...and to top it off... colimation fail.

[rocketandroll]

So...

As well as wasting four hours failing to get my guiding working, I also tried to get first light on my RC6 astrograph tonight...

it's never been used before... so, i took an out of focus shot on Vega to check colimation...

I think it looks wonky, tell me what you thing from the first pic:

I also wondered if vega looked right when I got the bahtinov mask on... pic 2:

Finally, here's a 20 second exposure on NGC891 (it is actually in frame but insanely feint) why are all the stars blurred out to one side like that? It also seems, after having focused it, it has slipped out of focus too.

Thoughts?

Ben

[Black Knight]

I've had the same "4 o'clock shadow", and I don't know what causes it. Several suggestions were put forward in my case, vibration and breeze amongst them. All I can say is the subsequent times I've been out imaging I've been extra precise about everything. Polar alignment, drift aligning, balancing, making sure everything is securely fastened etc, and I've been lucky it hasn't happened since. So maybe something I did sorted it out ... I don't know what though.

[JohnnyD333]

All I can say is that your collimation is just slightly off, but not enough to cause major issues. That 20 sec exposure is interesting, its very noisy for such a short exposure and just seems a little fuzzy. Give it another shot, your collimation is close enough and maybe your focus was a little off. Good luck!

[rocketandroll]

Cheers both

I think it's actually more of an issue that focus slipped during the session... the focuser seemed very loose and I thought I'd tightened it during the focusing... but obviously not enough.

As for the frame, it was admitedly at ISO 3200 and I have done a levels stretch on it and that i about 1/8th of the frame cropped to show the dodgy stars, not the full frame.

Good to know colimation isn't too bad then.

Ben

[rocketandroll]

Ok...

Having now de-coupled the guiding issues from the issues with the RC6... I am now keen to try and solve the RC6's colimation/focus issues.

So... here's a section of yesterday's image of the Pleiades showing the stars appear out of focus (large and slightly blurred) and all seem to have some kind of artefact or blur coming from the bottom right.

I've noted this distortion is in the same plane as the apparent colimation issue (ie top left to bottom right of frame) in the earlier shots... in both the camera was alignged the same against the scope.

What I am confused about is why the scope doesn't seem capable of reaching focus?

In all the shots I've taken the stars are never close to sharp, even when the Bahtinov mask says the focus is bang on.

So...

Is there something that could be wrong (eg: distance between the primary and secondary mirror, problem with the corrector lens etc) with an RC design scope that could cause it to be incapable of achieving sharp focus like this?

If so... how do I confirm that and how do i start to try and fix it?

Cheers in advance all.

Ben

[barkis]

Your collimation isn't right. The light path from the main mirror seems to be obstructed. Probably on it's entry into the focuser.

The out of focus image shows this, and when foscused doesn't look so bad. However, for for perfect images, the collimation will have to perfect, as in all reflecting scopes, particularly fast ones, although yours is f9, it needs to be spot on. Don't be satisfied with close enough.

I have a f10 sct, and if the collimation is even slightly out, Jupiter looks very mediocre. Totally transfomed when collimated.

Ron.

[kookoo_gr]

Deep Space Place - Equipment check this site for collimating an rc scope i am using it for my scope. These are latest test photos i took and they ar e the best colimation i have achieved so far. It needs more work but all in time

http://www.astrovox.gr/forum/download.php?id=31317

http://www.astrovox.gr/forum/download.php?id=31316

[rocketandroll]

Thanks for the thoughts Ron.

So, should colimation fix the 'out of focus' appearance issue as well as the asymmetry issue?

Is colimation on an RC or an SCT similar to a Newt? Never colimated anything before... anything RC specific I need to bear in mind?

Ben

Sent from my GT-I9100 using Tapatalk

[barkis]

Any interference of the light coming into your focuser can produce a result that could be diagnosed as poor focus.

You could have two issues, collimation, and a focus one.

The best approach is rectify the collimation, then determine if the other one has also been fixed. If not, then further investigation will be needed. Unless the front plate Assembly has been upset in some way, I don't see it being part of the problem. The secondary mirror does have adjustment of course, independent of the plate itself.

My own Meade SCT, only has secondary mirror adjustment to collimate the scope. I know the corrector plate has never been removed, therefore it is in the same orentation as when it left the factory. When correctors are removed for any reason, steps should be taken to ensure the go back exactly in the same position they were in prior to removal. Any rotational error could cause a problem.

Ron.

[kookoo_gr]

When collimating an RC scope keep in mind that you have three variables that contribute to the collimation 1)The secondary mirror 2) the primary mirror and 3) the removable focuser but when placed on the scope it acts as one unit with the primary mirror and the buffle. In order to eliminate the third variable at a very high degree you need to make sure that it is at the same mechanical axis as the rest of the scope. I checked it by placing the ota vertical to the ground and i inserted a film canister at the focuser, then with the help of bubble level i made sure that it was level. Here i found that when using a spacer for the focuser between the OTA and the focuser you can't make it level to the ground (see photo http://www.astrovox.gr/forum/download.php?id=31191 ) so i removed the spacer checked the level of the focuser and made sure it was level by small adjustments and used T2 extensions for the imaging system i use. alternatively you can buy TS Okularauszug Justierung - RCs von GSO & Astrotech 6' und 8' but you will need a spacer since it covers the collimation screws for the primary mirror.

When collimating the scope you will have to make small adjustments to the screws especially at the secondary mirror since the collimation can easily be lost and start all over form the begging. I found that using a film canister like at my newt helps for an initial collimation by following the instructions from the orion RC 8'' manual, don't use a chesire eyepice since it is highly inacurate.

Since it your first time collimating an rc scope i would suggest not to mess with the primary mirror yet since many at CN say that achieving collimation is really hard. My advice read the method i post above. Make note that it may take some time to collimate it (hell it took me almost a year to do it myself with bad weather and guiding issues i had but i am getting there)

Best of luck

[JimStan]

It may be different for an RC scope, but I would say that your picture of the "out of focus star " is TOO out of focus to really be very helpful.

To more accurately test your collimation, use the highest power eyepiece you have, and just pull the focus SLIGHTLY out. You should see a display that resembles a "target" with diffraction rings surrounding the central spot, ( which should be just a black dot, and NOT a big black circle ) If the target is perfectly round and the diffraction rings are evenly spaced and concentric, your collimation will be extremely close.

I read the instructions about an RC scope, and also advise you to do the easiest steps first, and see if you get an improvement, before going for the main mirror !

Good luck !

Jim S.