New to SCT/Not sure what issue is

[Dan Dimitroff]

I'm not really new to astronomy but I am to SCT scopes. I'm not sure what I am seeing.

I have a chance to purchase a C8 at a decent price, but when I was looking through it, I saw what I *thought* might be a sign of lens mildew. I know what mildew looks like on eyepieces. I have a Meade 10mm Plossl that looks like it has a bee honeycomb in a corner. I think it is a goner.

Anyway, when I was viewing through the C8 with a 25mm pl during the day, I noticed what looked out of focus tree branches/shadow in the edge of the bottom right. I was looking at the top of a water tower and there was a blue ghost line that outlined the image, which tells me I will need to collimate the scope, not that big of a deal to me. But when I moved to a 9mm eyepiece, the blurry branches took up more of the image. If I moved my eye closer to the eyepiece, they seemed to move back to the side. I tried moving the eyepiece around, but the branches stayed in the same place.

Looking at the corrector plate and primary mirror, I don't see anything. I thought I saw something maybe the size of 1/8" secondary, but only for a second and only while looking at its reflection on the primary mirror. Looking directly through the visual back to the secondary mirror, I don't see any issues.

With my refractor and Newt during the day, in a dark room and an LED light, I might be able to figure this out pretty fast but this isn't my scope (yet) so I don't have that sort of access or time to work with it.

Could it be that the secondary mirror is so out of whack that I am catching the edge of it when I use an eyepiece or should I just walk away?

[jenniferchristine]

Anything on the primary wouldn't show except as an overall blur.
I think the secondary would have to be loose to be so far off.
I'm thinking the primary mirror may have moved due to the OTA being dropped - and the central column may be out of line - but that would result in difficulty/roughness focusing.
Sometimes stuff in the diagonal or lens can produce ghosts - I had a tiny piece of thread tape show up like a tree branch. But it rotates with the lens when turned.
Cobwebs in the light train can produce strange effects too.
Take a knowledgeable friend with you and have another look. Is the secondary removable (it is on the meade - three screws on the backplate)?
Maybe secondary damage/dirt is visible if you take off the diag and look directly into the light tube

[Dan Dimitroff]

I think the secondary is on 3 screws. I will see about pulling it and taking a look.

Thanks.

[sxinias]

One thing that comes to mind is spider webs.

On my Celestron C6S, if you remove the three screws on the secondary mirror you are in deep yogurt as the primary will fall into the scope with a high probability of damaging the primary mirror. My Meade SCTs are different. Another approach is to leave the secondary alone and simply remove the corrector plate with the secondary attached. Before doing so, mark the position of the corrector plate in relation to the telescope tube as it is aligned with the primary mirror.

Misaligned SCT optics.

I've had two run-ins with SCTs with misaligned optics, both were Meades. In neither case did I see anything like the "dark branch" you are describing. However, thinking about it, it could be that the optics are so misaligned that you are seeing the edge of a mirror holder but ...... ?

Misaligned optics with my SCTs produced problems with image sharpness, light halos, ghosting, and collimation. The two SCTs I had with alignment problems .... one I purchased new in the 1980s (a true Halley Scope) and the other was a used telescope I salvaged from the scrap heap. Identifying the symptoms was easy, identifying the cause was more difficult, and I went down several false trails. In the end, both telescopes had a problem with the way the corrector plate was mounted at the factory ... one corrector plate was off centered and the other was cocked ... both due to manufacturing flaws and sloppy work. After identifying and fixing the alignment problems, both scopes perform very well today. Their images compete with my two Celestrons.

One fast way to check the gross optical alignment of a SCT is to simply look down the scope's tube from the corrector plate end. Place the tube almost level pointing slightly upwards from horizontal. Stand in front of the scope about a meter away and look directly down the tube with one eye. You may have to adjust the scope's elevation. Now slowly back away keeping the back of the secondary mirror centered in view until you can see the reflection of the secondary in the primary mirror. You may have to adjust the telescope's angle a little to keep everything aligned. At the point where you can just see the reflection of the secondary mirror in the primary mirror, move your head slightly until the back of the secondary is centered in its reflection in the primary. If the scope is at the first stage of collimation, you will see the secondary mirror and its reflection centered in the primary mirror similar to a collimated reflector's star test.

If the secondary and its image are not centered, you can adjust the collimating screws on the SCT to center the image to get a gross collimation ... at least sufficient to see if collimation is the problem. If you can not achieve a gross collimation using this method, misaligned optics are a likely reason. One of my two scopes could be collimated and the other could not so this is not a definitive test.

The Meade telescope (8 inch LX200 OTA) I salvaged from the scrap heap is now my number one viewing telescope. I don't use it for photography as I don't have a suitable mount for it. For photography I primarily use my Celestron scopes. My Halley Scope, a 4 inch Meade 2045LX3, is my true grab and go telescope sitting ready to go at a moment's notice and its OTA also serves duty as a camera scope from time to time.

Whether you should buy the scope or not is a good question. If you are handy, like to tinker, tolerant of taking risks, and can get a price low enough to make the gamble worthwhile, you may want to go for it.

[scopemankit]

Dan, Get the offending "thing" into view and turn the eyepiece. If the "thing" turns you have a problen within the eyepiece and not the scope.

[Dan Dimitroff]

Thanks sxinias. I'll give it a go (after it cools down out here). Like I said, there is some ghosting around the images during the daylight test so I know it has other issues that need to be taken care of. My biggest fear was some sort of mildew on a surface that I wouldn't be able to see. Finding that I have lost a couple of my favorite eyepieces recently to mildew, I am a bit gun shy.

Chris, I already tried the eyepiece test. In fact, it was the first thing I tried. I mentioned it in the original post and also that the "branches" seemed to come closer to center with a more powerful eyepiece.

[OleCuss]

Just a reminder to think long and hard before "pulling" the secondary on an SCT.

You may end up with some pretty severe alignment issues - but I'll admit I'm none too expert on that. But it is my understanding that in some cases the secondary may be ground to correct errors on the primary and that the precise rotatory positioning of the secondary can be very important. Again, I'm no expert on that - but I'd check with someone who knows a lot about the beasties before I'd yank out the secondary.