MPCC Disappointment - need help

[Uranium235]

Hiya,

I recently purchaced an MPCC to get rid of coma from my 150p, but if anything its swapped one bunch of problems for another.

Now, instead of coma, quite a few of the stars have red fringes - how is this possible from a newt?? Its really frustrating because I cant make it go away. Focusing has become more difficult than usual too with the newt.

My only idea of what it might be is collimation (and that an MPCC would make bad collimation worse), but ive done it twice now and the result was exactly the same. Has anybody else ever had this? I will attach a couple of images to show it at its worst.

Many thanks in advance for any advice.

Equipment used: 150p, CCD CLS, MPCC, 1000d full spectrum

[Euan]

Hmmm...weird

Does this happen right across the image or just around the edges?

[Doc]

I'm not an imager but have you tried removing the CCD CLS and just using the MPCC. If it doesn't make rings then you know it's the CLS filter causing it.

[tooth_dr]

At least yours fits! I bought one, arrived Thursday, and it doesn't fit my 250px.

But anyway let us know how you get on with this!

All the best

Adam

[steppenwolf]

The MPCC is a pretty well proven piece of optics and I haven't seen this particular problem before - I too would be tempted to simplify the light train to determine whether the problem is with the CLS Filter, the MPCC or a combination of the two cause internal reflections.

At least yours fits! I bought one, arrived Thursday, and it doesn't fit my 250px.

Adam, I use one on my 250SPX and it works very well indeed - when you say it doesn't fit, do you mean it won't fit the 2" EP holder or that you can't actually focus it?

[tooth_dr]

Hi Stephen, it won't fit into the holder, there is a lip inside it that prevents it from sliding it. I'm looking into getting it lathed today, looks like very little in it.

[Uranium235]

Thanks guys

I will try it without the CLS filter. I know the filter is fine on its own because ive taken subs with it and they were fine. I think steppenwolf may have come up with the answer - internal refections.

Could it be that light is passing through the MPCC, then some of it is reflecting back off the clip filter and back to the MPCC? I dont quite understand internal reflections, but it sounds plausable.

If it is, its a bit of a bottomer because I have to go back to having a filter before the MPCC rather than after it. Good job i didnt sell off my neodymium 2" then!

[Uranium235]

Ok, Ive switched things around now to 2" filter>MPCC>Camera. Have also laser collimated the newt, then checked it with a cheshire to be sure. My only concern is whether the secondary is correctly centred, it "appears" round when looking down the drawtube, but its hard to be 100% sure.

I took it out last night for a quick test, but the results are inconclusive as it was an unpowered setup.

Ive also been looking at a few other posts in regard to the CLS filter that state that red halos sometimes happen (which is exactly what i have). On really bright stars the halo is offset sligtly. These halos can also be found right across the image on brighter stars.

Anyway, we will find out tonight whether its worked or not - should be good weather for all

[MikeWilson]

Did it work?

I'm using similar kit to you. An Explorer 150P with Skywatcher LPF and the Baader MPCC. In my tests both with and without the MPCC I don't get any red fringing at all, and the coma has been significantly reduced. I hope you got your issue resolved.

[daz]

One trick for checking the secondary is to put a sheet of white paper inside the OTA, opposite the drawtube.

Then when you look down the drawtube, its easy to see the white edge around the secondary. You can then rack in/out to get a thickness of white that you find easiest to judge with

[Uranium235]

I didnt get around to solving it completely, I ended up putting a torch inside the ota so i could see what was happening. The trouble was, I should not have messed with the centre screw in the first place because it also moves the secondary up/down as well as rotates it.

Anyway, I took apart my cheshire to turn it into a simple cap and tried it with that. I havent tested it yet because its time i could be using to image with the frac (with a lot less hassle). I guess i could give it a buzz over the next couple of days because theres not much to do with the full moon about.

But Im not holding out much hope for it, if it fails again after this I might flog it off and completely cross to the dark side (APOs and CCD only). Will report back when im done.

[MikeWilson]

I'm a total newbie, but would it help to use a laser - instead of a Cheshire collimator?

[Uranium235]

I already have a laser, I use that once ive done a simple collimation using the cap/cheshire. Trouble is, even though it reads perfect with the cheshire and laser - it never seems to focus bang-on when I test it for real.

When its bang in focus (like when i first had it) the diffraction spikes are what could be described as "very pointy", what i seem to get now are split spikes and strange off-axis (red) halos on very bright stars.

Oh btw, I did to a test a while ago without a cls filter and without the mpcc, still no joy. Though it doesnt help with having a pants focuser (locking it just knocks the focus out again), once youve used a dual speed - using this one is like going back to the stone age!

[Uranium235]

Right, last night there was finally some clear sky to test the newt after what seemed an eternity in messing with collimation. Unfortnately, the MPCC & CLS CCD isnt a good combo for me because it still produces internal reflections that cant be fixed just by collimation.

Below are two shots of Vega, both unguided @ 30 sec. You can see the off axis halo that happens when I introduce the MPCC. Oh well.... live and learn ey?

[black5f]

Hi. I've only got back into astronomy recently. I made myself a laser collomator which I setup accurately by putting it in a lathe and shone it on a target about 200yds away, slowly rotating it to get perfect alignment. I used to spend ages collomating my old scope but my new one (150P) is just a dream, just a few minutes and it rarely drifts. My eyes are getting old now but I can't really see much coma to correct. What I have noticed whilst messing about with various camera setups is that you need to get the focal plane correct, if it is too close the focus tube extends into OTA by 1/2 inch or so. Mines a single speed crayford type and the outside of the focus tube is unblackend ali. Having this stick into the OTA it picks up all sorts of wierd reflections spoiling the image. Putting extra optics in seems to pick these up and give the impression that its the optics but in my case its usually because the focus tube is sticking into the OTA. I use the very cheap non AF macro tubes from ebay to get the distance right so the tube is clear of the OTA. Hope that helps. Tom