First Light with 2nd Hand Helios 120mm F1000 Refractor

[Meerkat]

Well, last night was the the first clear skies for over a week so I could go and use my new acquisition.

10 years ago I did have a computerised Celestron C8, (not GOTO but it did have dual drives and encoders etc.), and I was hoping that I would not be too disappointed with the smaller scope.

It sits on an EQ3-2 mount which although looks sturdy enough, I wasn't really impressed with because it was almost impossible to focus the scope due to to the wobbles. Had to move the focus out, wait 5 sec, move it in a bit, wait 5 sec, by then Saturn had gone out the field of view, twiddle with the RA manual knob, wait 5 sec, focus in.... well you get the idea.

I was using a Celestron 8-24 zoom and even added a 2X Barlow for good measure to give 4mm, (I know 250X is pushing it, but what the hell), even so I was very impressed with the scope after the vibes had gone. Even though I was sure I still hadn't got the thing in focus properly.

So, as far as the scope goes a big thumbs up from me, considering what I paid for it, but the mount.....

I really, really miss the drives as I couldn't believe how fast Saturn scooted across the eyepiece, and while looking through the eyepiece, I couldn't reach the manual RA, so had to get up each time!

Also, using the finderscope at the eyepiece end of a refractor isn't much fun, having to kneel down isn't good at my age.

So, any suggestions about the focus hassle, as it spoils the experience somewhat. I seem to remember that the C8 was much, much easier to focus and didn't move the scope around. The mount and wedge of the C8 was also much better. (Now wishing I hadn't sold it!).

Saturn was great to see though, and I could just make out the Cassini Division on occasions and some banding.

Steve

[ianporter13]

Steve,

how about a motor focus unit? that way you wouldn't have to touch the scope at all while focusing.

http://www.firstlightoptics.com/proddetail.php?prod=swautofocuser

Ian

[Meerkat]

Hi Ian,

Now that does look interesting, even though it'll increase the cost of the scope by 30%

Thanks,

Steve

[aries]

To wobble, or not to wobble?, that is the question.............

It's a no-brainer if the wobbles are that bad!

Have the same mount with a 900mm Newtonian, and it's fine, even at higher Mag's.

Nick............ :wave:

[dweller25]

Hello Steve,

If you attach a light chain to the front of the scope it will damp down vibrations quite quickly.

Also strip the focuser down and re-grease it.

Hope that helps.

D25

[Meerkat]

I took a look at the focuser last night.

Don't know what stuff was in there before, but it was like glue. So I cleaned all that off and put some ordinary Molybdenum grease on the rack.

While I was doing this I noticed the entire rear cell was "floppy", (no other word for it really), and the three screws were loose, so I tightened them up too.

Also, there is a thumbscrew on the top, and with this loose the whole focus tube rattled around, so I took it out to take a look.

There is a strip of plastic in a groove, which I assume is supposed to take up the clearance by adjusting two little grub screws. Unfortunately I do not seem to have the right size allen wrench, any idea what the size is?

Considering the whole of the rear of the scope was moving around I'm surprised I saw anything at all.

So now I've done all this I suppose it will need collimating? A collimating eyepiece did come with the scope, so another internet search is on the cards.

Instead of hanging a light chain, could I not just move the OTA to unbalance it?

Thanks,

Steve

[dweller25]

Instead of hanging a light chain, could I not just move the OTA to unbalance it?

Thanks,

Steve

It's worth a try.

[John]

If your tripod is the aluminum legs version then that will account for most of the shaking. The EQ3-2 mount is quite solid on the right tripod - an upgrade to a steel legged EQ5 tripod costs around £60 (cheaper 2nd hand) and would make a lot of difference (it will fit right on to the EQ3-2 mount head).

John

[RobH]

I have the F5 version of the Helios 120, on an EQ 3-2 mount with the wobbly tripod.

I found the same as you, too wobbly to focus.

This is what I put on mine, the EF unit.

http://www.switchedsystems.co.uk/

Works a treat and was like getting a new scope!

Cheers

Rob

[Meerkat]

I think I've decided to get the dual axis drives and an auto focuser.

I was thinking about going the whole hog and getting Synscan but that's maybe for the future.

Thanks,

Steve

[Ajohn]

Your focus problem is down to the magnification you are using - it's too much and the barlow may not be much good either if it's the one that they sold with it. Try a mag of about 150. That gives an exit pupil of 0.8mms (diameter of scope / magnification). That's in the ideal area for planets. You may want a bigger image but unfortunately you will just be blowing up detail that isn't there and lens aberrations and will not get a decent focus. Any detail will also be harder to see. If things aren't too good at 150 try an exit pupil of 1mm. My advice would be to spend your money on some decent eyepieces too. Anything by Vixen (even the zoom) or ebay ED's should be a good bet.

To check the scope look at a bright star with the magnification you are using. At 220 you should be able to see nice round defraction rings. The barlow may well mess that up though. An exit pupil of 0.5mms is about right for that and can be used to adjust the lens cell. Can't remember if the helios used an adjustable cell though. The skywatcher does.

John

[Meerkat]

Even at lower mags I was having trouble focusing due to the wobbles, not helped I found later, due to the end cell being loose.

I've also got a problem with the plastic strip that is adjusted by two little grub screws to get the right amount of drag, in that they don't seem to work and I have to use the thumb screw to stop the eyepiece end being "floppy" in the tube. (Anybody know the grub screw size?).

The scope did come with a collimating eyepiece so I've also adjusted the collimation to get it spot on, (it wasn't that far out to begin with though), it's also the same with or without the barlow.

Are you saying the Celestron zoom is a lot worse than the Vixen?

The stuff is arriving Friday, so hoping for a nice clear night....

Steve

[Ajohn]

This sounds like the "Star Travel" experience. :nono: Junk.

If the plastic bits are missing out of the focuser or worn they need replacing with something. Greased hardwood would be suitable if nothing else is to hand. Brass or plastic would be better. Or drill and tap some adjustment screws.

You can judge collimation roughly by looking at a moderate distance roof with something like a 4mms lens. If it's wildly out the definition will be off more in one direction than another. Can be cured by wobbling the focuser about for the best image and then tightening the screws. Any better than that really needs an adjustable lens cell. If you remove the lens shade you will see push pull screws if it has one. That adjustment needs a star or an artificial one. I've posted rough instructions for one of those elsewhere.

Your comments on the mount indicate that the eq3-2 is not the eq4. The 120 I have for sale is perfectly usable on the earlier tripod + eq4. Could also be that your focuser is just too loose in the housing. Agree about an ra drive being highly desirable.

John