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10" DOB Why can't I see the obstruction?

Started by BABOafrica, 02/19/2011 11:06PM
Posted 02/19/2011 11:06PM Opening Post
After quite a lively round (Joe's home-made 10" Dob) covering some basics, first let me thank all the fellow star gazers for their advice. I mostly remember how important the tilt of the primary is and how important it is to learn what you're looking at while collimating.

As far as collimating the Dob, I think I followed all your advice and at least the basic instructions available on the websites recommended. But maybe I missed something -- and there was a lot to miss!!!

I have one HUGE question nagging me. Last night I was out viewing, after a lengthy collimation session in the early afternoon. I was expecting to see some kind of obstruction in my view of the sky (because of the shadow from the secondary mirror on the primary).

With no eyepiece in the focuser, I can see the obstruction clear as day. With an EP inserted, I see no obstruction at all, regardless of what I'm looking at.

As far as collimation, the various circles all line up. I think I did that part ok.

Last night, I spent a lot of time looking at the moon. I even used an old barlow and it worked really well. I used all 3 eyepieces I have (none of very good quality but functional) and those include: EPs = 4mm, 8mm and 12mm.

Can anyone give me advice. I must have done something wrong.

Please remember that I constructed this scope myself, working purely off of pictures and website info. I have never seen a real Dob, must less actually used one.


In lumine tuo videbimus lumen.

8O Home-made 10” Dob / Home-made 4” refractor

EPs: Konig 32mm (1.25") / Zhumell WF 30mm (2") / Nagler 13mm T1 / Orion Sirius Plossls 25 & 10mm / Zhumell Plossl 9 mm / Meade MA 9mm
Posted 02/20/2011 01:36AM #1
You have done nothing wrong, under normal circumstances you should not see the obstruction.

The easiest explanation is simply that the obstruction is totally out of focus. The eyepiece is sharply focused on the focal plane, a few millimeters away, the telescope itself is focused at infinity, you can put your hand in front of the scope and it only shows up as a slight dimming of the image.

If you look at a bright star and then defocus the image significantly, you will see a large disk with a hole in the middle, that is the secondary obstruction. But if you are in focus, you will not see it.

Posted 02/20/2011 11:17AM | Edited 02/20/2011 06:19PM #2
If you used a long-enough focal-length eyepiece you Would see a shadow of the secondary mirror superemposed on the focused image, but such a long focal length eyepiece would also be impractical due to its' having an exit pupil much larger than your eye can manage, so much of the scopes' 10" aperture would be wasted (the maximum human eye pupil dilation in a dark-adapted situation is about 5 to 7 mm depending on the individual & as previously mentioned a 32mm focal length eyepiece provides 7 mm diameter exit pupil in your scope so is the lowest power eyepiece you can use most effectively). An easy way to actually see the effect is to place a finger directly in front of one of your eyes with Both eyes open; notice it appears as if you are looking thru that finger when looking at something across the room. In such a way the larger primary mirror seems to look thru the smaller secondary mirror even tho the secondary is directly in the primarys' incoming light path. This Secondary obstruction is the inherant defect of a Newtonian reflector, a refractors' inherant defect is defocusing of the various colors of light since from the center to the edge all magnifiying lenses (even eyepieces) resemble Prisms in cross-section (prisms turn *white* light into a rainbow of individual colors). Unobsructed reflectors have Coma (a flaring of star images away from the central axis) as their inherent defect. Each such defect affects the image similarly so given equally well-made & same-size optics, each telescope type performs equally per image resolution. Newtonian optical design reflectors such as yours (Dobsonian refers to the mounting arrangement) are easier & lower cost to implement than equal aperture refractors or tilted-component off-axis reflectors due to fewer optical surfaces to make well and glass quality (reflectors do not look thru their glass). Hope this helps.
Posted 02/20/2011 11:48AM | Edited 02/20/2011 06:21PM #3
Joe, in your scope a 12mm eyepiece magnifies 95X, an 8mm provides ~140X & a 4mm gives 285X. I would think the 4mm could be used effectively only rarely (image becomes dimmer & less sharp at high magnifications) when the Earths atmosphere that you are looking thru is very steady. And even 95X is rather high for much Deep-Sky (nebulae & galaxies & star clusters) viewing. When you acquire a 20mm wide field (or even a 16mm wide field) type eyepiece & either a 28mm, 30mm or 32mm lowest power eyepiece you will be thrilled even more! So you still have something to look forward to beyond your 1st view. 32mm will give you 35X (28mm = 40X), 20mm = 57X & 16mm = ~70X in your scope; 32mm will give the widest True Field of view possible in a 1.25" barrel size & would provide great viewing pleasure while casually sweeping the Milky Way. True field is the actual area of sky seen & Apparent field is the angular width encompassed by the eyepiece itself (Wide Field types have greater angular area); a wider Apparent field does provide a wider True field vs same focal length eyepieces of lessor apparent field.