Posts Made By: Jarad Schiffer

The most common f-ratio for 8" scopes is f6. I think this is just because it makes a nice 4 ft focal length, convenient to put on a dob mount and get a reasonable eyepiece height. I have also seen some 8" f4 scopes, mostly for use on an equatorial mount (f4 to get tube length down to make it easier to mount). There is no true "shortest" f-ratio - you could make it as fast as you want.
The issues that go into deciding on an f-ratio are:
1 -Tube length. Longer f-ratio, longer tube, more weight, more leverage on the tube requiring a beefier mount, etc.
2 - Maximum FOV. Shorter f-ratio give a wider maximum FOV for a given focuser size (1.25" or 2").
3 - Secondary size. The longer the f-ratio, the smaller the secondary can be made while giving adequate illumination. Note that there are limits to this, as discussed in another thread in this forum.
4 - Coma. As you get to fast f-ratios, coma increases. This can be dealt with by using a coma corrector down to f4 or so, but that adds expense.
5 - Expense. It costs more to make a fast mirror to high standards, and the fast mirror will need things like a coma corrector, and expensive eyepieces to handle the steep light cone without astigmatism, etc.

There are trade-offs either way. Most 8" newtonians I have seen are f6. I have seen a few at f8 that were optimized for planetary viewing, and a few at f4 that were optimized for wide field viewing and/or photography. I haven't seen any faster than f4, but that doesn't mean you couldn't make one. It would be pretty much a dedicated wide field scope, though, with a large secondary.

Hope this helps,

Jarad

This becomes an issue at or below f5. A 35mm Panoptic will have a 7mm exit pupil at f5 (or with an f4.3 scope using a paracorr). At f4 (with paracorr), you might want to make your lowest power eyepiece either a 31 nagler or 27 panoptic or similar. On the other hand, once you go above f7 or so, the 2" focuser will limit your maximum true FOV instead of exit pupil (i.e. you will be unable to reach the widest TFOV with any commercially available eyepiece).

Jarad

I would just attach the curved spider to the square tube. At this size a single vein 180-degree arch would probably be stable enough (there was a nice article a few month ago inS&T describing a homemade 8" scope with a curved spider like this).

You could still put the baffles in, but I would make the baffles out of something lightweight instead of 1/2" plywood. You don't need all of the baffles listed in Newt, the important ones are the two above and below the focuser, the one above the mirror, and the tube wall behind the secondary (I like to add a ring baffle behind the primary mirror too, so light can't reflect off the ground into the light path).

Hope this helps,

Jarad

Interesting idea. I don't think it is worth the larger secondary at 18", since you can make an 18" with an eyepiece height of around 5'6" the normal way. Might be nice for a 30-40" scope to avoid those cherry picker ladders, though.

Jarad

Well, if you already have a good 15", I am not sure why you would want a 12.5" of probably lesser quality (meade vs. Swayze...). If you want a smaller scope because the 15" is too much to drag out, I don't think the 12.5" is going to be that much easier to use, especially if it is a solid tube. If that is the case, I would say go down to something like an 8" f6, 6" f8, or small refractor for a second quick look scope.

Jarad

I set the scope outside at 7:48 PM, pointed at the sky at about a 40 degree angle up (I just left it pointing basically north). Here is a picture of the position outside (the car isn't mine, my in-laws were over to watch West Wing with us....):

I have a 13mm T6, but no T4's. From what I understand from others, the T6's are lighter, smaller, and cheaper, but with less eye-relief than the T4's. Some people say they feel more "immersed" in the view in the T4's, but it is very subjective (some people have said they prefer the T6). I chose the T6 for the price and weight issues. Both are excellent eyepieces, though. The T6's have 12mm of eye-relief, which I find comfortable. If you prefer more eye-relief, you might want to go with the T4. Probably no way to tell for certain without actually looking through them - try to get to a star party so you can see for yourself.

Jarad

You don't need the panoptic-barlow interface with a powermate - they effectively have one built in.

That said, I agree with Bob. I would go for either a 35 pan or 41 pan for your low power (or 40 Pentax XW). These will give you a wider field and get to somewhat larger exit pupils (you are at f10, so even the 41 pan will only be a 4.1mm exit pupil). The 41 plus the 2x powermate would be 20.5, and if you get the 27 you will have 41, 27, 20.5, and 13.5, which is a pretty nice progression, close to 1.4x steps. I would skip the 16 nagler - too close to the 27 doubled to be worth it. You could add in a 9 or 11 nagler T6 or 10 radian for the high powers (or use the 9.7mm plossl you already have). I would sell the 26 plossl, unless you want it for public star party use. You will like the 27 Pan much better.

Hope this helps,

Jarad

With a 10" f4.7, assuming the tube is 12", you should be fine with a 2.14" secondary. The one you have will be a bit of overkill, although there is no reason you couldn't keep using it. Protostar is known for high quality digonals, so they would be a good choice if you want to downsize it.

If the binoviewer requires extra travel (i.e. needs the focal plane to come out from the focuser) that would affect your choice. Download Newt 2.5 and play with the numbers - it is a free program, excellent for this sort of thing. Whether or not you would want to downsize the secondary with the binoviewer depends on the numbers - you will have to make some measurements and plug in the numbers to see. If the binoviewer uses a built-in barlow, remember that it will need a smaller fully illuminated field anyway.

When you change the focuser and/or secondary, you may have to move the primary further down as well to bring the focal plane closer to the tube (if the binoviewer doesn't need it further out). You can try to do this by backing down the collimation screws, but you will probably have to drill new holes for the primary cell mount an inch or so further down the tube. That's easier than redrilling the focuser and spider, though.

Hope this helps,

Jarad

Precision Applied Products. They have a website: http://www.precisionapplied.com/

They coated a 10" for me a few years ago and did a very nice job. Good service, quick turnaround, and they did the secondary for free (just put it in the chamber with the primary).

Jarad