Stargazing Telescope alteration (Russ or turbo-1)

[Chronos]

It would be more accurate to say that with the lens, the effective focal length is 1000mm. When you remove that lens the actual focal length (which IS a property of the curvature of the mirror) should be shorter - perhaps in the 400-500mm range. You'll have to measure that focal length and subtract the average focus tube extension to determine the how far the secondary mirror needs to be from the primary.

You have it backwards, turbo-1. The primary mirror is figured at f8.8 [which is much cheaper than say f4.4]. Removing the relay lens will significantly increase the focal distance of the telescope.

 

[Saladsamurai]

So, it will be longer. Well that's cool. I already have a larger scope with an f4.9.
So the longer focal lengths are usually better for planetary viewing, right?

 

[Chronos]

The intrinsic magnification is greater, hence you will get more magnification from a less powerful eyepiece. For planetary viewing, this is an advantage. Short focal length eyepieces have a narrow exit pupil [more difficult to use].

 

[turbo]

Righto! I will try to try it tomorrow weather permitting (stupid New England!).

Thanks y'all. I'll let you know how it turns out.

I'm in stupid New England, too (central Maine) and we had 40-50 mph winds almost all day. Luckily, they were fairly consistent and not gusty, or the resonances in the gusts would have weakened/toppled trees. Amazingly, our power stayed on all day. Yay!

 

[chemisttree]

So, it will be longer. Well that's cool. I already have a larger scope with an f4.9.
So the longer focal lengths are usually better for planetary viewing, right?

No, Turbo had it right. It is a short focal length telescope with an integral barlow yielding an effective focal length of 1000 mm. Remove the barlow and the effective focal length will be lower.

 

[Chronos]

Incorrect. The relay lens is not a barlow.

 

[chemisttree]

I spoke with the technical service guys at Meade and they tell me that it is. Go to http://www.meade.com/starterscopes/eq_series.html" The focal length of the mirror looks to be about half of the regular model (fl=900 mm).

The company's motivation appears to be the use of a smaller mount.

 

[russ_watters]

The company's motivation appears to be the use of a smaller mount.

Just to clarify - by "smaller mount" you mean shorter tube. That's why they are called "short tube" reflectors. You get a longer focal length than you would normally expect out of a tube that length -- by using an internal Barlow.

Chronos, I think you may be stuck on the f/8.8 - That's the specs of the telescope, not the specs of the mirror. It would be useless to label/market the telescope with the specs of the mirror if the "relay" lens is not intended to be removed.

 

[turbo]

Just to clarify - by "smaller mount" you mean shorter tube. That's why they are called "short tube" reflectors. You get a longer focal length than you would normally expect out of a tube that length -- by using an internal Barlow.

chemisttree has it right, russ. It is more expensive to accurately figure a faster mirror, but by using that mirror in tandem with a permanent barlow, you can approximate the performance of a scope with a longer f/l. The trade-off is that you get a short OTA with less vibrational momentum to damp, so you can use a cheaper, lighter mount and get acceptable results. The extra cost of figuring the steeper mirror is more than offset by the savings they get by using a cheaper, lighter mount, especially if the mirrors are figured someplace like China. My 6" f:8 A-P APO has a long tube, a heavy objective cell in the front and a heavy cast/machined focuser at the rear. As a result the mount has to be much beefier than you would need for a 6" f:8 Newtonian. Mounting it on the tripod is quite a handful, but it is rock-solid even in windy conditions and damps very quickly.

 

[Chronos]

The scope uses a long focal length [ie, cheap to make] mirror with a correcting lens to reduce the focal length [which shortens the tube]. I do not mean to be disagreeable, guys, just trying to clarify the sales-speak.

 

[russ_watters]

We're pretty sure that that isn't the case. It might help clarify if you could explain the basis for your conclusion.

I don't even think what you are suggesting is possible (it requires the relay lens to refract the light to shorten the light cone without changing the angle of the cone - draw a diagram of that). And even if it was, it would have some big problems, such as cutting off part of the light cone, leaving a scope with tunnel vision and not using its full aperature.

 

[Chronos]

Long focal length mirror, corrector lens, shorter tube . . . what part of that is too hard to understand?

 

[Chronos]

I suggest http://www.meade.com/catalog/ds2000/ds_meade.html
Search for 'relay lens' on that site and draw your own conclusions.

 

[russ_watters]

Long focal length mirror, corrector lens, shorter tube . . . what part of that is too hard to understand?

The part of that that makes sense...it isn't there. And no offense, but your lack of explanation implies you haven't thought it through.

For example, the picture you linked clearly shows the relay lens lengthening the focal length, not shortening it. Before the relay lens, the angle between the rays is larger than after.

And think about what would happen if you flattened the mirror and made it a longer focal length - the light rays drawn wouldn't hit the secondary mirror because they wouldn't be bent enough!

Also, the description is exactly like what we are suggesting:

The optical design of Meade Digital Series reflecting telescopes includes an achromatic relay lens, permitting longer optical focal lengths to be housed in shorter optical tubes.

It doesn't say a longer focal length mirror, it just says a "longer optical focal length". Ie, normally a scope of that physical size would have to have a shorter focal length. This one doesn't because the relay lens lengthens it.

 

[russ_watters]

Here's a link that's a little more explicit:

The two most common problems with inexpensive Meade scopes seem to be mounts that aren't stable, and short Newtonians that use a relay lens (permanently attached barlow) to get a longer focal length. [emphasis added]

http://answers.yahoo.com/question/index?qid=20071122041842AAnXToe

Here's a diagram of a Barlow lens. Compare it with the diagram of the "relay lens" on your link (flat front, concave back): http://www.bpccs.com/lcas/Articles/barlow.htm

Another link: http://excelsis.com/1.0/entry.php?sectionid=12&entryid=413

Optics use a relay lens (like a built in barlow) to achieve longer focal length...

 

[russ_watters]

Well anyway, I'm glad you pushed me to research it because lookie here what I found:

How to convert a hybrid short tube Newtonian to an enjoyable RFT [rich field telescope]

http://www.cloudynights.com/documents/convert2.pdf

It's exactly what the OP was looking for! The guy even used the exact same scope! Among other things, it has the true specs of the mirror: 114mm, f/4 (448mm fl). And detailed (holy cow, are they detailed!) instructions for making the conversion.

 

[turbo]

Russ, you're fighting an uphill battle, I'm afraid. I love observational astronomy, and I'm a board-certified optician, and I have found that there are more misconceptions about astronomical optics that any other field that I have been involved in. It doesn't help that the purveyors of astronomy gear can get really vague about the specs of the equipment that they sell (they rarely manufacture it - that is done overseas). Note Orion's lead for their BT70 "Premium Binocular Telescope": "Probe the Starry Skies with 3D-Like Depth of Field". I have had good experiences with Orion's products and service, but what idiot writes their ads? Depth of Field? Is the "Depth of Field" so impressive that you have to focus once on the Moon, again on the planets, depending on how distant they are, again on the more distant stars, and lastly on the more distant galaxies? This crap is insulting to the amateur astronomy community.

 

[turbo]

Well anyway, I'm glad you pushed me to research it because lookie here what I found: http://www.cloudynights.com/documents/convert2.pdf

It's exactly what the OP was looking for! The guy even used the exact same scope! Among other things, it has the true specs of the mirror: 114mm, f/4 (448mm fl). And detailed (holy cow, are they detailed!) instructions for making the conversion.

You leap-frogged me, Russ! Thanks for that very well-documented conversion. I think that this issue has been addressed as thoroughly as possible, and Casey should have a good idea what might be awaiting him during a re-build.

 

[Chronos]

I concede, Russ. I gave links and you still found a way to insert fiction into the mix. I withdraw from this thread.