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Tasco 12TE: Using 1.25 eyepieces w/ a .965 focuser

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#1 SpaceNetworks

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Posted 12 September 2013 - 11:11 PM

I have been using my modified Tasco 12TE 60mm / 700mm focal length refractor with the original diagonal and .965 eyepieces (see the attached photo). Terrestrial and celestial views are good, focus is clear.

In an effort to also use 1.25 eyepieces, I purchased a hybrid .965 to 1.25 ScopeStuff diagonal and some standard plossls. However, I'm not able to bring images into focus. With the focuser tube drawn in as far as possible, images start to take shape but are still blurry (see the second photo).

Am I doing something wrong; have I miscalculated; is there a solution to this?

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#2 SpaceNetworks

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Posted 12 September 2013 - 11:12 PM

With the focuser tube drawn in as far as possible, images start to take shape but are still blurry:

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#3 terraclarke

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Posted 13 September 2013 - 08:45 AM

Haven't done anything wrong really, its just that the optical path for your new configuration of diagonal and 1.25 inch e.p.s is several inches longer that that of the original configuration. You will now need more in-focus than you have given the length of the tube. The alternative is to shorten the tube. This seems to be more of a problem with the smaller Tasco's than with other Japanese 60 and 75mm telescopes. Primarily because of the configuration of their focusers and compound drawtubes.

#4 SpaceNetworks

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Posted 13 September 2013 - 01:00 PM

...the optical path for your new configuration of diagonal and 1.25 inch e.p.s is several inches longer that that of the original configuration. You will now need more in-focus than you have given the length of the tube. The alternative is to shorten the tube.

Thanks, Terra. How can more in-focus be achieved? I'd prefer not to shorten the focuser drawtube.

#5 terraclarke

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Posted 13 September 2013 - 01:43 PM

Your options as I see it are

(1) shorten the tube (the main tube).

(2) shorten the focuser drawtube (but then you have to rethread it.

(3) move the objective cell closer to the back, (but again, that is going to require cutting and either re-drilling or re threading the main tube depending on how the cell is attached.

The bottom line is that with your new configuration you have too much distance between the objective and the eyepiece so that the focal plane is falling short of where it needs to be with regard to the eyepiece.

The easiest option is to shorten the main tube from the focuser end. (My original suggestion- i said tube, meaning the main tube, not the draw tube.) Just be very sure your cut is perfectly square and all new drill holes are precisely aligned. (Unless it threads in, in which case you will need to re-thread the tube and that will require a machinist.) But it looked from your pictures that the focuser is attached with set screws so all you need is a drill and a tap.

The only other option is to use a Barlow which will move the focal plane further back from the objective, but it will also increase the power and darken and reduce the field. Not a very good solution. I'd just cut the tube. After all, its a nice little scope but its not an Alvin Clark. You obviously want to use it more than display it or you wouldn't change the optical configuration in the first place.
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#6 fjs

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Posted 13 September 2013 - 02:26 PM

If you want to shorten the tube but are hesitant to chop the original, you might be able to get one of these to work. Even if you have to cut one down, at least you can put things back to original if desired.

http://www.cloudynig...ct=54212&sor...

#7 charen

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Posted 13 September 2013 - 04:08 PM

Try a prism diagonal as opposed to a mirror version. They have different light paths and focus travel can be an inch or so different.

Chris

#8 Jeff Phinney

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Posted 13 September 2013 - 08:28 PM

Your options as I see it are

(1) shorten the tube (the main tube).

(2) shorten the focuser drawtube (but then you have to rethread it.

(3) move the objective cell closer to the back, (but again, that is going to require cutting and either re-drilling or re threading the main tube depending on how the cell is attached.


(4) Observe objects that are much closer to you whose focus is not at infinity. :smirk:

#9 JonH

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Posted 13 September 2013 - 09:52 PM

Hi Eric - you could also swap out the objective for one with a longer focal length. Surplus shed do a 60D 800F lens but its not an achromat and there are heaps of 60/900 doublets available (although they will give you the opposite problem). I have seen 60/800 achromat scopes around so you might get lucky and find a lens that fits. I had the same problem using binoviewers with a 4"unitron and eventually got to focus via a Jaegers doublet with a slightly longer focal length.

Good luck - Jon

#10 SpaceNetworks

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Posted 13 September 2013 - 10:12 PM

Thanks for all of your feedback. I can always continue with the original Tasco diagonal, and buy additional 0.965" eyepieces (the only Tasco eyepiece I have that is worthy of continued use is a circle T 20mm.

But I'm also now wondering if it's possible to use 1.25" eyepieces with the 0.965" Tasco diagonal. Are there any sleeves or adapters that make this feasible?

#11 Datapanic

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Posted 13 September 2013 - 11:57 PM

You could try a .965 to 1.25 inch adapter and use a .965 diagonal. No guarantee that it will work, but it does add much less focus travel than a hybrid diagonal. I did it that way, less diagonal, with a Tasco 11TE reflector and had no problems with an 18mm 1.25" ortho.

#12 bdc52

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Posted 14 September 2013 - 12:14 AM

Seconding the prism diagonal. I use one on my 700/60 and all of my eyepieces from 40mm to 6mm reach focus with a bit of room to spare. Could you borrow one from somebody to check it out first?

#13 SpaceNetworks

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Posted 14 September 2013 - 09:10 AM

I ordered an Orion 0.965 to 1.25 adapter, and will see what that yields.

#14 terraclarke

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Posted 14 September 2013 - 11:24 AM

The problem with going from 0.965 to 1.25 inch eyepieces in a 0.965 inch diagonal is that it will effectively move the eyepiece back (away from the objective) a distance equal to the height of the eyepiece barrel of the 0.965 inch diagonal. So what you gain in in-focus with the smaller diagonal you loose with the adapter. When I read your original post, I assumed that you were using a prism diagonal (that's all I ever use with any of my vintage long focus refractors) as you only mentioned that it was a hybrid and not a hybrid mirror diagonal. (I used to have a hybrid prism diagonal that was quite nice- Orion used to sell them.) It is true that you will gain in-focus if you switch from a mirror to a prism diagonal of the same physical size, (the gain will be a factor of about 1.5xL) due to the difference of the index of refraction of glass (BK-7 glass has an index of refraction of 1.5168) as compared to the index of reflection of the mirror which is equal to 1, to the ratio of 1- (1/n).

I might add that there are also some very excellent 0.965 inch eyepieces out there in the used market if you search around; you will be able to find Kellners, Plossls, and orthoscopics made by Vixen and Towa without too much trouble. Some of the best eyepieces out there are made by Zeiss and they are 0.965 inch. We have developed a prejudice against them in recent years with the shift over from long focus to short focus telescopes. Yes, they do have a reduced field and shorter eye relief, but if you learn to use them you will be amazed by their performance. Just like it ain't your daddy's Oldsmobile anymore, nor is it his Huygens and Ramsden eyepieces.

Good luck in whatever path you choose.

#15 SpaceNetworks

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Posted 14 September 2013 - 02:17 PM

The photo in my original post shows the position of the focuser drawtube when a distant terrestrial image is in focus, and for lunar/stellar objects it's practically the same. By placing the Orion .965 to 1.25 adapter behind the Tasco .965 diagonal, and using my standard Celestron 1.25 plossls, will focus be achieved?? I'll find out soon. And if that doesn't work…

I see that Agena Astro has a GSO hybrid prism diagonal:
http://agenaastro.co...m-diagonal.html

But if I need to stay with the Tasco .965 diagonal, which cleaned up nicely, like you said, Terra, I can look at acquiring additional .965 eyepieces. Siebert Optics appears to have some nice ones:
http://www.siebertop...-eyepieces.html

#16 fjs

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Posted 14 September 2013 - 03:23 PM

I see that Agena Astro has a GSO hybrid prism diagonal:
http://agenaastro.co...m-diagonal.html


Not sure you would want an erect-image diagonal. If you go with the Siebert eyepieces, I would sure like to hear a report. I'm considering them also.

#17 terraclarke

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Posted 14 September 2013 - 05:03 PM

Yes, as Frank says, don't get an erect image prism diagonal. Amici prisms add additional surfaces of internal reflection and are generally not good for astronomical observations- especially of bright objects where you get spurious reflections or for high magnification- they make the image break down. I think if you use the original 0.965 inch prism, you may be able to achieve focus with your 0.965 to 1.25 adapter, at least for some simpler 1.25 inch eyepieces. Otherwise, your options are:
1) shorten the tube,
2) change out the objective
3) change out the tube
4) stick with better 0.965 inch eyepieces.

I'd be inclined to stick with option 4.

Terra

#18 SpaceNetworks

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Posted 14 September 2013 - 06:05 PM

Just for future reference, does anyone know a source for a hybrid .965 to 1.25 diagonal with a simple 90-degree prism (i.e. NOT an Amici prism)?

#19 photiost

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Posted 15 September 2013 - 12:40 PM

Try a prism diagonal as opposed to a mirror version. They have different light paths and focus travel can be an inch or so different.

Chris


Indeed.

Someone at our club had a similar problem and they tried an "older" B&L 1.25"prism diagonal which brought a 12.5mm Ortho and other eyepieces to focus.

I have also noticed that some of the older prism diagonals seem to have a shorter "collar" so the eyepieces sit a bit lower (almost touching the prism) and are thus positioned closer to the objective as well.

I would bring your scope to a local telescope shop and try different diagonals (new + used) you may get lucky.

Let us know how it turns out.
.

#20 SpaceNetworks

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Posted 15 September 2013 - 01:09 PM

I'm now seeing two possibilities:

1) .965 focuser - Tasco .965 diagonal - .965 to 1.25 adapter - 1.25 ep
2) .965 focuser - .965 to 1.25 adapter - 1.25 prism diagonal - 1.25 ep

#21 Larry Geary

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Posted 15 September 2013 - 09:05 PM

I'm now seeing two possibilities:

1) .965 focuser - Tasco .965 diagonal - .965 to 1.25 adapter - 1.25 ep
2) .965 focuser - .965 to 1.25 adapter - 1.25 prism diagonal - 1.25 ep


3) .965 focuser - .965 to 1.25 adapter - 1.25 ep

#22 pogobbler

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Posted 16 September 2013 - 01:35 AM

I've got a 60mm scope with a .965" focuser. When I got it, I went out and got a .965" to 1.25" adapter and, luckily, my 1.25" eyepieces came to focus with it, but, you know what? I ended up getting some .965" Plossl eyepieces from Hands on Optics and prefer them with the scope. The quality is fine and the size just fits the scope better and doesn't upset the balance of the scope. The eyepieces are around $30 and perform just great. That'd be my recommendation instead of cutting the tube.

#23 Joe Cepleur

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Posted 16 September 2013 - 08:34 AM

...bring your scope to a local telescope shop and try different diagonals (new + used)



It may be easier to find a nearby club, with members who will be eager to help you test their accessories in your scope. Some mail order houses may also have return policies favoring ordering lots of parts, and keeping only those that work.

My rig (albeit with a different brand and model of telescope) is:

Blue Fireball 0.965"-to-1.25" adapter
Celestron 1.25" prism diagonal

#24 tag1260

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Posted 16 September 2013 - 09:26 AM

Also, have you tried other eyepieces? I know I have eyepieces that will work in one but not another when I use adapters.

#25 SpaceNetworks

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Posted 19 September 2013 - 10:44 PM

I received a .965 to 1.25 adapter, and tried it with my Tasco 60mm:

.965 focuser - Tasco Circle T .965 diagonal - .965 to 1.25 adapter - 1.25 ep
Outcome - unable to reach focus

So I tried Larry's suggestion: .965 focuser - .965 to 1.25 adapter - 1.25 ep
Outcome - reached focus with 3/4" of travel left for the focuser drawtube (see attached blurry photo, taken at twilight)
But I doubt I would be willing to do an impression of Galileo (i.e. straight-through viewing) very often.
Does anyone think a 1.25 prism diagonal could be added to this optical path, and reach focus? I dunno.

Getting additional .965 eyepieces to go with my Tasco Circle T 20mm is starting to look like the best option.

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