| review: Newtonian Reflector Dobson Discovery 10 inch (appr. € 1200,-) |
The Discovery Dobson is a 10"/f4 rich field one.
The extremly short tube is easy to transport.
It's made from cardboard so it is not very heavy.
The rocker box is solid and the movements on the teflon pads are smooth.
There is no possibility to balance the tube but it came perfectly balanced.
This is true even for the heavier wide angle eyepieces.
The focuser is a rack & pinion one and made from aluminium.
It's japanese solid quality, something you can work with.
There is no visual shifting when reversing the focuser movment.
The end of the focuser is a t-thread (M42x0.75).
I used a Baader Deluxe 1.25 inch eyepiece holder with a brass ring to secure the eyepiece.
The scope comes with a Telrad finder.
In my opinion this really means finding and not searching.
Especially with the Telrad finder charts for the Messier objects.
This mirror is of short focal length and therefore you have to collimate the optics very carefully and you have to use eyepieces of the better kind.
I collimated the mirrors with my Cheesire and fine tuned it with a laser.
Three big spring-loaded screws are holding the cell in place.
Big holes in the cell help to cool down the mirror when it was stored inside.
But the baseplate under the mirror is so close to the holes that the air cannot flow perfectly.
Maybe a little fan could help here.
But the Discovery cools down in a reasonable timespawn like 30 minutes or so.
In the winter time it will be longer.
The mirror cell is made from wood and slipping very precisely into the tube.
Four screws are holding the cell in place.
It turned out that the mirror was only slightly out of collimation after transportation - thanx to the rigid mirror cell.
I read a lot of reviews about other discoverys having good optics so I was keen on the first light.
Discovery now claims their primary mirrors to be lambda/5 or better but this scope is one generation before.
I started with my cheapest 25 mm Plossl eyepiece which came with my TAL2 telescope.
The performance was OK, but at the edge of the field the stars were slightly unsharp.
This eyepiece performed well on longer focal ratios like f/8 but had problems with the f/4.
So I took several Meade Series 3000 Plossl.
I found them to give the same image quality.
I changed to the Baader Eudiaskop (a.k.a. as Celestron Ultima) and this was *much* better.
Nice field of view and very sharp to the edge for an f/4.
My last try were my Pentax SMC XL eyepieces.
This is the finest quality I have.
And they are wide angle.
And now the view was really breath-taking!
I simply love wide angle views when they are reasonable sharp and an f/4 is made for that!
The several clusters in Cassiopeia, especially the double cluster were like true treasures.
The whirlpool galaxy was not very high in the sky but still clearly visible as two spots with a softly structured haze in between.
Conclusion: Good mechanics, good optics, but f/4 demands for above average eyepieces!
More about the optics:
After 1 year or so I invested in another Dobsonian from the hi-price league.
So the Discovery didn't have much to do for a while.
Later I got 2 mounting rings and gave it a try for CCD imaging.
I found out that the focuser-cardboard connection was not rigid enough.
So I screwed 2 pieces of aluminium from inside to the tube.
Now the focuser is fixed to the tube and to these pieces.
That worked fine even for longer exposures.
The quality of the primary and secondary mirror turned out to be excellent for that price.
Here is an example of the fully resolved globular star cluster M2.
If you have a very close look at the stars you will note that they are not 100 percent round.
While this effect was never visible in an eyepiece the extremly small pixels of my CCD camera did.
The effect is caused by the main mirror which sometimes is pinched a bit by the wooden base plate it is glued to.
This effect seems to be related to the humidity of the air.
Another example: The pelican Nebula
But again: this is a Dobsonian designed for visual use!
Search more images taken with the Discovery.
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