12" Dob first light double images on Saturn?

[syousef]

Hi,

I'm hoping someone with a bit more experience than me can answer whether I should be concerned. I bought a Bintel 12" (GSO) dob on Saturday, checked and slightly adjusted collimation, attached locking castors yesterday, and finally got a view through it today. I've had an 8" since 2000 and a 10" since 2002/2003 sometime, so I'm not completely unfamiliar with scopes but i do not observe as much as I'd like to.

I used the supplied 26mm 2" to align a Rigel Quikfinder tonight, so that happened to be what I had when I first pointed the scope at Saturn. For about 5 minutes I was quite worried. I saw the most horrible ghost/double image of Saturn I've ever seen. Both images equally and dazzlingly bright. Using the aperture mask dimmed the image and eliminated the reflections. I checked collmation with a laser collimator, and while it wasn't perfect it was pretty close (within maybe 3-4mm on the center ring - assuming that's correctly positioned - and perhaps 4mm out reflected back onto the collimator). I tried throwing a bright star out of focus - it too seemed to double when viewed in focus - out of focus no hint of diffraction rings - seeing a complete mess. Fainter stars still seemed point like in focus but anything particularly bright was awful.

So I tried my 15mm GSO Superview, barlowed and unbarlowed and the ghosting was no where near as bad. Even switching back to the 26mm - while the ghosting was still there was not as bad!?

So I turned to other objects. The Jewel box being my favourite I got the increased detail I'd expect moving up 2" and having a newer mirror. I was able to make out what Stellarium is telling me was a magnitude 14.6 star with averted vision (but of course it wouldn't surprise me if it was variable). I was making out stars that didn't show up in Stellarium. All nice and point like, no issues. Very pleased. One look at mars confirmed the seeing was still a mess. Total mush - even considering it's on the opposite side of the sun this was horrible.

Getting back to the ghosting on the bright planet Saturn, before I bother the folks at Bintel, was it just the seeing causing issues? Or should I be worried I have issues with my mirrors or the mechanics? Am I just not use to a larger brighter instrument? Should I just be sticking to 1.25" for planetary viewing.

Thanks for reading...

[erick]

Doesn't sound right at all, but I've no idea what is causing it? Eyepiece problem?

[pgc hunter]

Did you allow your scope to cool? A warm mirror or poor seeing shouldn't show ghosting though, but rather a blurry, 'boiling" image at focus.

If you're getting ghosting, it could be the eyepiece (those cheap 2" GSO EPs are not the best at F/5 and certainly can introduce abberrations to the view). However, you mention that the view with the same eyepiece improved after a bit of time, I suspect that you did not allow the mirror to cool.

You also mention that your laser collimation was off by up to 4mm. That will matter at F/5. You should also note that the collimater itself might be out of adjustment. It's easy to check for this, put the collimator in a V-block, aim it at a wall a few metres away and rotate it, if the dot makes a circle motion then your collimator is out of alignment.

It's better to use a cheshire eyepiece. You can align the secondary quite accurately using the outline of the bottom of the cheshire tube, then align the primary by centering the centre spot donut on the black cheshire dot.

[syousef]

I did look at it with the cheshire. Alignment seemed spot on. I have also checked the laser collimator by turning it fairly recently (2-3 months ago).

[malclocke]

I get a similar effect with the 30mm 2" superview that came with my 10" GSO DOB. The view with my 9mm Plossl is fine, so I think it's the superview eyepiece that is the problem. It could also be compounded by not allowing cool down time in my case, I don't remember trying to use the superview on Saturn later in a session, I usually just jump straight to my 9mm plossl.

This review also reports a similar problem due to not seating the 2"-1.25" adapter properly http://www.cloudynights.com/item.php?item_id=2582. Obviously this isn't the case here, but it could be a similar issue ... maybe.

If the sky ever clears here I'll have another look at Saturn with my superview after a good cool down and report back.

[mental4astro]

It's not the eyepieces. Changing the EPs and seeing the same effect confirms this.

Primary mirror could be held too tightly in its cell causing deformation.

You may have chanced upon a very nasty jetstream rocketing in front of Saturn & Mars as they are close together. This has happened to me once or twice before on bright extended objects like planets and the Moon. The effect can provoke a twining effect due to extremely different atmopheric currents. Somewhat like some crystals can cause a double image of that lies underneath. My high school physics is a bit dim nowdays to explain it better.

I suspect the latter from what you described that your view of the Jewel Box was fine.

Mental.

[alocky]

Any chance the secondary has been installed with the aluminised side on the inside of the holder? If the effect is independent of eyepiece I can't imagine what else it could be.
Cheers
Andrew.

[barx1963]

If the mirror clamps are too tight it causes astigmatism which shows up as severely distorted stars. As there are 3 clamps you will tend to get triangular stars. They will look horrible even when as precisely focussed as you can get, and defocused images look like misshapen triangles.

I cannot see how collimation would create this effect.

One possible way to test the EP would be place the image of Saturn nearer the edge of the field and rotate the EP and see if the "ghost" moves. If it moves I would think it would be a prob with the EP.

Malcolm

[barx1963]

Oh and to check for tube currents, defocus a brightest star and you can see the currents within the image.

Malcolm

[Shiraz]

yep, if it changed over time, sounds like tube currents - how long did you leave it cool down? As Alexander mentions, could also be the current horrible jetstream

[syousef]

The scope hadn't been out for long at all. Perhaps a minute.

Well I managed to get the scope out last night....but only at the end of the night. I was out with the 8" at a school with my astronomy club and only got the chance once i got back. There was no way I was gonig to get a proper star test out of anything.I could barely make out the diffraction rings it was that vile. But I still got reasonably good in focus views...

Don from Bintel offered to exchange the eyepiece for a 30mm and I think I'll do that, perhaps ask for the 42mm instead if that's not pressing the friendship.I don't need two 2" 26mm eyepieces. (I'd recently bought one) but the 42 woudl round out my set nicely.

The actual views I've been getting the 2 times I managed to pull the scope out are great given the poor seeing. I really need a decent night to do a proper star test. I still want to rule out any issues with the primary mirror.

[syousef]

I'm definitely not seeing triangles but the diffraction spikes do go in different directions when on opposite sides of focus.

Part of my problem is my eyesight isn't perfect. And the other is lack of experience. I've not looked through many scopes and the ones I've owned I've owned for years. Yet another problem is I don't have decent high power lenses - my 10mm and 6.3mm plossls are my best options. I have a barely usable 4mm one, but it's not good fora star test.

[syousef]

How should the diffraction spikes on a star look as you move from out of focus, into focus then out of focus in the opposite direction. Should the diffraction spikes keep their orientation? Shoot off at 90 degrees to prior, or is the direction of the spikes either side of focus meaningless?

[AstralTraveller]

My old solid-tube 10" used to do this after hot days. If it's the same effect it's tube currents, especially those right on the surface of the mirror. It would always be worse when looking near the horizon than looking straight up. After it cooled it worked great. Unfortunately when I built that scope thermal management wasn't generally considered and I'd never heard of it.

[Astro_Bot]

@syousef: Does this help?

Backyard Astronomer's Guide - Appendix A. Star Testing (http://www.backyardastronomy.com/Backyard_Astronomy/Downloads_files/Appendix%20A-Testing.pdf) (PDF)

[syousef]

Yes it does actually. One of the better articles I've come across.

Also the diagrams at the end of this page are useful, but they're "too perfect" to match what you'll really see, especially on a bad night.
http://www.willbell.com/tm/tm5.htm
Also a whole book! on the subject...wow. Anyone own this?

I'm certainly not seeing anything as extreme as anything in that article - no triangles, . I thought there *might* be a slight amount of astigmatism, but I had only tried at low power. With a 6.3mm eyepiece and without barlows or anything else the discs seem to flatten in very slightly in the same direction on both sides of focus. If there is an issue I think it's very minor.

I also tried using the software "aberrator" here:
http://aberrator.astronomy.net/

What I really need I'm not going to get - and that is someone with a lot of experience with telescope mirrors to take a look for me.

[syousef]

Thanks to everyone who commented. I've read every comment and appreciate it.

[GrahamL]

I used to find my 10" would settle quite quickly when taken outside .

My 12" if I can't get it out late afternoon takes a good couple of hours to throw up nice steady views, a minute in probably isn't a good time to be having a decent look at really bright stuff .

[bigdobman]

It's winter....you may have been looking over the neighbours chimney.

[syousef]

None of the neighbours have chimnies, but an interesting thought.

I've had the scope out again one more time. I know have wheels and manual azimuth circle + wixey angle meter attached. Can get things within a 42mm eyepiece


I swapped out 26mm for the 42mm at Bintel. Had a long chat with Don on Saturday. He was very friendly and helpful with a number of things. I bought the scope with decent after sales service in mind and so far haven't been disappointed. Nice change from after sales service I've been having on electronics (nothing to do with astronomy) of late.