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Bresser 127 Mak, f/15; Two Baffling Qs

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#201 KerryR

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Posted 23 December 2018 - 10:05 PM

Joe, I'm curious... did you DPAC your Mak? How did it look?

I'm probably just missing the obvious, but... could someone define "DPAC"?



#202 precaud

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Posted 23 December 2018 - 10:30 PM

It's an acronym for Diurnal Perturbed Astronomer Cyndrome  Double Pass Auto-Collimation. 



#203 Asbytec

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Posted 24 December 2018 - 04:40 AM

Great discussion! I really enjoy reading this thread. I recently bought a Orion 180mm Mak (older style) and thinking about doing some modifications. One of the bottleneck I noticed is that the primary mirror retaining ring is fairly wide, limiting the possibility to reduce CO even if the secondary baffle is removed. Anyone done calculations on how much the secondary baffle should be shortened?

Thanks!
Gu

https://www.cloudyni...ary-baffle-mod/



#204 Joe1950

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Posted 24 December 2018 - 07:15 AM

I did not as of yet, John. That’s on the to do list. I have to use oil as a flat since the largest glass flat I have is 3”. But with patients, it works.

 

Gu, there is a link to a thread in post # 8 by Norme that shows a very comprehensive way of calculating the size of the baffle optimized for astro viewing.

 

What I did was take the secondary baffle off altogether and tried it that way. I didn’t feel there was any light scatter in the FOV, even on the moon. But with the 1.25” diagonal in place I could see a a ring of light around the secondary image with no eyepiece in place. 

 

So, just to be on the safe side, i didn’t do the calculations but cut it roughly in half as shown in post 68. Not scientific by any means but it reduced the CO by a good amount, yet there is no ring of light seen around the secondary with a 1.25” diagonal and no eyepiece.

 

When I took the baffle off the glue was easy to remove by using a microfiber cloth and alcohol. That was the case for me, but may be much different in other scopes, so proceed slowly and with caution.



#205 KerryR

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Posted 26 December 2018 - 12:38 PM

Regarding secondary baffle glue (I asked about this earlier): The baffle on my Orion gray-tubed 127 was held on with gray foam double sided sticky tape. Alcohol didn't seem to touch it. I ended up using an acetone soak.

I filled a shot glass all the way on to the top, the rested the lens/mirror on the lip of the glass, the lip of the glass was larger than the center spot by a wide margin, so the entire center spot and foam remnants were completely submerged. I let this sit about 30 minutes. The acetone dissolved the remaining foam and glue to the point that it could be wiped away fairly easily. It still took a fair amount of gentle work with acetone to get everything clean. It came out quite pristine- no visible sleeking or spotting.

On my scope, the silver spot was actually a bit wider than the base of the baffle, so I may pick up a teeny bit of additional FOV.

On axis, I can't see around the spot. Off axis, I can. My plan is to add an annular knife edge between the ota body and the visual back to cut this region out of the off-axis field, while trying to maintain the scope's current aperture of 115mm (as measured with the flashlight test).

 

EDIT: I also replaced the rubber material between the rear cell and tube, which had become inelastic. I used the ridged rubber tubing used for holding window screening in the screen frame. It's a little thicker, but it fit nicely, and gave a little additional leeway while colimating.

 

EDIT AGAIN: My annulus at the rear opening idea won't work.


Edited by KerryR, 26 December 2018 - 01:20 PM.

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#206 Joe1950

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Posted 26 December 2018 - 04:12 PM

Nice work, Kerry. That white/gray foam double sided tape is nasty to remove. You have to have nerves of steel to use an acetone soak! But we’ll done.

 

I’m glad you mentioned that ring between the tube and rear housing. My scope did not have that and was left loose. When I tightened it the focuser ends up on the left side.

 

I made a rubber ‘O’ ring, and it works, but it’s too thin and some of it binds in the threads. I’ll try tour screen rubber stuff. Good idea.

 

I guess the best place for the annuals ring would be at the front of the primary baffle tube. Make a good light baffle also. I did flock the inside of my primary baffle tube, with Edmund Optical, non adhesive flock paper. It’s the thinnest stuff you can find at about 0.25mm. It’s cut to press fit inside the tube and stay put. Works well.



#207 Joe1950

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Posted 26 December 2018 - 06:30 PM

For those who may not be familiar with the DPAC test, here is an animation of it with a refractor. Any type scope can be substituted.

 

 

The light passes through the essential optics (that is only the optics native to the scope, not with additional star diagonals or other optics added) of the telescope twice, so the observed or photographed Ronchi pattern is twice as sensitive as is a single pass situation.

 

Since the source light is placed at the primary focus of the optics, when it emerges the system on its way to the flat it is 'auto collimated.' That is the light beam is collimated (made parallel) by the nature of the test.

 

The collimated light is reflected by the flat and then passes through the optics a second time (double pass) and comes to a focus.

 

It's a very sensitive test. Straight jailbar Ronchi lines are optimum, but not always seen. That doesn't mean the scope or optics are bad, just not perfect. If you place the Ronchi grating right at the focus, you can duplicate a double pass autocollimation Foucault test! Again just like the Foucault test but twice as sensitive.

 

This is my proud DPAC of my C80ED, done in green light (~570nm) with a 150 LPI glass etched grating.

 

 

Tests are usually done with a green light source such as a green LED. If, for example, you test an ED or apochromat lens with red or blue light, the amount of correction may be different than at green! This would lead you to a false assessment.

 

For mirror type Newts, SCTs or MCTs, the color doesn't matter, but green is still a good choice.

 

To quantify a Ronchi pattern that is not optimum with straight lines takes a great deal of skill and experience. Smooth, slightly curved bands with a good edge would, however, indicate a very good optical quality with a small amount of under/over correction. The bands would curve in opposite directions, inside and outside of focus.

 

For those who do not have a large enough of an optical flat to do the test, a rig can be set up to place the scope on and point down at a pan of oil.

 

The set-up must be very sturdy, however. The oil should be just a few mm thick to dampen movement and wider than the tested scope to avoid the edge of the pan.

 

The stand where the scope sits is fashioned with three adjustment screws to adjust the scope and align the return beam. Unless you do all this on a concrete floor, anyone walking around in the home can cause the oil to move. Or air drafts can move it. Also the whole thing should be at a steady ambient temperature for the needed time so that everything is stable, or the reading may be off.

 

The oil in the pan is actually not perfectly flat. It has a radius of curvature that matches the radius of curvature of the Earth's surface! Needless to say, flat enough.

 

 

joe


Edited by Joe1950, 26 December 2018 - 06:33 PM.

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#208 Simon B

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Posted 27 December 2018 - 01:24 AM

EDIT: I also replaced the rubber material between the rear cell and tube, which had become inelastic. I used the ridged rubber tubing used for holding window screening in the screen frame. It's a little thicker, but it fit nicely, and gave a little additional leeway while colimating.

 

The same thing happened with my Orion 90 mak, I bought these, its a perfect fit:

 

https://www.amazon.c...r/dp/B00A784A74

 

 

Perhaps something like this might work for a 127 mak:

 

https://www.amazon.c...s/dp/B00AKW5V6Q

 

https://www.amazon.c...r/dp/B00A77KHCM

 

https://www.amazon.c...s/dp/B00LGIRINM

 

 

I found the 2mm to be perfect for the 90, I don't know if a thicker ring would be more ideal for the 127


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#209 Joe1950

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Posted 27 December 2018 - 01:32 AM

Good find! I’ll have to take some measurements and see what might be the best. 

 

Thanks much,Simon!


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#210 Simon B

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Posted 27 December 2018 - 01:47 AM

No problem Joe : )  the place where the O-ring sits is about 102-3mm in diameter, so I looked for an O-ring a couple mms smaller, perfect

 

It was a bit annoying to keep it in place as I screwed the rear cell back on, it would sometimes come unseated and slip inside the OTA, but with a bit of patience I managed it eventually


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#211 Joe1950

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Posted 27 December 2018 - 03:47 AM

It grabs. If you put a layer of grease on it, it doesn't bunch up. I had a piece that was just a length and not a circle. But a complete O ring at the right thickness would be best, as you have done!


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#212 Simon B

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Posted 19 March 2019 - 01:02 AM

So I bought one : p   I replaced my C5

 

The mak is proving to be more of a planetary specialist, startest is a bit better, stars are more pinpoint at higher powers, presenting a sharper image - this was my hope when choosing to switch from the C5 to the 127 mak, and I'm glad I did

 

But as you guys have pointed out, yes it has mirror shift. This is my only real gripe with the scope. I can live with it though, since my purpose for this scope is as a planetary instrument - so just putting it on my Vixen AP, aiming at a planet, achieving perfect focus with a single eyepiece, and letting the mount track the planet for an hour or so as I enjoy the view.

 

I think the mirror shift would become annoying if one were to jump around the sky looking at many different objects at different powers, which would require refocusing with different eyepieces every time. I may in the future add a mini 1.25" helical focuser to the visual back

 

 

BUT, most importantly it has very good optics, and a full 127 aperture! Unlike the Synta 127 120


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#213 Simon B

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Posted 19 March 2019 - 01:23 AM

I can definitely see that the secondary baffle is quite a large cone, causing the CO to be unnecessarily big, at ~40%. I may trim it down as you guys have, or remove it completely.... or just leave it alone? If it ain't broke, don't fix it? Hmm

 

Anyway, looking forward to some Jupiter time with this guy : )


Edited by Simon B, 19 March 2019 - 01:32 AM.

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#214 Asbytec

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Posted 23 March 2019 - 07:38 AM

I like MCTs for some reason. I've had a blast with mine. If you do anything to the baffle, I think trimming it is the better mod than removing it (like I did.) It's not for the faint of heart, hate to potentially mess up a perfectly good scope. Proceed with caution. Well, we jump out of perfectly good airplanes for the thrill of it, though, don't we. I mean, not me... :) 


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#215 Joe1950

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Posted 25 March 2019 - 02:37 AM

I’m looking forward to the planets making the swing also. I have a Paul Rini modified binoviewer and from what I’ve seen of the moon, the planets should be outstanding. 

 

I still havent made the jacket for it though. Have the materials, but been too lazy. crazy.gif  I have a lot of small projects in the works, but the energy department is lacking.


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#216 Redbetter

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Posted 25 March 2019 - 02:49 AM

I am keeping this thread in mind for when I have the cojones to do the sort of mod Joe and I discussed early in the thread.  I like the idea of trimming the baffle on both ends to widen the effective secondary mirror spot while also reducing the secondary obstruction...better illumination and better contrast = Goldilocks for an otherwise over-baffled scope?  If combined with some sort of flocking/inner baffle tube blackening this might be a real winner IF the optics are otherwise good across the resultant effective aperture.


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#217 Joe1950

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Posted 25 March 2019 - 08:09 AM

Could hit the sweet spot with that idea, Red!

 

I did darken the primary baffle tube and I believed it made a difference. Jupiter views later last season, even at the low altitude were excellent.

 

I used a trick from baffling the primary tube on the C-90 Mak. Edmund Optics, in Barrington NJ, makes a flock paper that is very good and quite reasonable.

 

It comes in both adhesive and non adhesive backing. The trick is to get it the right length and a little larger than the inside circumfrance. Then making small trims get the flock paper to fit snug as one side butts the other and works like a charm. All can be done from the back port of the scope.

 

When we were experimenting with the C-90, the Edmund flock paper was the thinnest we measured; thinner than other flock paper brands and painted sandpaper.

 

Very easy to do. If you want to try it and need any, send a pm and I’ll send you some. I have a good bit left!


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#218 JimFR

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Posted 25 March 2019 - 08:10 PM

The only obvious issue I have with mine is reflection at the edge of the miniscus and it’s housing.  The plastic edging of the front cell is extremely shiny and I can see heavy reflections from off-axis light when I have the back off.  This shows up strongly in the eyepiece. Flatting that down should get rid of big chunk of contrast loss.  Blackening the edge of the miniscus should help as well.  I’d consider shortening the secondary baffle but have a feeling that it would be a marginal gain.


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#219 Joe1950

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Posted 25 March 2019 - 11:52 PM

That would be important. I’ll have to check for that as well! Thanks Jim.




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