Quote:
Originally Posted by mental4astro
But one can use a technique called 'drift' where one puts the object to be viewed at the edge of the field of view, or just outside it, and allows the object to drift across the field of view without shaking. Here, wider apparent field of view eyepieces help with the length of time that drift happens.
Mental.
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I agree with everyone else's comments that a 10"/F5 dob is a great first scope that will see you good for a couple of years at least and more than likely 3 yrs to 5 yrs. The critical thing is "DARK SKIES". If your skies are dark enough you can see quite a bit of detail in many DSO's including Galaxies, Nebula, Clusters and Planetaries.
In regard to Alex's comments I have quoted above regarding observing using the drift method, I wanted to expand on that.
Let me say that probably well in excess of 50% of dob owners observe using their dobs in this fashion. I am not one of them and I never will be.
This graph on the Televue Website shows how the spotsize increases as you move off axis in some common F-ratio reflectors.
I think many people who observe in this manner are not aware (or don't care) about the image degradation and reduced limiting magnitude that occurs at and near the edge of the field of view in Newtonians with an F ratio faster than about F5.5 due to comatic blur. In a 10" F5 Newtonian using a fairly standard 12mm eyepiece the field of view is only diffraction limited across about the inner 40%. Whilst the view at and near the edge of field is not horrendous, it is not diffraction limited and it is blatantly obvious to most observers that it is not quite as sharp as the view in the centre of the FOV. What is important however is that due to the increased spot size (from 10 microns at 30% to 30 microns near the edge) and the fact the light is spread over a larger area and thus diffused, the ability to see dimmer targets and features near the edge of the field is much less than it is near the centre. This is easy to test. Find a dim star that is on the verge of visibility in the centre of the FOV, push it to near the edge of the FOV and it will more than likely not be visible if you have in fact chosen a star on the verge of visibility in the centre of the FOV. HN40 the multiple star complex at the centre of M20 (Triffid nebula) is a good target to test this on in with a 10" scope. In a decent 10" scope under good seeing conditions and dark skies you should be able to see the A,B,C and D stars in HN40. Push them to near the edge and you will not get D.
I recommend spending some time tinkering with your scope to get good smooth movement with both the altitude and azimuth bearings, then spending the time learning to hand track at high powers. Whilst 3 of my 4 Newtonians have a servocat fitted (one of them doesn't) I can still hand track every single one of my scopes at 500X plus. Most 10" Newtonians will be flat out getting to 500x. If this is the way you want to go there are lots of things you can do to the bearings to get the scope tracking smoothly and just for the record putting armour all or beeswax on one or all of the bearing surfaces isn't how you do it. Plenty of on line articles on re doing the bearing surfaces with ebony star laminate and virgin Teflon bearings. I spent a bit of time working on my 10" GSO dob to get it to track nicely, but it now tracks as well as any premium built dob. I put extra bracing on the base so that it couldn't flex outwards, used ebony star and virgin Teflon on all the bearing surfaces and actually shimmed all the sides of the bearing surfaces with virgin Teflon. If your 1/2 handy as a woodworker you could even build a new base and if you used the right design paramaters (read larger altitude bearings for starters) you could build a scope base that hand tracks beautifully at 500X plus.
Cheers,
John B