10 inch Classical Caseegrain

[cristian abarca]

Hi guys I have started a new project. A 10 inch F17 Classical Cassegrain that will convert to a 10 inch F5 newtonian. I cut the hole out of the mirror yesterday and here is a picture. I would have included a photo of the 2 blanks for the secondary but I put them away for safe keeping and I can't find them. When i do i'll post a picture of them.

Regards Cristian

[Dennis]

Hi Cristian

Wow - it's great to see that there are still glass pushers out there. :thumbsup: :thumbsup:

I just don't have the patience and I admire it in ATM'rs. Your project looks technically very rich and challenging. I wish you every success and a good run through all the obstacles that will surely pop up. Keep us posted on progress.

Cheers

Dennis

[cristian abarca]

Hi guys just finished my primary mirror for this project. The figure looks good but I'll get a second opinion before I call it finished. It turned out to be F4.55 instead of F5. This will change my overall dimentions a little but not too much. Next week I start on the secondary. For those that are interested here are the measurements of the various zones. Zone 1 = 0 Zone 2 = 2.1 Zone 3 = 3.9 and Zone 4 = 5.6. It basically means it's 1/20 wave.

Regards Cristian

[Merlin66]

How will you test the secondary? I had the same problem with a 12" f5/f20 Cass. system and successfully used Gaviola's method. I've still got the test details somewhere if you need them. Was the simplest test method I could find.

[cristian abarca]

Thanks ken I'll take you up on that when I'm ready.

Regards Cristian

[joe_smith]

Cristian 1/20 wave is awesome mate well done.

I have some glass here I am going to grind its 6.5 inch x 1 inch. I started a 10 incher but the tile tool became a bit thin during fine grinding and become very brittle and unusable I put it away and haven’t look at it since. But your posts have gotten the bug in me again and I'm going to finish it off, after the 6 incher. Where do you get your grits and pitch from ???

[Satchmo]

Christian, There's another slick way to Null test a Cass secondary by moving the light source much closer than infinity and increasing the spacing between the secondary and primary during the test. This can be a good way of fine tuning th ewhole scope as the primary mirror is in the light path too. Not too hard to calculate with raytrace software.

Mark

[GrampianStars]

I've a 10" blank just itching for some grit
would make my 1st dob out of it

[cristian abarca]

Thanks guys and thanks Mark too. I just started reading up on the test you describe and I will certainly be trying that. In fact I will be trying everything that comes my way. Joe I get my grit from my local astronomy club ASV. I have also bought grit from telescopes and astronomy in South Australia. Google telescopes and astronomy, click on the result,scroll down and click on Dobsonian Making and Information & Home Telescope Building,click on 3 Home Construction and scroll down until you find the what you need(I find the web site a little difficult to navigate). The prices are reasonable as you only need one bag of each grit and they deliver pretty quickly too. I don't know what the pitch they sell is like but Garyh purchased some pitch from somewhere and he managed to finish a mirror successfully. You might try asking him about the pitch. I know burgundy pitch is the favoured one but I think you can only get it from overseas. Joe a bit of advise about the 10 incher. When you make a new tile tool make sure the plaster is the shape of the mirror, before you set new tiles on it. I made the mistake of not doing this and it took me ages to work the tiles to the shape of the mirror . I wasted a lot of time. If you want I can tell you how I added plaster to the existing tool and it didn't come off.

Regards Cristian

[Garyh]

Hi Guys,
Great to see how well your figure came out Cristian, should be a ripper of a scope..my mirror is still away being coated....
I got the grit and pitch from Telescopes and Astronomy in S.A you can get bulk grit from Shell Lap Supplies also in S.A but you have to buy by the kilo... good for clubs but..
Interested to see how your new scope progresses..I shall keep a eye on how it comes together...
CS..Gary

[cristian abarca]

Hi guys got back from my telescope makers meeting and my mirror was found to be very good (1/20 wave). I left it so it can be coated. I'll get it back next week and I will post some pictures of it. I'll also post some pictures of the secondary blank. I'm actually quite happy with the result so far.

Regards Cristian:rofl:

[joe_smith]

Thats good Cristian 1/20. when are you going to start the 2nd piece of glass and what is your plan of attack for it :)

[cristian abarca]

Thanks Joe. I want to start this weekend by cutting the two pieces of glass that I have. I have one pyrex blank that is 100mm diameter and a plate glass tool that is 150mm diameter. I need both to be 70mm. I'll cut them on saturday and if everything goes well I'll start grinding on sunday. I'll go through the whole process until polishing and then see how I tackle the next step. There are a few options available. I'm trying this out to see if I can make this whole project work. If it does then I'll do the same with the 50cm pyrex blank I have.

Regards Cristian

[joe_smith]

look forward to seeing you get there. grinding 70mm disks should be fun ;) are you doing it by hand or machine? A 50cm Cass on the way? :thumbsup:

[cristian abarca]

Hi Joe I'm doing the mirrors by hand and the 50cm well, soon. Any way just to let everyone know that I got my mirror back today and here is a photo. I know I have a chip of it but that will go under the primary cell hoder so it won't affect it. It was my carelessness during rough grinding that caused it.

Regards Cristian

[cristian abarca]

Hi guys, here are a few photos of the secondary mirror (for this telescope),tool and biscuit cutter. Also some pictures of the cutting process and finished product.

Regards Cristian

[joe_smith]

Happy grinding Cristian, keep us up to date on how you are going :thumbsup:

[cristian abarca]

Thanks Joe. I've spent most of the day grinding the back of the secondary flat and polishing it. I need to make it clear at the back because I need it see through for the testing. Here is a photo of the final figure of the primary. Sorry for the qualit but my photographic equipment is modest and my skills lacking a little. The lines are all uniform any blemishes are due to the photo quality. The photo was taken with the mirror on a stand so the lines are supposed to bend.

Regards cristian

[cristian abarca]

An update on my progress in making the secondary cassegrain mirror. I just finished the concave hyperbolic reference and tested it with a ronchi screen and it looks fine. I'm going to do an inteference test on the secondary using the concave mirror as a test base. Hopefully the secondary will come out reasonably spherical when I finish polishing it so that I don't have to do too much retouching when I start to figure it. This is the most challenging part of the project. I'm going to start building a new tester for the secondary in the next couple of days so that it's ready when the mirror is polished. Still a bit to go but hopefully I'll have it finished before the end of october.

Regards Cristian

[joe_smith]

good to hear its going well mate :)

[Merlin66]

The Gaviola secondary test doesn't require a master for interference comparison, only a std magnifying glass of similar diameter. Uses the Foucult/ Ronchi test set up. Well worth considering.

[Satchmo]

Cristian

As I imagine your Cass is F12 or slower speed , I'd recommend do the final figuring on the secondary using a real star with a Ronchi grating and the whole telescope assembled. At F12 or slower beam the Ronchi grating at focus would be a fairly sensitive test ( but not that useful at F5 or faster ) .

Mark

[cristian abarca]

Thanks Mark that is the final test that I will perform. (I haven't finished the telescope yet, still waiting for gears for my mount from the US) At the moment I just want something to tell me that I'm heading in the right direction with the polishing. It's like driving blind I can't tell if the figure of the mirror is heading towards spherical or not. At this stage I've never managed to make a mirror that is spherical after polishing, there is always a slightly TDE or a slight hole or hill in the centre. I like to test as I'm going along and modify my stroke accordingly so that I don't end up with a really bad figure. The first mirror I made took me about 12 hrs to fix and it was touch and go whether I went back to fine grinding or try to fix it. I persevered and finished it but since then I have become a little paranoid and like to check after every hour of polishing. I don't trust my strokes maybe I should I've done enough of them.

Regards Cristian