View Full Version here: : Primary mirror locking screws
14-05-2012, 10:27 PM
Hi. just a quick question. How tight should the locking screws be after collimating? I was doing mine quite tight. I have been having trouble getting round stars lately and I am sure it is a combination of coma and poor guiding. I had the locking srews a lot looser the other night and my stars were alot rounder. I dont know if that was it. It has been very windy and I am still playing with PHD settings and my results are not yet consistent. But basically I am wondering if overtightening can cause funny shaped stars.
I cant add any images at this stage as my pc is crashed and I have to type from my phone........
14-05-2012, 10:50 PM
Absolutely YES!!!! If you have them too tight you'll be distorting the shape of the mirror and your stars will look, er, rubbishy. As you've found out.
If you're not moving the scope around much (i.e. in a car) I would just let them off altogether. Far better to learn how to collimate so you can tweak the alignment of the mirror when needed rather than bending the mirror out of shape out of fear of having to touch it. Otherwise, if you really must apply them, they should juuuust be touching the back of the mirror. No pressure at all!
14-05-2012, 10:59 PM
Fantastic! Thank you. that explains why stars appeared to scew of an random directions! Question number two. I am using a baader mpcc. it is spaced out exactly 55mm from the sensor. I am getting coma in only one corner of my image? Why only there? All my imaging gear sitting square on the drawtube as far as i can tell.
14-05-2012, 11:05 PM
Sorry, not familiar with that, will have to handball it onto the next person...
14-05-2012, 11:55 PM
Next person! :)
How are you collimating your telescope.
image tilt, what telescope do you have? which leads to my next question
Focuser what focuser do you have?
Lock nuts... take them out those evil little things! a few good springs and they are not needed
15-05-2012, 09:32 AM
Brendan. I am collimating with a glatter laser, and also star testing afterwards. My scoe ips a 10" Saxon with a standard crayford on it. I thought maybe coma was present in only one corner because the focuser may not be sitting "square" in the lightpath, if you know what I mean. If so how do you adjust that accuratly?
15-05-2012, 10:20 AM
Okay So, if you are wanting to use the newt for astrophotography, there are a few things you need to do.
First of all and its my bane, my pet hate. Lasers for astrophotography and newts get you about 70% collimated. decent images start forming on your chip set at about 98%.... Sell the laser my friend because for the same amount you purchased the laser you can buy the right tools that will take you the whole way for collimation. I have no links nor any spin offs but cats eye collimation tools (there are a set of three) Autocollimator, Sight tube and collimated cheshire are the best for the job.
Don't get me wrong lasers may have their place somewhere in somebodys mind but they are a poor substitute for the right tools.
Saxon is skywatchers slightly poorer brother and as such there are things that you should do.
First of all the primary mirror cell needs modifications. Head down to bunnings and get yourself 3 new springs all up it will set you back 15 dollars. While your there get some Neutral cure silicone.
Pull the Primary cell off (6 screws around the cercumfrence), split the primary mirror from the main body of the cell. Take off all the mirror clips and pull the primary mirror out of its immediate cell. place 3 dobs about the size of a 5c piece when squished out of the silicone you just purchased just to the outer side of the cork pads the primary mirror rests on. just for a bit of safety near the middle do the same. Now the mirror clips can be totally left off (better resolution Zero possibility of astigmatisim, past experience).
Now when putting the cell back to gether, give the original springs to a kid to play with, they may be usefull enough for that because they sure as hell aren't usefull for what they are ment for. Put your new springs in. and put everything back together.
This will raise your mirror up the tube slightly.
Now to the top end. The secondary mirror if I am not mistaken will be 58mm (minor axis) which is fine for visual use but useless for photography. Your fully illuminated circle (FIC) is so small you will be down around the 80% light loss by the edge of a KAF 8300 chip set which is an extremely popular chip for cameras at the moment.
for around 50 dollars you can have a GSO 70mm secondary mirror. this brings your fully illuminated circle to approximately 25-30mm depending on your back focus requirements.
Focuser.... although your standard 2" craford works its pretty bung holioed! you will start getting issues with vingnetting even with a KAF8300 chipset because the light cone starts clipping on the focuser tube. Minimum size you should be looking at is a 2.5" and if you can afford it a 3" focuser. Moonlight seem to have a decent priced setup and the top of the line is Feather Touch and FLI focusers. Both FT and FLI have some really nice positives.
lastly try thinking about off axis guiding (OAG). With the OTA itself being thin steel quite often the tube flexes considerably. I was always fighting tube flex untill i moved to OAG, as soon as that happend, pin point stars and good data started happening.
Hope that gives you a bit of a guide as to where to look. once again though loose the laser, proper tools don't require star testing for newts...
15-05-2012, 10:46 AM
Thanks for all that information! Very helpful. I will try get some new springs today. I have already placed some silicone around the mirror because it was vwey loose in its cell. I know you hate the lasers haha. Pretty sure you have mentioned that before lol.
I have considered OAG but heard alot about difficulties finding guide stars and focus issues. Havent made my mind up yet on that.
So do I perhaps spend money on my current tube or get a new higer quality one prefitted with the quality parts for imaging?
15-05-2012, 11:38 AM
OAG issues arise from people who do not know what they are doing. its quite plane and simple!
A ruler and some knowledge will drop you pretty much on the focal point.
Another misconception that OAG's do not work is that people think you can put any dodgy sensor on it and happy days. Truth is that your not working with 50mm of aperature, your more working with 10mm and as such you need a sensitive or a decent guider cam.
QHY5 and OSSAG work but only when there are good star fields, when you get over to say Antenna galaxies, the pickings are slim you will not find a star in your FOV.
Sbig, Starlight express load star are the types of camera that you want to be working with. Light, small, extremely sensitive very fast intergration times.
If you where looking at moving up to a better newt you will be looking in the range of about $3-5000 with Orion optics CT or AG range of newts. everything else you will pretty much suffer from the same issues and even then with the CF tubing and the like you would want to look at OAG just for the reason it is so accurate!
15-05-2012, 01:09 PM
What is your opion on a mead lx200 10" with focal reducer and OAG. Coma free.
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