Chromatic Aberration on Jupiter in 8" Dob???

[slime]

Hi crew

My first post and I can't believe how crazy it sounds.

I've a Bintel 8" Dob, bought about 6 years ago, and have just cleaned the mirrors for the first time. The past 2 nights I've had it out, spending some time on Jupiter and am seeing a blue coloured flaring on the side of the planet. The eyepieces used were 32mm and 11m Televue plossls and a 25mm GSO plossl. I believe that I've collimated it well using a cheshire first, then a laser to finish off. I've also been reading about star testing and using a 9mm GSO plossl, the rings seem fairly concentric, except for the shadow of the secondary, which is slightly off centre. I let the scope cool down outside for about an hour, but did leave the tube cover on. Even after about an hour of viewing, the flaring was still there. I could focus Jupiter quite well and see the 2 large cloud bands and make out about 4 other thin bands. I have slight astigmatism in my left eye, about 0.25, which is my viewing eye.

Given all the info above, does anyone have any clues as to what I'm seeing and what is the cause?

Thanks
Simon

[AlexN]

what did you use to wash the mirrors? any possibility its left some very fine residue on the mirrors?

If the secondary shadow is slightly off center, take a look down the aperture of the scope and see if the secondary is well centered in the tube. If not, adjust the spider vanes (via the 4 thumb screws on the outside of the tube, then re-collimate...

Is the blue flaring off to one side? or the whole way around?

[slime]

Great suggestion, its certainly possible.

I have the books Astronomy Hacks and Starware 3rd edition, which I've read over and over. One of the last steps in the mirror cleaning instructions is to rinse the mirror with a mix of distilled water and a drop of detergent. So maybe there is some residue left. I can try rinsing the mirror again, this time with distilled water only, probably not till the weekend though.

The flaring is consistently off to one side. Stars looked fine as did the few clusters and nebulae I've been looking at.

Does the secondary have to be perfectly centered looking down the tube? I completely disassembled and cleaned the scope also, and probably put the spider vain in a different orientation from before. I asked a guy at Bintel this once and he told me not to worry about it. That was when I first got the scope and was still fighting the collimation demons.

[AlexN]

I like to have my as centered as possible.. although i dont measure it.. just as close as possible by eye...

The orientation that you re-fitted the spider is not of much concern, just making sure its centralized and properly aligned to the focuser, and the primary is properly aligned to the secondary...

That should fix your problems.

[Dennis]

Hi Simon

Sometimes you can see “false colours” when an object is low, usually less than 30° above the horizon and also when the seeing is bad.

Cheers

Dennis

[Screwdriverone (Chris)]

Hi Simon,

If you look at some of my previous posts you will know that almost all of my problems have been caused by collimation problems first with the alignment of the secondary and the focuser.

These Chromatic abberrations you talk of are almost 100% due to slight misalignment of the optics when you took them out to clean them, they didnt line up perfectly when they went back in.

My suggestion is to collimate with cheshire/laser tool as close as you can and then at night on a BRIGHT star or Jupiter, wind the focuser ALL the way out with a low power eyepiece and then adjust very slightly the tilt of the primary mirror until the distance around the secondary shadow is even ALL the way round.

Make sure that when you do the slight alignment adjustments that you keep the star in view. If you get to the edge of view when adjusting, stop then recentre, check and continue, the last thing you want to do when fine tuning is lose the star/planet you are looking at. Periodically return to focus and check if the CA is still there. Once you fine tune it all up, you should (and I mean should) see a dramatic reduction or even elimination of the CA.

If you look closely, when you switch from near to far focus just off actual focus, you may see the flaring move 90 degrees from in to out. This should disappear when you tune it all up.

Hope this helps.

Chris

[slime]

Thanks heaps guys, back to square one. I was observing Jupiter near zenith last night so I'll try realigning the secondary and recollimating, with an attempt at star collimation, if no success then rinsing the mirror.

I must say that I wasn't that impressed with the outcome of my mirror cleaning as the primary doesn't look like a super polished mirror. When viewed from a side angle, it has a fine, uniform patterning\smudging all over, perhaps from living very close to the ocean. Though looking into the mirror, my reflection is very clear. The secondary isn't flash either.

What would the effect be if the secondary was slightly pinched from over tightening the retaining bracket? Could this also cause some flaring?

Cheers
Simon

[AlexN]

if you pinch the secondary you could possibly get flaring... any curvature in the secondary mirror is going to cause odd visuals.

[rmcpb (Rob)]

If you rinsed your mirror with a mixture of distilled water and detergent then you have to pull your mirror again and rinse it off with pure distille water. Otherwise you will always have a hydroscopic film of detergent that will cause problems.

[Blue Skies (Jacquie)]

Originally Posted by slime

Does the secondary have to be perfectly centered looking down the tube?

No, and it shouldn't be centred. If you you've done the right thing with your collimating it should be off-centre just a little. This is the offset, something that naturally occurs with today's modern short-focal length telescopes. To have perfect collimation the secondary should be moved away and down from the focuser to correctly intersect the light cone coming from the primary. Using a chesire will get this result automatically as part of the collimating process. The movement down usually needs to be calculated. It's more important a for big scopes, such as 16" and above, where its a significant shift.

A search of this beginners forum using "offset" should find some old threads where this has been discussed numerous times, and there will be some good explanations as to why there as well (better than I could do off the cuff).

In the old days, and with the older, longer focal length you did have to aim to have it centred but not anymore.

As for the flaring I would have said it was the eyepieces but if you're using televues I'm a bit stumped, sorry. Jupiter is bright, though, and its not uncommon in my experience to see a little bit from this planet.

[slime]

Well, a couple of weeks later and I'm all sorted. I rinsed the mirrors again under the tap and finished off with distilled water only. While I was at it I also laminated the dob base with ebony star #50, greased the focuser with white lithium grease and flocked behind the focuser. Still got to replace the stock teflon though.

Looking at Jupiter tonight was wicked! At 150x (8mm TV Plossl) I could get good focus and see 4 bands with some detail in between. My best views of this planet yet.

On a side note, I read a few comments on cloudy nights about the same issue I was having and it was mentioned that there seemed to be a bit of water in the atmosphere at that time. Beyond my understanding, but I thought it worth the mention.

Thanks heaps for all your suggestions!

Cheers
Simon

[AlexN]

Glad to hear its all sorted out Simon!