction of 20% compared with one of 25%? Theoretically the smaller figure will allow better resolution and contrast, but in practice the difference isn't that great between 20 and 25%. You would notice more a difference comparing 15 or 20% with say 30% or more, like some SCT's have.

You can also get diagonal mirrors of 54mm diameter by the way. It may cost a bit more, but could be safer than 50mm. If the diagonal is too small, you will notice it on the longer focal length and wider field eyepieces, where the true field is too wide for the diagonal mirror to field (or trap all the incoming light rays). The centre of the field will be just as bright, but the outer edges diminish in brightness a bit, when the diagonal is too small.

I have a 12.5" f6 mirror and use a 54mm diagonal which works well. I imagine you would get away with a 54mm too, even considering your mirror is f5 compared with my f6. But your primary mirror has a smaller diameter, so this balances it out pretty much.

Maybe someone else with a GSO 10" f5 could comment on the diagonal size normally used with these, but I suspect they are somewhat oversize from memory.

Regarding wide angle e/p designs, they can put more of a strain on faster optical systems like f5 or less (f4, etc) because the focal plane of these is not! It's curved and not a plane, and so you can benefit from coma correctors like Televue's Paracorr. Of course, you need to keep in mind that the better wide field designs you get, the better they correct for these types of aberations, especially towards the edges of the field of view. You get what you pay for basically. However, you can still enjoy most wide field designs with newtonians of f5, as most people already do.

Hope this helped a bit,
Cheers.

[Roo]

Thanks skies2clear,
I'm learning a lot! Can you tell me where you bought your 54mm diagonal mirror?
Regards,
Andrew.

[cristian abarca]

Hi Andrew I have a 10 inch F4.5 and my secondary mirror is 63mm. You can reduce the size of the secondary by having less distance between the secondary and your eye. Use a low profile focuser or a focuser like this http://www.kineoptics.com/HC-2.html . Of course you have to weigh up the costs involved.

Regards Cristian

[skies2clear]

Hi Andrew, please take Cristians advice about reducing the secondary size by using a lower profile focusser and positioning the eyepiece closer to the secondary. You do this by using the smallest practical diameter tube for your scope, and position the secondary as far away from the primary mirror as possible. Use the previous advice of others about tube diameter, etc, and run some numbers through the Newt program till you feel comfortable and understand what is happening.

As Cristian said, you have to weigh up the cost of all this, ie. a low profile focusser, a 54mm diagonal cost.

I got my 54mm from Chi Qin Co in Sydney, and they advertise in the back of most Australian Astronomy magazines. Alternatively, you could try Claude at Astronomy Electronics Centre in Cleve, SA. Both carry good quality pyrex secondary mirrors with enhanced coatings and overcoated with SiO or quartz. Probably cost close to $100 though.

Always glad to help a fellow astronomer!
Feel free to ask more questions anytime,
Cheers
Nick