Mead LX90 8" Needing Help

[London_David]

I've got focus problems, and I'm not sure exactly the cause.

I have been given an 8" LX90 GPS - probably around 2000-2005 vintage - because the previous owner wanted something smaller and lighter. It's rarely been used since it was heavy and mainly has sat sitting in a warm relatively constant temperature living room and moved 3 meters outside four or five times a year, if that. Physically it looks in great shape with no markings and just a little dust around the edges. I used it a few times in the early 2010s and I remember it working well, except for a sloppy focus mechanism. That said, I was inexperienced then so I don't know for sure.

Over a few nights at Christmas and New Year I tried using it again to see what shape it's in, expecting it to just work as before. Unfortunately, I couldn't get it in focus. No matter what I did it was always fuzzy. Smoothly turning the focus knob, stars and planets would start to come into focus, then would become a fuzzy blob, then go out of focus again. I looked at the moon which was seemed to be okay in lower magnifications, but at higher magnifications there was notable chromatic aberration. There was no detail on Mars only a fuzzy orange blob. Jupiter's bands weren't really visible and certainly no detail (or even a sharp edge to the planet). Stars would never come to a point. They'd always be blobs at higher magnifications and never seemed crisp at lower magnifications. 

This is using a Baader Hyperion Zoom 24-8mm and Televue Delos 6mm through a WO Star diagonal. The fuzziness started being notable around about 14mm in the zoom. Though once you saw it, you could just about notice it on fully zoomed out too. This was particularly true of the chromatic aberration you could see when looking at the moon. As far as I know I did everything correctly. I let the thing cool down, there was no dew or fog on the front plate (though dew and frost was an issue on some nights when I used a hair dryer to get rid of dew occasionally). I even took the diagonal out of the chain using the Baader and it didn't make any immediately visible difference to the image.

The best I can describe is that it looked similar to when there's dew on my secondary mirror on my Newtonian. The Meade image was probably blooming a bit less than when that happens, dew is usually quite obvious, however here at first glance using lower magnifications it seemed almost okay. There was no dew or frost on the Meade front plate or the optics that I could see.

While I always remembered the Meade focus mechanism as very sloppy and horrible to use, historically that didn't effect me getting the image into focus. Plus there was the chromatic aberration. I looked at the collimation on a star by going fully out of focus and looking at the concentric circles, and it seemed to be the correct pattern, and tube currents seemed okay. I may not have done that test right - I'm not sure, I have a newtonian and a couple of refractors, this is the first SCT I've got. I watched a YouTube video and tried it out in the cold, so I may have missed something.

I had a William Optics GT80 next to it and my first thought had been that seeing was bad. The humidity was between 80-95% across the different days I tried it but on the forecast but the arc second resolution on the forecasts was okay and I've seen better views in worse conditions with my 6" Newtonian. To the eye stars lower down had a bit of twinkle but were pretty solid high up and I was at a dark site without much light pollution. The WO seemed fine. Jupiter would show clear banding and looked better than on the Meade (though to be fair that was using lower magnifications and I've had better results from the WO scope elsewhere). Also, the problem with the Meade was consistent across several nights.

Could seeing really have been similarly bad every night? Jupiter was low in the sky but Mars and the Moon were not. I also used the WO scope with a camera and while the seeing definitely wasn't great, I still think it has to be a problem with the Meade. The previous owner also said that they could never get it in focus either, and had told me previously they struggled to see Saturns rings clearly. However, I was thinking the previous owner needs glasses and sometimes struggles to find focus with my refractors - which are fine - so I wasn't convinced anything was actually wrong until I tried it myself on Jupiter and Mars.

Does anyone have any ideas? Could the seeing have just been that terrible? Was I just cranking up the magnification too high? Could I have had dew or frost somewhere I didn't think about? Where would the chromatic aberration come from? 

What should I do to test it? Or fix it?

Possibly relevant to what I do going forward: I've wanted an SCT for a while and was considering a C8 until I was offered the Meade. My intention was to defork it (I have a CEM60 on a permanent pier) and get a Baader Steeltrack focus for it. My interest is galaxies and planets. So I'd probably be putting a focal reducer and a camera on it 50% of the time and visually looking at planets for the rest. The C8 is still better for me overall than the Meade -- however, I'd have to spend a lot more money to get one! This one I hoped might just need some mounting rings and a focus upgrade. If it's all too complicated to fix though, maybe it's better selling the whole thing... and buying a C8.

Any thoughts are welcome!

Edited January 7, 2021 by London_David

[knobby]

It's difficult to say without looking but I know my C8 edge takes a good 90 minutes with a fan running before I can use muck more than an 18mm Ep.

You really will need a good cool down / good seeing to use a 14 mm ep.

---edit--- If you feel it's a focusing problem, maybe try a clothes peg on the know ... it will give more precise movement. Also try running the focus all the way in and out several times, maybe the grease has gone 'gloopy' with age.

Edited January 7, 2021 by knobby

[London_David]

Thanks for a quick reply!!!

Hmmmm... One night I was out testing it within about 30 mins. Another night it was out for about 2 hours before I started using it. No fans though. There was no memorable difference in the performance in either night.

I've heard stories about SCT's being a bit soft. Could it be that I'm over-magnifiying? It's incredibly disappointing if it is - my cheap 6" Newtonain has notably better views than I had last week!

Edited January 7, 2021 by London_David

[Martthebass]

I had one of these back in the day.  My skies were typically Bortle 8 where I lived at that time and I found I had the best views at lower power using (the supplied) 26 and 40mm eyepieces.  I occasionally used an 11mm Nagler and found my sky quite often couldn't take it. 

A lot of what you say strikes me of themals in the tube....mine was definitely fuzzy for the first hour or so, a little longer if it was brass monkey outside.