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Collimation or seeing.....help?

Solar_observer

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Solar_observer

Solar_observer

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I was on a high this afternoon as I collimated my 200p dob for the first time. It was out as I inserted a collimation cap and the centre dot of the cap was outside the paper donut. I spent over an hour following astro baby's instructions and finally got everything lined up. See pic and see what you think?

So left my scope cooling for 30 mins earlier and was excited too see some improvment. Io moon and shadow transit around now.

im using a 6mm tmb and a 12mm bst with both up until I adjusted got really sharp views and very good contrast. Now the contrast is not there and no where near as much as BEFORE collimating earlier. Im also using the 2 inch adapter and a revelation self centering adapter for the first time and thought it could be this so reverted back to the standard size but with the same results. I really got everything centered so I dont understand whats happened. Can anyone help?

Sent from my GT-I9300 using Tapatalk 2

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Spec-Chum

Spec-Chum

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Well, I've woke up all bright eyed and bushy tailed and took another look at this pic.

I think myself and Jason are on about different things here.

I was suggesting you move the secondary closer to the OTA opening to centre it in the focuser, but looking at the pic more closely I can see what Jason means now. The primary reflection is not central on the secondary mirror, indeed you can actually see the full clip and part of the tube on the left of the reflection. So you're not gathering 100% of the light.

:)

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Solar_observer

Solar_observer

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Im just concerned that im going to continue to make it worse. It was ok when I started but like I said, when I put the cap in it was clearly NOT aligned. But I suppose at least it will get me accustomed to the procedure at least.

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Jason D

Jason D

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Well, I've woke up all bright eyed and bushy tailed and took another look at this pic.

I think myself and Jason are on about different things here.

I was suggesting you move the secondary closer to the OTA opening to centre it in the focuser, but looking at the pic more closely I can see what Jason means now. The primary reflection is not central on the secondary mirror, indeed you can actually see the full clip and part of the tube on the left of the reflection. So you're not gathering 100% of the light.

:)

I also woke up this morning with fresh eyes.

Had the OP used a quality laser or a quality sight-tube with cross-hairs then the secondary mirror will need to move lower but it is obvious based on the attached photo the laser beam will not hit the primary center and the cross-hairs will not point to the primary center. In this case, it is hard to determine the proper movement of the secondary mirror.

To determine the proper movement, the primary mirror center spot reflection needs to be centered with respect to the focuser. When that happens, the primary mirror reflection will be centered under the focuser. Now, think of the secondary mirror as a window overlooking the primary mirror. Determine how best to move the window (secondary mirror) to view the whole primary mirror. See attached animation (you might have to click on the attachment to play the animation).

The OP is using only a collimation cap which is not the best tool to center the primary mirror reflection under the focuser.

Having said all the above, I do not see any gross miscollimation in the photo to justify a poor contrast as others have already mentioned.

Jason

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Planetesimal

Planetesimal

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I would say collimation is quite close, close enough to not cause any real problems. I would suspect lack of contrast was far more likely to be caused by bad seeing.

I second this... I was out last night with the frac - no collimation worries and no dewing of the optical surfaces, but seeing / transparency was awful... The views of Jupiter through my 8mm were fuzzy and lacked contrast - I'm sure the OP's problems were caused by atmospheric conditions. Only one way to find out though! :)

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KevG

KevG

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I second this... I was out last night with the frac - no collimation worries and no dewing of the optical surfaces, but seeing / transparency was awful... The views of Jupiter through my 8mm were fuzzy and lacked contrast - I'm sure the OP's problems were caused by atmospheric conditions. Only one way to find out though! :)

Thrice agreed..... I was out last night (not too far from the OP's location) the seeing was particularly bad. So hopefully it was a result of the naff atmospherics, and not collimation.

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Solar_observer

Solar_observer

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Thanks guys for the input. Just ordered a standard cheshire from flo as I want too do the job, as best as I can. Is it best to put my standard 1:25 inch focuser tube in when I use either my cap or cheshire or can I keep my 2 inch one installed with the revelation self centering adapter fitted?

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