First Light n00b

[night-vision]

Hello all,

Went out tonight even though the weather wasn't the best, but boy was I impressed as to what to expect on a good night.

Had a look at the Moon, Messier 007, Butterfly galaxy, Jupiter just to name a few, I had a little trouble with the two star align mainly with finding the correct objects in the sky and then in the eyepiece:whistle: so the goto was a little off on the first few attempts, I also did'nt realise how much I would be swaping the eyepiece's around, next time I will have them at arms reach:lol:

But it was a great first experience and cant wait for some clear and still nights:thumbsup:
One question though, when I focused on an object it moved in the eyepiece in the direction in which I turned the focuser, is that normal?

whoohoo
night-vision

[iceman]

Hi night-vision, sounds like you had a great night.

When you say the object moved when you moved the focuser, can you explain a little more clearly? Or maybe I can guess..

What scope do you have, and what type of focuser does it have?

If I had to guess, I'd say it was a dob with a rack'n'pinion focuser, and maybe the tension is too tight so when you focus, it actually shifts/wobbles the scope a little bit?

[Dennis]

I see from your signature that you have an LX 90. The optical tube on these ‘scope is a Schmidt-Cassegrain which has a moving primary mirror and a fixed secondary mirror.

When you focus using the focus knob, the primary mirror slides up and down the central baffle tube and this is what contributes to the slight movement of the image when you focus. Provided the movement is only small, that is okay and there is nothing wrong.

Cheers

Dennis

[iceman]

ah I didn't look there :doh:

The old mirror flop/slop.. One reason I really don't like SCT's unless they have the upgraded crayford focuser.

[night-vision]

Hi iceman, Dennis,

Yup it is a LX90 and it was a fantastic first experience untill the rain came.
The movement was small as far as I can tell, if I focused on Jupiter say
with a 15mm eyepiece Juptier would move out of the eyepiece FOV and I had to manually slew the scope back onto the object and continue to focus on it.
How much difference would a crayford focuser make? Because even though it was my first light it was annoying enough for me to now consider an upgrade in the very near future.

Cheers

[erick]

Sorry to drop in on your thread, night-vision, but I'd like to understand as well. I take it the normal mode of focussing an SCT is this mirror movement up and down the OTA and the eyepiece is in a fixed mount. I gather that an option is to set the mirror to a certain location in the OTA, and add a draw tube focusser (Rack & Pinion, Crayford - whatever) and do your focussing with that instead? Is that how it works?

[night-vision]

Hi erick,
No Problem, you bring up some good questions as I'd like to know exactly how it works also.

[Meadehead]

Hi Night Vision, :welcome:

I'm a noob also and have a similar scope, but I'm not exactly sure what the difference is with the OTA between the LX90 classic & LX90GPS. I only have a 26mm eyepiece at the moment which gives a magnification of approx 77x and I also experience some image shift when I focus but I don't lose the image as you do, maybe with a 15mm eyepiece at 133x it may react the same.

A suggestion given to me to lower the amount of image shift was by running the focus mechanism to its full travel about 20 times in order to re-distribute the grease supporting the mirror "slew" on the central baffle tube. So far I ran the focus mech twice as I counted about 90 twists with my wrist each full travel and had to have a rest besause it hurt:lol:, so I plan to try this over the next few nights to see if it helps.

[night-vision]

Hi Meadehead, :hi:

Thanks for the input, I will definitely try what you suggested as I've been looking at the crayford focusers on the internet and they aren't cheap:eyepop:
and I think I'd rather a sore wrist atm rather than spending those kinda $$$ straight away.


Cheers

[Mark]

Congrats on your LX90 purchase. I'm an ex-LX owner. I just sold my LX90 on the weekend.

Mirror shift while focusing is something an SCT owner must get used to. There is no escaping it. Some SCT's have horrid mirror shift, others are quite good. Even if you do acquire a Crayford, you will still need to do some of your focusing via the main (mirror) focuser. The best way to do it ,IMO, is to 'coarse' focus the object with your main focuser, then use the Crayford to fine focus. Keep in mind that whichever Crayford you choose, you will lose pretty much all of your OTA to fork-base clearance. Be careful when slewing to a new objects - things will go crunch!

The best piece of advice I can give you is when using the main focuser, try to bring objects into focus by turning the focus knob anticlockwise. This will be pushing the mirror 'up' the OTA, reducing any slop to a minimum.

Clearest,
Mark

[night-vision]

Hi Mark,

Great advice

Thank you

[ngcles]

Hi Night-vision,

Congrats on that first night, sounds like it was a blast.

I am no great Schmidt Cassegrain expert but I do know a bit about telescopes generally and I use several different Meade LX200 Schmidt Cassegrainians a lot at Sydney Observatory. The advice offered by others earlier above is good advice and I agree with it. The spreading of the greasy-goo by racking the focuser in and out is particularly good advice and might help quite a bit (not always though!)

Mirror flop (to some degree) is part-and-parcel of S/C ownership. Mirror flop while using the 'scope visually is a bit of a pain -- coping with it while imaging with a CCD has driven many a good man to drink. Commercial S/Cs are focused by moving the primary mirror up and down the tube. The Crayford option eliminates this by moving the eyepiece instead, but does introduce other problems like the mount clearance as noted by Mark above. You do your coarse focusing as normal, fine focus with the crayford.

One thing I think no-one else has noted is the _degree_ of the mirror-flop problem you are describing. You describe Jupiter flopping right out of the FOV of a 15mm ep.

Let's hope I get the maths right here but:

2000mm fl 'scope / 15mm fl ep = 133x magnification

Assuming a 50 deg AFOV standard plossl is used:

50 / x133 = 0.37 degree field of view, equating to 22 arc-minutes diameter true field.

Half of that 22 arc-mins that is 11 arc-minutes (or more) -- this is a pretty big flop (assuming is went all the way from centre to out of the FOV). Seems to me at least to be somewhat worse than average.

This mirror flop _could_ be a significant contributor to the alignment problem you described. If you use two stars far apart (as you should) for the align, a flop can (and in my experience often does) occur while you are slewing from one star to the other instantly adding a significant error to the alignment. There may be (and likely are) other contributors to the accuracy of your align, but that degree of mirror flop will not help it at all.

We don't know where you bought the 'scope (or even whether it is new or second-hand) but after I'd tried the fixes suggested above, I'd be checking this degree of flop with the service agents of the manufacturer, or the vendor who sold you the 'scope to see if they can help.

Don't panic -- it can be rectified or greatly minimised with a few simple adjustments.


Best,


Les Dalrymple
Contributing Editor
AS&T

[Meadehead]

Check this out NightVision. It's a breakdown of the mechanism and will demonstrate why we have mirror shift. I found it useful.

http://jan.eaglecreekobservatory.org/focuser.html

[night-vision]

Hi ngcles,



It was :thumbsup:

I will definitely do this and see if/how much difference it makes.

I did'nt think it was normal, it seemed stange that what ever I was trying to focus on at times went completely out of FOV.

At the moment I'm using Hadar and Canopus/Rigel for my two star align untill I'm better acquainted with the night sky. Then I'll use Hadar and something further to the North, any suggestions on which two stars to use? As you mention I dont think with so much mirror flop it's helping with the alignment.

The scope is second hand and around 5 years old, I did take it to Bintel before first light for a check over and a collimation, I can see myself back there after I've tried all of the above.

That is great news, I was a little worried.
Thanks for a great post Les.

night-vision

[night-vision]

Thanks for the link Meadehead,

I will try this at some stage, I like the idea of the 'Jiffy Focuser' also.

[Meadehead]

[QUOTE]At the moment I'm using Hadar and Canopus/Rigel for my two star align untill I'm better acquainted with the night sky. Then I'll use Hadar and something further to the North, any suggestions on which two stars to use? QUOTE]

After your two star align, slew to a known object in the North, then hold the Enter key for 2 seconds, then press enter again, this will achieve higher accuracy in that part of the sky.

[night-vision]

Hi Meadehead,

I didn't know of that, I will try this next time out cheers.

[ngcles]

Hi Night-Vision & All,

Assuming the LX90 has a similar database to the LX200, a good pair to use early in the evening at this time of year would be Achernar (Alpha Eridani) and Enif (Epsilon Pegasi -- a 2nd magnitude star about 1/2 way between the bright star Altair and the Great Square in Pegasus) which are both in the list of alignment stars for the LX200 -- I assume they are in the LX90 database of alignment stars too.

The other stars you mention are okay as well, but these two are in the early-mid evening each a bit either side of half-way between the horizon and zenith and more than 150 degrees apart. I think from memory the minimum recommended distance for alignment for the LX 'scopes is 45 degrees (two adult hand-spans and a bit) -- though in my opinion, the further apart the better (within reason) . Avoid aligning on stars close to zenith -- they make for poor alignments and ones close to the horizon as well because of atmospheric refraction.

Make sure you follow all the other instructions in the manual and that the tripod is on a very firm footing that is not "springy". Likewise make sure all the nuts and bolts etc holding the tripod together are firmly screwed-up (but don't over-tighten either) and the tripod spreader (with the bolt that screws into the base of the 'scope) is fitted properly.

Assuming all this and that you are taking care in centring your alignment stars reasonably accurately during the course of the align procedure (use medium to high power for this), my next suspect for the alignment issue is indeed mirror-flop.

Even if there is no actual flop during the align procedure, if a flop occurs while you are doing a goto to a subsequent object of interest, it will have pretty similar effect. After doing an align, do a few test slews to objects with an obvious centre -- at this time of year, NGC 104 (47 Tucanae) and M2 are good (there are plenty of others too) to see if it is all behaving as it should.

Hope you get it all together real soon.


Best,


Les D

[night-vision]

Hi Les,

I'm slowly working through the adjustments as time seems to be on my side at the moment with cloudy skys. I've trained the drive, turned the focuser 20 times or more back and forth (I used a cordless drill for this) Only thing left to do is EZ-Focuser mod (install new bearings ect) and maybe the 'jiffy Focuser'.

Next clear night I guess I'll see how bad my mirror shift really is, with my choice of star alignment I actually thought the further away the better, thanks for clearing that up for me. I havn't had a chance to check whether Achernar and Enif are in my autostar although I'm sure they are and I'll definitely use these next time out hopfully my alignment will be a little more accurate.

Thanks

night-vision