Blurry unintelligible image - Mak Cassegrain

[Twelvecarpileup]

 Picked up a Skywatcher 127mm Cassegrain off Craigslist today, took it out to spot the skyline and get the finder scope configured and ran into something I've never had before.

The image was completely unintelligible. Pointed at the buildings across the large bay, I couldn't even make out the buildings just a grey blur... pointed at the ships about 5km's away, just a blue blur. Focusing does nothing. can see the light coming through if I take out the eyepieces, and I've tried it with my eyepieces.

Emailed the seller, but considering she essentially ran away while I was checking it and she was counting the money not really expecting much. I've owned five scopes in my time, refractor's, reflector's a nice dobsonian... but I've never ran into a problem like this.

Does anyone have any idea what could be causing this?

The mount is still good (Steel AZ4 mount, working properly) so it's not a complete loss, but I'm wondering if this is a fixable problem or if I got screwed.

[Dave In Vermont]

I have no idea what could be responsible for this problem. And without personally seeing the scope, I rather doubt anyone really could diagnose it. I presume the main mirror is intact? How'd the corrector-plate look? Any visible damage? So forth.

I'd suggest you find a local astronomy-club and ask about attending a meeting and bring the scope with you. Someone there would likely be able to help you.

Good luck -

Dave

[nightfisher]

The 127 has a huge range on the focus, give it another try, best to take focus all the way to the end of travel then start working back

[ronin]

Only thoughts but as mentioned there is usually a lot of travel and so a lot of winding with the focuser, so it could be a case of "wind, wind, wind". Next is that your eyepieces and the scope have image and object planes that do not meet up and maybe never will so you never get a good focus. The final one and I half think the most probable is that whoever has movewd the focus adjustment so much that it has disengaged the threaded rod from the main mirror assembly.

If the last one then I do not think it is a huge job to fix but does not sound simple. You would need to get the mirror assembly out, through the rear of the OTA then reinsert the adjuster and then put it all back in. I half think the assembly is sort of self contained so is not a case of dissasemble the unit while in place, makes sense for the nature of the manufacturing process. Put corrector in the OTA, Invert, lower the mirror assembly in, tighten all screws up. I would begin to suspect that the previous person has had the threaded unit come out and simply sold it on.

As you say BC I would guess possibly somewhere around Vancouver or that Vancouver is accessible, unfortunately it seems that the 2 astronomy shops in Vancouver have closed down, actually pretty close in time to each other. Meaning no place to take the scope to and enquire.

Search out a club if it makes sense, I know BC is large. You could find someone at one that it knows the problem and knows what to do. As said not exactly an unknown event. The RASC site is likely the start point.

[Dave In Vermont]

I wouldn't suggest trying to disassemble the scope if this is the case, and winding the focus all in/out doesn't solve the problem. At least not without understanding exactly what you're doing. And why. But there is a true 'Guru' of Cassegrains you might contact on the web - Doc Clay Sherrod - who could help you if anyone can. This is if you can't find a knowledgable person in your local area.

Here's a link to his site and his forum:

http://arksky.org/index.php

The 127mm Maksutov is usually an excellent telescope. So this is quite rare regards the problems you're having. I hope the seller spends you're $$$ taking skydiving-lessons. And forgets the parachute....

Please let us know how it's going?

Dave

[spaceboy]

Before any thing I would at least try it on the night sky and try each eyepiece. If you let the scope cool completely try focusing as suggested and you still don't get a sharp star then drop in a Cheshire collimator to see if everything looks aligned. It is possible to collimate a MAK so this suggests that they can also loose collimation. I think it is a simple process and how to guides can be found on the net. If all else fails then I would consider removing the primary and checking the focuser is still attached correctly. Just make sure you make reference marks on the cell and OTA before removing the rear assembly.

[happy-kat]

You only mention using eyepieces. You need to insert a diagonal first and then put the eyepiece in the diagonal. I don't think you would reach focus without the diagonal.

[Peter Drew]

I'd address this problem slowly. The Maksutov has a long focal length and will produce higher magnifications than often expected. Your problem could be simply one of poor seeing over great distance during the day at this time of year due to heat turbulence. Maksutovs don't like heat, night time use requires considerable "cool down" time for the optics due to a similar effect. As has already been mentioned, this telescope is usually a good quality reliable instrument.

[spillage]

+1 for making sure you are using a diagonal. 

[Cosmic Geoff]

+1 for above comments. In addition the best way to get a totally out of focus Mak or SCT into focus is to point it at a small but bright light source, e.g. Jupiter or Capella, with an eyepiece and the diagonal installed.  If you see a bright ring with a dark centre, wind the focus knob in the direction that makes the rings smaller, till you get focus.  If you wind in and out and nothing is changing, that would suggest that the focus mechanism is not working.

A Mak should be collimated, or close to being collimated, as the only adjustment is the screws at the back for the main mirror, which won't be out of collimation by much unless someone has monkeyed with it. To eliminate this, take out the eyepiece and diagonal, and look in the hole. What you see (in a SCT) should be symmetrical.

[Geoff Lister]

+1 for above comments. Another quick check is to look in to the objective end of the scope. If you have one eye central, you should see that eye reflected in the rear of the secondary mirror, and, in the primary mirror, an enlarged image of the bit of your face surrounding the eye (in my case, not a pretty sight).

If you move your eye to the side, the primary mirror should look even. The central, black, tube should have about 30 to 50 turns of a fine thread visible on its outer surface, and it should be possible to move the grey sleeve, (sitting by the mirror, at the bottom of this thread), by multiple turns of the focus knob, to cover, or reveal 1 turn of the thread.

If you have access to a Barlow lens, it may be worth trying it with your eyepieces, as it should shift the point of focus.

Geoff

[Twelvecarpileup]

Hey everyone, thanks for the replies. 

 

I did slowly adjust the focus one way all the way and back (spent an hour doing this while cursing) with no change at all. And I am using a diagonal as well as eyepieces I've used on my other scopes. As for focusing on stars or a bright object even that's not possible as it's that blurry. It's really wrinkling my brain since I've never seen anything like this before in my other scopes. 

Someone did reach out to me to try and get me in contact with the person who ran the local telescope store in Vancouver before it went out of business, as he does repairs on the side still. So hopefully he can get it figured out. 

[michael8554]

If you look at the ships 5Km away with a low power eyepiece and diagonal, and move the focus knob, you should see some movement of your blue blur due to an inherent SCT/MCT problem - mirror flop.

If this isn't happening, the focus rod has become detached from the mirror.

On Meades you can re-attach without taking the primary mirror out, but don't know about Skywatchers.

I wouldn't worry about collimation until you've got the focus sorted.

Michael

[Peter Drew]

Try pointing the telescope vertically and then turn the focus knob clockwise, the knob should eventually stop turning as the primary mirror reaches the end of its travel. If the knob has come detached from the focus rod it will keep turning for as long as you turn it.

[Dave In Vermont]

Do look into the corrector-plate at the large mirror as you turn the focus-knob. You should see the main-mirror moving as you wind the knob - just to be certain it's on it's 'sled.'

Dave