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Unknown Maker 152mm Short Tube - Need Advice

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#1 oddog

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Posted 01 February 2013 - 07:13 PM

I have this factory sample short tube 6" achromat I got a while back, photos below.

No makers name but obviously Asian sourced, pretty sure it isn't Synta because of the finder bracket shoe.

Initially the view through it was AWFUL (caps on purpose), hazy, flowery, unsharp, poor contrast, a star test looked like a triangle outside of focus and a very fuzzy triangle inside of focus. Now I understand a 6" fast achromat will not give the greatest image on a star/planet or Luna, but this was way out.

I tried flopping the lens elements around in all combinations and found the front element had been installed backwards. That improved things but the view was still bad, stars were seagulls everywhere.

I noticed if I pulled the front element away from the rear (made tabs of tape) the image got steadily worse so I decided to thin out the spacer between the elements. The factory spacer in this case was a 9 thou (.009) thick plastic ring. I removed it and put in its place three evenly distributed pieces of Scotch tape (1.5 thou, .0015). This improved the image dramatically, bringing the scope into the realm of what I was expecting (sharp, some false color on bright objects).

Unfortunately this also allowed the center of the elements to touch each other (rainbow pattern in the center that expanded and contracted as you pressed on the front element) so tonight I am trying two layers of Scotch tape (for 3 thou, .003, 1/3 of factory spacing). I will let you know the results.

By the way as I was testing the new thin spacer material, and as I had tabs attached to the front element, I tried rotating the front element in relation to the rear in 15° increments. There definately was a position where a stars image was optimized, so if anyone is experiencing less than pinpoint stars you might want to try this. Just put a piece of tape shaped so a portion sticks up as a tab on the front element near the edge (so you can have some rotational leverage). Loosen the clamping ring (most scopes with this kind of issue will have a dew shield that just pulls off thus exposing a plastic ring you can loosen with your fingers). You need to make sure the spacers between the elements don't fall out or re-position themselves (they usually are stuck on one element) and also to make sure the rear element isn't just moving with the front. Usually there is a piece of dust on the back element that you can use to judge this. Don't worry about the tape damaging the coatings, todays coatings are very hard and a little lighter fluid on a cloth will clean up any residue. Of course I marked the sides of the elements (sticker with pen drawn line) to note this relationship. While you're in there you should paint/magic marker the sides of the elements.

So what I need to figure out is the focal length of this OTA. I am sure there are simple ways to do so but I don't know what they are. Can anyone advise?

Also, the scope is fitted with a 1.25" visual back that screws into the large adapter that screws into the focuser tube. The focuser tube is 2-15/16" OD (is this considered a 3" focuser?). The part that threads into the focuser tube has 2.825" fine threads. Might this be some sort of industry standard such that I can find a 2" visual back for it?

The part that screws into the focus tube has T-mount threads (interesting...) that the actual 1.25" visual back threads into.

Thanks in advance for any and all input.

Tom Duncan

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#2 oddog

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Posted 01 February 2013 - 07:16 PM

The rack & pinion focuser tube with backs...

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#3 oddog

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Posted 01 February 2013 - 07:17 PM

And the two threaded backs/adapters...

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#4 oddog

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Posted 02 February 2013 - 01:39 AM

So I tried the scope out tonight and its a no-go. The sea-gulling is more apparent than when the elements were touching each other. It was nearly non-existent with the thinner shims. Scope could be an OK Milky Way scanner with a long EP but not as satisfying as it should be, especially for its size and weight.

Tom

#5 tubehead999

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Posted 02 February 2013 - 11:39 AM

Yeah..under-corrected. Should be a minimum of eight spacers around the periphery to support that heavy crown. You can use more if you wish.

#6 GlennLeDrew

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Posted 02 February 2013 - 01:12 PM

If you have a loupe with measuring reticle, measure the lunar diameter or the separation between any two stars whose angular separation is known (as many planetarium programs can do). The focal length equals:

linear separation / TAN(angular separation)

A crude, quick method to obtain the f/ratio:

eyepiece focal length / exit pupil diameter.

Note, however, that this presupposes no aperture clipping is occurring, as sometimes happens when a diagonal is installed and the focuser is racked far enough inward for the inner opening to clip the on-axis light cone.

#7 David-in-China

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Posted 03 February 2013 - 06:10 AM

Your scope is from Ningbo Zhanjing Optical Instruments.
http://www.nbzhanjin...tsd.php?pid=485

I have the same scope with their EQ-4 GEM, but still await first light. It came with a 45 erecting prism and typical budget 6mm and 25mm Plossls.

The focuser on mine was rough, but managed to improve it a little. The GEM was rather coarse, but massaged it a bit, too. Solid tripod with SS legs, and heavy duty case which weighs a ton.

There's a better diagonal, decent 2X Barlow, and better eyepieces coming in the next two weeks. I'm anxious to see how this budget scope performs. Admittedly, I'm not overly optimistic, but it wasn't expensive.

Looking forward to hearing more of your progress.

#8 Jon Isaacs

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Posted 03 February 2013 - 06:43 AM

Glenn:

How about just bringing it to focus at infinity and measuring the distance from the objective to the focal plane??? Bright the scope to focus, use a tape measure to measure the distance from the objective to the center of the diagonal (actually the intersection of the optical axis of the telescope and the eyepiece) and the distance from the center of the diagonal to the top of the diagonal. It's not exact but it's an easy measurement to do with reasonable precision.

Jon

#9 John Jarosz

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Posted 03 February 2013 - 09:05 AM

Hmmm. The webpage with the scope says it's F5, but they claim a focal length of 900mm. F5 should be 750mm.

#10 Jon Isaacs

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Posted 03 February 2013 - 09:26 AM

Hmmm. The webpage with the scope says it's F5, but they claim a focal length of 900mm. F5 should be 750mm.


Either than or the focal ratio and focal length are correct but it's actually an 180mm F/5... :)

I kept looking at the 900mm focal length and thinking, that's not right but it didn't ring a bell.. :(

Jon

#11 GlennLeDrew

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Posted 03 February 2013 - 12:59 PM

The appearance suggests f/5, unless there is an additional lens toward the back end, which I highly doubt.

And yes, Jon, the simple tape measure of focal length is viable, as long as no other 'Petzval-like' optics are present. In such case, one measures from a point estimated to be about 1/3 the objective thickness behind its front surface.

#12 AlienRatDog

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Posted 03 February 2013 - 02:48 PM

How well does it ride on that portamount?

#13 David-in-China

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Posted 03 February 2013 - 05:27 PM

The invoice for mine says f/5 750mm, as did the package label.

#14 oddog

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Posted 07 February 2013 - 02:32 AM

I will try putting more single thickness scotch tape along the edge, hopefully that will bring back the image quality and keep the elements apart.

I have settled on this scope being a 750mm f5.

I used this scope on a Voyager manual alt/as mount, it handles the weight and size fine, just have to be a bit more careful when using it.

Tom

#15 watcher

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Posted 07 February 2013 - 02:42 AM

Where is this scope available?

#16 oddog

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Posted 14 February 2013 - 09:10 PM

14 piece of single layer Scotch tape around the edge keeps the glass from touching but the image still is just OK at low powers, blooms at higher powers. Off to the bay it goes tonight, starting at $99.99, as-is. Maybe someone can do something with it I couldn't.

Thanks all

Tom






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