National geographic 76mm dobsonian collimation problem

[5ash (Philip)]

I bought one of these for $50 in Aldi to look at the stars with the grand kids. Was not expecting great things but low power views of terrestrial objects look OK. However after looking at the collimation of the secondary with the focuser I think the views could be much better. When looking into the eyepiece the secondary appears to be about 1 cm too close to the top of the optical tube. I want to correct this by putting in longer adjustment and centre screw , does any one know what thread size these screws are? I'd imagine when I fix this I will have to look at the adjustment of the primary. I like challenges.
Hope someone can help regards philip

[jenchris (Jennifer)]

Take out a screw and pop down to the hardware store.
Should be fun to play.

[5ash (Philip)]

Thanks Jennifer that's what I normally do , just thought someone else might have travelled the same path.
Philip

[mental4astro (Alexander)]

This little scope is typical of so many potentially nice scopes made to a price and give lip service to function.

I also picked up one of these and found the same problem with the secondary set way too high above the focuser towards the aperture opening. And the adjustment screws are hopelessly inadequate.

The solution I came up with was to remove the entire secondary assembly (it's only one screw holding the single arm 'spider', , for access to the secondary holder. Then removed the central screw so the secondary mirror is totally free. Then it was a matter of packing enough washers between the secondary and the collimation screws until the secondary mirror is far enough down to be centred in the focuser - I think I ended up using 3 or 4. BUT there's a catch! You will need to replace the original central screw with a longer one. I was lucky enough to find a suitably long one in my bits and pieces box.

This will allow for the existing collimation screws to be used as they then push against the top most washer.

I hope this helps you out, Phil.

Alex.

[5ash (Philip)]

Thanks for the tips Alex , will take out that screw and try to match it with a longer one + Washers.
Thanks philip
PS am getting nice widefield pics with that little scope you sold to me .

[PeterAnderson (Peter)]

I've just got to get my two bob's worth in because I was quite stirred up. I got mine at OZscopes for $59 at the same time as some other stuff. I was giving it to one set of grandkids. Boy, was I disappointed! I didn't see it was my function to take it apart, merely to try to adjust it as best I could. Of course the grandkids and beginners might not notice much difference... but we do.

Here is my review:
[I]The unit was purchased as a gift and I spent a couple of frustrating hours trying to adjust the secondary into a position where the optics were 'on axis'. I failed. I could simply not adjust it far enough, so I settled for as much as I could checking star images along the way.
Now the secondary is between 18mm and 20mm minor axis. Let's call it 20mm and is situated 23cm from the mirror and 12cm from focus (Focal length 35cm.)
A bit of trigonometry will reveal that in order to see the full mirror from the central point of focus, the secondary should have a minor axis of 26mm, and if I was designing the optics with the same mechanics, I would say around 32mm would be an absolute minimum in this design, but I would put the focus much closer to the side of the tube. As it stands the effective aperture is no more than 58mm and possibly as small as 53mm! Very poor design especially for an international organisation. A 76mm telescope - HUH!

The eyepieces had a film on their optical front surfaces, but this cleaned up easily. The barlow was okay. The eyepieces further are very cheap in plastic mounts with small field and this also let the unit down.

Given the appalling alignment, star images are interesting, but the Moon was okay with the 20mm eyepiece which I suspect provides a slightly higher power than the nominal X17.5. Using the 20mm plus barlow or the supplied 4mm, even the Moon does not quite get the the crisp image stage. Why supply these accessories if the alignment is so pathetic? (You should have seen it before I started tweaking the optics!)

I haven't looked at the Moon filter or CCD/DVD software, but the tiny planisphere is a 40 degrees North unit. There are lots of good warning stickers about not looking at the sun. The compass is a gimmick as are the altitude and azimuth calibration marks - but they look good...

Overall, the price is amazing for all this material, but though it presents very well on your mantlepeice or wherever, and the motions are smooth, the actual design is appalling and quality of the eyepieces is poor.

I suppose I got my values worth at $59.95, but not a bargain.

If you want to see this concept done well on a smallish unit, just have a look at the Skywatcher 130/650 collapsible Dob unit (I have one), but there you need to add a shroud or mask when it is extended to avoid stray light.
(Posted on Jan 17, 2016)

This would be a very strong contender for the 'Crap Product of the Year' award.

[5ash (Philip)]

I followed the advice about the secondary screws, centre spotted the primary and used a cheshire eyepiece to see how much out of adjustment the primary was. The primary definitely was out of alignment so I loosened the screws holding it and forced it to move to a closer alignment. I then noted the direction each screw moved , removed the primary and filed the holes in the directions indicated. Then refitted the primary and carefully forced it into alignment whilst observing with the Cheshire. Tightened the screws and added a ring of electrical tape between the primary holder and tube. Now this little scope gives quite respectable images at low power , using better eyepieces.
Regards philip