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Go Back   IceInSpace > Equipment > ATM and DIY Projects

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  #1  
Old 18-09-2018, 07:13 PM
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Lognic04 (Logan)
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Making a 6 inch f2.8 dob

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

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
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  #2  
Old 18-09-2018, 07:14 PM
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Catch up on progress here: https://www.cloudynights.com/topic/6...-f3-mirrordob/
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  #3  
Old 19-09-2018, 08:01 AM
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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.
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  #4  
Old 19-09-2018, 08:42 AM
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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.
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  #5  
Old 19-09-2018, 08:54 AM
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Ok. Do you know when i should switch to tile tool? 3mm? more?
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  #6  
Old 19-09-2018, 09:10 AM
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Merlin66 (Ken)
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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.
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  #7  
Old 19-09-2018, 01:10 PM
bratislav (Bratislav)
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6 inch f/2.8 - homemade coma corrector

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.
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Last edited by bratislav; 19-09-2018 at 02:37 PM. Reason: spelling
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  #8  
Old 19-09-2018, 02:28 PM
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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
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Old 19-09-2018, 02:36 PM
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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.
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  #10  
Old 19-09-2018, 04:29 PM
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Wow, 10 surfaces!!! Eek!
Does different glass do anything?
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  #11  
Old 19-09-2018, 05:50 PM
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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.
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  #12  
Old 19-09-2018, 06:21 PM
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This is the reflection test at Roc.
As you can see, a wierd figure!!!
Also after even a 30 minute grinding session, the sag hasn't changed in fact the roc went from 109 to 112cm?!
some funky stuff going on!
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  #13  
Old 19-09-2018, 09:16 PM
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Didn’t the guys on Cloudy Nights say that is a sign that the metal isn’t soft enough or something?
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Old 20-09-2018, 05:50 AM
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What do they say.... worry less, grind more!
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  #15  
Old 20-09-2018, 06:40 AM
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Well, I have to worry a bit - the roc went up!
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Old 26-09-2018, 04:30 PM
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240 grit now!
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  #17  
Old 07-10-2018, 09:37 AM
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So there are 3 or 4 pits (big ones)
I'm guessing these are random ones and should go away after starting 25u?

Last edited by Lognic04; 07-10-2018 at 11:09 AM.
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  #18  
Old 07-10-2018, 10:58 AM
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Write to Sony tell them that they could help by sending you one of their curved sensors.
Alex
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  #19  
Old 07-10-2018, 11:09 AM
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good idea
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  #20  
Old 07-10-2018, 12:37 PM
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Quote:
Originally Posted by Lognic04 View Post
So there are 3 or 4 pits (big ones)
I'm guessing these are random ones and should go away after starting 25u?
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
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