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Oops! Wrong one.....

You could change yours to "and more TALs than the wife knows about" or "and a desire for more TALs than the wife knows about" .

Solar modding folk and someone wanting to use binoviewers might want to chop a bit off the tube. But yes, the focal length of the lens stays the same. Someones had a brain-puff and decided doing that

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How long should the white tube be from focuser shoulder to the end of the lens cell. Mine is 750mm.

Now there's a question. I don't have any notes of how long mine was. With this being a different focuser to the new crayford one, the tube length may be different.

If someone could measure the visible white painted portion of the newer crayford equiped RS, that shouldn't be far away from the length of this older one was, when un chopped.

Andy.

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If the scope has just been shortened then it's worth checking that the baffles in the tube don't vignette the objective lens and reduce the effective aperture. I believe there are a number of ways to to this, the simplest being to look though the scope with no eyepiece in but from a point which is around the focal plane of the scope (where an eyepiece would come to focus). If you can see the whole of the objective lens (ie: including the 3 spacers that separate the objective lens elements) then I think the scope is not vignetting.

I'd guess that your scope has been shortened from the objective lens end.

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Do as John suggests. Focus on the most distant thing you can with a low power eyepiece. Fluffy clouds, believe it or not are a good way to get focus at infinity ! Now remove the eyepiece and stick your eye right on the diagonal opening. Can you see what John says above?

There is another method that can be done indoors, if you have a laser.

With the scopes focuser racked out to where it would be, if focused on a star, position the scope about 12"/300mm away from a wall, pointing straight at it. With an eyepiece in the diagonal, hold the laser about the same distance from the eyepiece. Maneuver it until you get a lit disc showing on the wall. Unless you have extra arms, get someone to measure the diameter of the disc. It should be 100mm.

Andy.

ps: I use the 2nd method when making up adapters for my 'home-brew' finders. A good test. I got the idea off of a thread on 'Cloudy Nights'. It's down at the moment, but as soon as it's back up, I'll check to make sure the method is correct.

Edited by AndyH
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Measuring for clear aperture in an optical system.

Insert low power eyepiece, focus on infinity(fluffy clouds are good!), position scope 12"/300mm from wall, same for laser/single LED torch, maneuver torch 'til circle appears, measure circle diameter. If it's not the same size as your scopes aperture, then there's some viginetting going on.

8485502978_db60d17183_c.jpg

:smiley:

Edited by AndyH
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IT LIVES !!! IT BREATHES!!! IT SEE STUFF!!

First light (really bright light) today assures me all could be ok. I took the scope out in the garden and put it on the CG-5 GT. I didn't bother to power it up just for the quick test, but wish I had done. The sun moves quite quickly !!

Anyways, I put my 80mm Baader solar filter from my st80 into the end of the dew shield and taped it in position so no nasty accidents were going to happen. I removed the diagonal, looked up the tube and targeted the sun. Popped the diagonal and the E-lux 25mm plossl back in and focused. BINGO ! there it was. The drawtube was only extended about 50mm and the Sun filled almost half of the FOV.

I adjusted focus bit by bit until I could clearly make out 2 large sunspots with some smaller spots and a squiggly dark line at various places on the disc. I swapped the 25mm for a skywatcher 10mm ep and looked again. I had to refocus and now the disc was much larger but also moved too fast to get a good focus and study anything. Thats when I wished I had powered up the mount. All too soon the fir tree in the next door back garden started getting in the way, so I'll try some stars tonight if its clear.

So, whether the tube has been shortened or not, it still works fine. I am feeling pleased.

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If someone with a 100RS and the same focuser as mine could measure the visible white tube so I can compare, it would be very useful.

See post #335

I hope this helps.

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Thanks Tony, it did help. Measuring from the same points as in the pics, my scope is 772mm.

I left the scope out in the garden today while I went back to work. I got home at 9pm and went straight out for a looksee. 1st stop was the moon with the E-lux 25mm plossl and it was a cracking sight. Very crisp image with what I would call, decent contrast. I swithched to the SW 10mm for a closer look and that was every bit as good. Then with Jupiter lurking nearby, I just had to spend a few minutes viewing it while dinner cooked.

With the 25mm plossl I could clearly see 4 moons and very faint bands. I switched to the 10mm and although closer, could not get decent definition of the bands, but could see all 4 moons clearly. I did notice diffraction spikes with the 10mm, but that could just have been me not getting perfect focus. I put in the 2x barlow, but without tracking I could not get a steady image while I focused, and i got cramp in my legs from squatting so I stopped for dinner.

Still well pleased with the new scope. I wish I had bought this one years ago when I had the chance, but got a 100mm reflector instead.

The one thing that really stood out for me, was the lack of colour fringing that I read about so often with achromats. There was none at all. Not on the moon or Jupiter. No purple fringe at all. Is this normal for a Tal?

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Tal 100Rs and RSs are known for the surprising lack of CA and false colour despite the fact the glass isn't ED or APO. On the few nights I have observed with my 100R (the 100RS has not been out yet) I was not aware of any real problems with colour fringing when observing the Moon or Jupiter.

It is a terrific scope and for the money, even new, with the fact you get an excellent finder, diagonal and decent eps, well it really is a no-brainer.

Really pleased for you that you have a scope there that is performing so wonderfully ... and for the price!!! A steal...

Edited by TonyD
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Yes I'm very pleased with my steal.

I've yet to work out exactly what it is. The tube is well baffled and I can see the air spaces aroung them if I tilt the tube, but looking straight along the tube from the focuser end it is pretty black with no stray light leakage. It has a 2" focuser, but not like the RS ones I have seen and the objective is purple-ish like the the 100R. I don't know whether its a modified 100R or 100RS but the views so far are really nice. With a bit more time and a decent ep they could be outstanding.

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