DIY 18” truss dob, UTA size?

[Kunama]

Originally Posted by Dbroz

Attachment 232675
Hi everyone, thanks heaps for all your help and input. I thought I would attach a photo of my UTA, I wouldn’t have been able to do it without your help

Thanks again. I will upload more photos of the finished scope in a few weeks, hopefully I will be done by then. I did have an accident and dropped my secondary mirror on my concrete driveway and it’s now all badly scratched up. But I will get anew one soon.

Once again thank you all

Daniel

Looking good Daniel, keep the pictures coming.

[Dbroz (Daniel)]

Almost finished, just a few more adjustments and a bit more painting, I was able to focus with my 25mm eyepiece last night but I could not focus with any eyepieces below 25mm, my poles are still about 30-50mm too long. I will be cutting them down tonight and hopefully be able to get the rest of my eyepieces into focus. If anyone has any suggestions on how I can improve this telescope to look or function better I am happy to hear your thoughts. Collimation holds well as I turn left and right, but up and down I do notice that the secondary does move slightly when I go from horizon to zenith. It doesn’t move much but it moves enough to allow the laser to go off the centre mark by about 15mm. I’m guessing it’s the weight of the secondary. I did use a thick foam double sided mirror tape to hold the secondary mirror to the holder. I might change it to silicone once my new secondary comes in. But if you have any suggestions about anything that would be great.

Thanks a lot guys.

Daniel

[Kunama]

How much tension do you have on the spider? I have mine tight enough to play G sharp......or an octave above

[Dbroz (Daniel)]

My spider is pretty tight. I’m pretty sure it’s the thick tape I used, the mirror is a bit loose , I was playing around with it before and noticed that the tape is too thick and it makes the secondary wobble. I got the poles to the correct length, so all eyepieces can reach focus, but I’m still having issues with the 10-6mm eyepieces, the higher I go in magnification the more the objects come out like a blob or blur. With the 25mm I can’t seem to get a sharp image, the bright stars are looking like arrow heads or seagulls. Could this be due to my secondary mirror being so scratched up as it was dropped on my driveway and has deep scratches everywhere? Or is it collimation? Or did I not let the mirror cool enough, my primary is 50mm thick, and I only had it outside for about 30 minutes before I looked through the eyepiece. And as I move up in magnifation it gets worse with each increase.

Thanks
Daniel

[Kunama]

Daniel,
Sounds like you have a few things going on in there....
Start by securing the secondary so it doesn't wobble about, then if you have a laser, check that the reflected laser spot hits the centre of the primary and doesn't move about when you move the scope around.

Depending on what you use for collimation, get the primary collimation done. I use either a barlowed laser or Cheshire eyepiece at the moment. Then move the scope around to check that the collimation holds while you do this. (Check for any movement in azimuth and altitude)
I have my spider/secondary set so that my laser spot will only move off the centre of the primary if I shake the scope and even then the movement is less than 1mm.

At F5 you will still see coma at outer half of the field (seagulls) without a Paracorr, this will be more pronounced with some eyepieces...

Your 10mm and 6mm eyepieces are pushing the magnification up to 225x and 375x and that will require the mirror to be cooled to ambient and steady seeing.

A 2" thick primary, if exposed to warm temps during the day, may never get to thermal equilibrium at night, forced cooling by fans will help but much depends on how fast the outside temperatures are dropping.

If you have a large fan (typically used inside house) and 240v available you can use that to cool the mirror much faster.

I doubt that the scratches would be the culprit to the extent you're describing, my money is on the collimation shifting, coma inherent in the optic at F5 and lack of thermal equilib.... The 'woolly' stars become smaller and smaller points as the mirror gets closer to ambient

[Dbroz (Daniel)]

Thanks Matt

I worked out for sure that the thick tape is definitely not good as the weight of the secondary moves as I bring the scope up and down. I’m using both a laser and Cheshire. I will try to cool the primary as best I can. As for coma should I get a coma corrector? I was told a F/5 shouldn’t need one. When I look a say Jupiter for example, with the 10-6mm eyepieces it seems as there is a double image and they are slightly off resulting as a big blur. Is this coma too? It’s hard to explain exactly what I’m seeing. I also don’t know how well figured my mirror is. It is 20 years old, the original owner still had it in its original box as he never got around to building his scope. It was made by Nova optics. Do you think it would be wise to test the mirror? And if so can I do this myself at home? I would even know how to test the mirror. I might join the asto association club if they are allowing new members. Anyway I appreciate all the help you have given me so far, mich appreciated

Thank you
Daniel

[Rod]

Hi Daniel

Your scope is looking great!

I have a 12 inch F5 and don't use a coma corrector. I have one but just don't feel the need for it so I don't think that's your issue.

I agree with Matt it's probably a collimation issue brought on in part by the tape. Check also for how stable your truss system is. It looks well made but I found with mine that if some of the knobs weren't tight enough the truss could slip a little.

I'm sure you know this but alight with the Cheshire first to check all the optical elements look concentric. Just use the laser for the final touch up.

I would recommend you join the ASV. You can do that anytime and members of the Instrument section can help you. They can also test your mirror. If you are in the south eastern suburbs, MPAS is a good group too with many helpful members.

Rod

[Kunama]

I doubt that what describing is from the primary mirror. I am no expert on Newtonians or on any scopes of that matter but until you rule out the secondary movement you won't be able to get the collimation right.

Is the issue the same with other eyepieces?

The lack of detail at higher powers could simply be too much magnification for the seeing conditions...

Joining the ASV is the way to go there is a wealth of knowledge and experience at the ready to help diagnose issues like this.....

[Dbroz (Daniel)]

Yeah the issue is the same with all eyepieces from 15mm-6mm, the 25mm is ok but still not pin point, the seagulls are not just at the edge but in centre as well. But like you said it’s most likely due to collimation. The scope is so high that I collimate while horizontal, once I begin to lift it will come out of collimation. I have plenty of tome to work it out and fix. Thanks again for all your help

[Kunama]

Try to collimate with it at around 35º-45º rather than horizontal then you know the mirror is resting agains the cell's glides.

[Dbroz (Daniel)]

Thanks Natt for all your help. I got the poles at perfect height and all my eyepieces come into focus now. Wow what a view, I can see so much more than my 12”. It’s amazing the difference between the 2. Now all I need is my new secondary and to properly make my ground board and finish painting the inside and outside of the box. Then I need to make a shroud. But apart from that I am so happy with the results. I will send another pic once it’s fully finished. Thanks again for everyone’s help and input. I couldn’t have done it without you all.