Making a 6 inch f2.8 dob

[Lognic04]

Hi all,:hi:
After completing the 10 inch, after looking at Mel Bartels website I was inspired to make a 6 inch f2.8 ultra fast scope!!! :eyepop::thumbsup:

I am currently hogging it out, at about 2.8mm sag now but I have to reach 3.4mm. I think the plan is to get to about 3mm sag, pour the tool with dental plaster directly onto solid unglazed tiles and remove the last 0.4mm while making it spherical. Orr get to 3.4 then move onto tiles?
Hope you enjoy this thread!!!

Also, is making a coma corrector feasible? It seems pretty simple optically from what I have seen (only 2 elements). Hopefully Bratislav or Stefan can answer this question :D

[Lognic04]

Catch up on progress here: https://www.cloudynights.com/topic/630268-making-a-6-inch-f3-mirrordob/

[Lognic04]

Because i am using a subdia tool to hog, there is this severe "lip"where it transitions from being untouched to where it is, do i need to grind this away before going to the tile tool or can i cast it with the lip?

According to the light reflection test, ROC is 108cm, FL is 54cm and sag is ~2.7mm according to Mel Bartels website.

[Merlin66]

IMHO you need to grind out the lip and get a uniform curve across the blank.
The fastest “comet seeker” I built was. 6” f4.

[Lognic04]

Ok. Do you know when i should switch to tile tool? 3mm? more?

[Merlin66]

Logan,
I actually used the tile tool from the beginning, rough grinding through to fine grinding - get down smoothly to the required sag then over to the lap tool.

[bratislav]

It would be actually possible to make quite decent (in fact more than decent) two simple BK-7 lens coma corrector for a 6" f/2.8 .
Catch is, you have to make it as a system , that is mirror will have to work together with lenses. You wouldn't be able to use it alone, lenses are essential part of the system.
Consequence of catch #1 is, mirror will have to be hypebola. Twice as deep as paraboloid (which is hard enough as it is at f/2.8 ).
So here's the prescription - mirror is ground at f=450mm (that is, f/3). This actually makes it easier to figure, as well as asking for much smaller secondary. The eccentricity would be -2.1 .
I also made lenses easier to manufacture. First lens is plano-concave. Lenses touch at the edges (also to make it easier to manufacture, no need for precision spacers, but make sure you bevel edges well). Second lens is biconvex, BFL is a generous 87 mm .
Performance is excellent (better than diffraction limited) across 6 mm circle which is larger than the field stop of a high power eyepiece (say 2.5mm Nagler Type 6).

BTW, don't even think on skimping on eyepieces. Minimum would be Nagler line (or similar), even better, use Ethos. Most eyepieces will work rather poorly at f/2.8 even on axis.

Having said all that, this is not a project for a beginner. Deep hyperbola, tight control of lens radii and wedge, near zero mechanical tolerances for alignment and collimation ... not a job for someone without an access to a well equipped machine shop.

So, I'm sorry to say, but you will have much better chance of a success if you just buy a Paracorr2. It will be much cheaper in the long run actually, and you will be able to use it in your larger fast mirrored Dobs later.

[Lognic04]

Dang :(
How do commercial coma correctors work with parabolas then?
That design might be useful though because someday I hope to make a 6 inch f2.x astrograph :D

[bratislav]

They use a lot more expensive glass, and a lot more of it. Paracorr2 has 5 lenses, most likely some ED as well as Lanthanum.

[Lognic04]

Wow, 10 surfaces!!! Eek!
Does different glass do anything?

[bratislav]

Of course not, Uncle Al, Roland, Yuri and people at Takahashi just use these expensive glass types to rack-up the prices of their equipment ;)

BTW, when you arrive at making a fast astrograph give me a call again. CCD/CMOS are a lot more sensitive at blue/UV and deep red/IR parts of the spectrum than our eyes. Two lenses of the same glass type just won't cut it.

[Lognic04]

This is the reflection test at Roc.
As you can see, a wierd figure!!! :eyepop:
Also after even a 30 minute grinding session, the sag hasn't changed in fact the roc went from 109 to 112cm?!
:help: some funky stuff going on!

[troypiggo]

Didn’t the guys on Cloudy Nights say that is a sign that the metal isn’t soft enough or something?

[Billyboy78]

What do they say.... worry less, grind more!

[Lognic04]

Well, I have to worry a bit - the roc went up!

[Lognic04]

240 grit now!

[Lognic04]

So there are 3 or 4 pits (big ones)
I'm guessing these are random ones and should go away after starting 25u?

[xelasnave]

Write to Sony tell them that they could help by sending you one of their curved sensors.
Alex

[Lognic04]

good idea :lol:

[Rod]

Unlikely. The finer the grit the longer it takes to remove a pit. Persevere with 240 or 320 till the pits are gone. The danger is that these indicate there will be other large pits you have missed.

Rod

[Merlin66]

I agree with Rod...
Keep grinding until there are no visible pits when using a loupe.
Mark the rear of the mirror where the pits are seen with texta to help you check that area later....

[Billyboy78]

x 2

[Lognic04]

Thanks guys, I will continue.
Question, the pits seen larger towards the edge but definitely from 240 grit, like there are more anomalously big 240 grit pits towards the edge? The light reflection does *not* discolour either going towards the edge?

[Rod]

Hi Logan

Not sure why some 240 grit pits are bigger than others. I think it more likely the grind is incomplete. Do what Ken suggested. Then you have a much more objective test of when to move to the next abrasive.

Rod

[Lognic04]

I looked again and not as bad as I remembered.
I think I have been grinding too much towards the center and need to find the fixed post sweet spot.

[Lognic04]

Ok, 240 grit pits are gone :)
Ugh, cleanup!! :(

[Lognic04]

Started 25 micron!
Is my slurry too thick? The mirror needs water added every 30 seconds and dries out in under 2 minutes!!!
I'd say I'm at 1:10 ratio? Maybe 1:8?

[LewisM]

This thread-umentary need pictures at every step, otherwise it seems like the ramblings of a teenage astro-nerd... :) :P :lol:

Didn't happen if there's no piccies :D

[Lognic04]

Lol, will take pics when I get home :)

[Lognic04]

https://youtu.be/0AfBcW-CIrk

Short video of grinding :)

[Billyboy78]

Looks good to me. Maybe just give it a spray with water so it doesn't dry out. If it does dry out it will leave scuff marks on the glass. Don't ask how I know.

[Lognic04]

It seizes when completely wet too?

Grinding is going well, got 45 mins in today so 1hr total. Maybe a half hour to go? Does that sound reasonable for 240 > 25u/~500 grit? See pics.

[Billyboy78]

Hmm...try a sharpie test. Sounds like the discs may not be matching up and you're getting an air lock. Now is the easiest time to fix it before you get to the fine grits.

[Lognic04]

Well it goes ok then I think if the mix gets too thin or some other variable it slowly slow down, locks then unlocks after 3 seconds stalled and makes a really harsh grinding sound until slurry added. Any thoughts?

[Billyboy78]

If it were mine I would ditch the spin for a bit and do some MOT just to make sure it is uniform and heading toward the sag target. Keep using the turntable, just ditch the fixed post.

If you have adjusted the overhang recently this will upset the spherical figure and may take a while to normalise, and lockup while it does its thing.

[Lognic04]

Ahhh that explains it, I went from close to chordal to neutral pretty quick (only 40 mins to adjust at 240 grit)?
Also I moved the tool in a tad because there was too much removed at the edge and it definitely does lock more than before.
Damn, my tool is 75% so mot is out of the picture. I can try some tot? How do I use the subdiameter by hand?
Thanks

[Billyboy78]

Maybe just continue as before with the #240 and stay on it until the discs mate up, but do a sharpie test to see where you're at. What is the sag at the moment, you should be headed for a tad over 3mm. What are the channels like on the tool? They need to be around at least 1mm deep so it can 'breath', this too can cause lockup.

I recall 'in the old days' I was doing a 16", desperately trying to deepen the centre and ended up wearing the entire mirror too thin.

[Lognic04]

Ahh, that maybe why - I have no channels!!
I will also do a sharpie test (will it ever grind off at 25u though?)

[Billyboy78]

Number 1 problem is the discs not mating. Because they are not mating and there are no channels, vacuum pockets are forming and locking the discs together. STAY ON #240 UNTIL (1) locking ceases and (2)sharpie test shows uniform mating AND (3)sag is close to 3mm and (4) there are no pits anywhere. (yes, I'm raising my voice:))

Sharpie will grind off at 5um

[Lognic04]

I think i should do a sharpie test now to see how they are mating, by the way at 240 grit there was never any locking? points 3) and 4) are already true :) I made sure this time there are no 80 pits (i did an extra 30mins to grind the last ones away)
Should i channel my tool?
Its not so much going back that i fear its the cleanup :)

[Billyboy78]

Yes, just use a ordinary saw or hacksaw, and a single cut will do it. It will only take a minute.

[Lognic04]

Ok cool. the gaps between my tiles are too thin for a hacksaw though? (i didnt plan on channeling)

[Lognic04]

Ok so I did the sharpie test at 25u and it is perfect contact (even dimming). I think it must have just been the channels, I dug new ones out with my fingernail.

[Lognic04]

Did 35mins, total 1hr50m. 25u is done!

[Lognic04]

Hmm, FL wandered to 48cm over the course of 25u - should I do anything?

[Rod]

F2.8 with a 20 mm eyepiece gives 21x. F3.2 with a 20 mm eyepiece gives 24x.

The field with a 100 degree eyepiece at 21x will be roughly 4.76 degrees. The field with same eyepiece at 24x will be about 4.16. So you lose 0.6 degrees by staying at F3.2. Personally I think a 4 degree field is huge so I would leave it at F3.2. It will be a little easier to figure too.

Rod

[Lognic04]

Started 9 micron grinding. Have done 15 mins so far, no rush because i'm picking up the polishing stuff on the weekend :)

[xelasnave]

You better start building one of these☺https://youtu.be/75AoWl3jhcQ
Alex

[Lognic04]

Wow, thats pretty spectacular!!

[Lognic04]

Alright - finished 9u and therefore fine grinding!!!
Pouring the lap tomorrow and going to make a video :)

[Lognic04]

Ok, lap is poured! Gotta sort out some contact issues after school.

[Lognic04]

Polishing is going well, 2 hours done now but center has maybe 15% more to go and edge 40% as a rough guess? I am working mot. Also my channels are closing fast (soft acculap), which means channeling,which means pitch getting thinner, and now the pitch is id estimate 6 or 7mm thick? I think this lap will get me polished out but not into figuring. Unless i somehow pour new pitch onto the lap?

[Lognic04]

Mirror is polished out after 4 hours!

[Merlin66]

In my experience....
Just double, triple check the outer edges for minor small remaining pits Bright light and loupe eyepiece...:thumbsup:

[Lognic04]

:thumbsup:

Figuring has started, edge is just flat and im having a hard time with mike lockwood's reccomended strokes (not doing much to correct the edge!)

[Lognic04]

How do you guys go about fixing kinks?
I need to fix this one:
https://cdn.discordapp.com/attachments/254187783499939841/511068514656714762/IMG_1217.jpg
https://cdn.discordapp.com/attachments/254187783499939841/511068510504615946/IMG_1211.jpg

[sfarndell]

Hi Logan

My experience is more with larger mirrors (18" f4), where you work with sub-diameter laps, so I'll share what I know:

More information is needed:
1. Have you polished enough? 4 hours is relatively short if polishing by hand.

2. Are you inside / outside focus? If you're inside focus, then you have some issues (use short strokes with small overhang slightly off-centre to lower the centre)

3. If you're outside focus, then it's not so much as a kink, but a spherical centre and some parabolisation towards from the 70% zone and you don't seem to have a TDE, which is good news! F2.8 is a pretty aggressive / short focal ratio. Check mel-bartels' website for the matching ronchigram to determine if you've over-parabolised the edge. If not, all you need to do is use a subdiameter lap (4") and parabolise the centre (short chordal strokes with minimal overhang close to COC), then smooth with full size lap.

[sfarndell]

entered your details here https://www.bbastrodesigns.com/ronchi.html

You'd also need to tell us what your Rochi LPI are. I assumed 4 per mm, ROC of 840mm. Using this you seem to have an over-parabolised edge. I'd return to a sphere using the full-size lap with a 1/3 D stroke, offset from centre. Likely you still need to polish out the mirror anyway so this shouldn't take too long.

[Lognic04]

Hi Scott,
Sorry for not replying earlier!
I now need to add parabolization to the 0.7 zone according to Bratislav, he says either full size scrubbing 0.7 down, or a subdiameter.
I'm waiting for my mentor to reply on cloudynights though.

[Lognic04]

So I had to return to sphere, and have been fixing a tde for the month. I only need about 1 more hour to fix it though! Woo!
Rod helped me make the beautiful wooden birch octagonal tube, it looks great! Now it needs a coat of polyurethane :)

[Rod]

Hi Logan

The tube’s come up well. Before you put finish on it, wipe down with turpentine over it. This will help you see if there is still any glue in the wood that needs removing. The tube might then need a quick rub with fine sandpaper as the turps may raise the grain.

Rod

[Lognic04]

Thanks Rod!

[Lognic04]

Wow, haven't posted here in ages!
So I spherise the mirror, fixed the tde and started parabolizing. I'm very close now, like 1/4 wave!
Tube had to be redone because the polyurethane didn't dry. Almost ready to star test!

[Lognic04]

Lot of figuring, and finally the mirror is done!!! Can't wait to use it!:eyepop::D
Left : outside roc Ronchi, right: Ross null image
I'll get interferometry data soon too!

[Atmos]

That’s looking pretty darn good Logan!

[Rod]

Congratulations Logan.

A lot of work and good on you for persevering through some frustrating experiences. How is the star test appearing to you?

Looks like you might need to mask off a little at the edge but that’s no big deal.

It will be great to see it under a dark sky one day.

Rod

[Stefan Buda]

Good effort Logan, but don't forget that these tests are not very sensitive at this sort of f ratio. You may be surprised if the interferometric test shows a half wave raised zone near the edge.

[Lognic04]

Hi Stefan, we will have to wait and see.

[Lognic04]

This is by far the closest and smoothest it has been so far, with an average conic of -1.02 so far from igrams which is pretty good

[Lognic04]

Still having trouble with astigmatism being induced by air currents, but the main point here is the smooth, uninterrupted -0.99 conic!

[Stefan Buda]

Most of that astigmatism is probably caused by your interferometer. I'm using a tiny lens and prism to minimise the displacement between the two beams. My spacing is something like 6mm. How much is on yours?
Anyway you have to make 5 sets of measurements, rotating the mirror 72 degrees between the sets, then average each set, derotate each set and combine for a valid final result. A big undertaking.

[Lognic04]

Thanks Stefan. For now I disable the astigmatism Zernike. The mirror might need some smoothing unless the ke says otherwise

[Lognic04]

Looking at stars, passes the snap focus test nicely! clear point where it's in focus!!
With 5% uncoated reflectivity, it reminds me of views through a 60mm frac!