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Various Questions from a Relatively New Big Dob Owner

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#1 robcac26

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Posted 25 January 2021 - 08:27 AM

I picked up a 16" New Moon a few months ago and am really enjoying it so far but there are a few things I am wondering about.

 

1.  I'm familiar with cooling the mirror prior to using it, but when it's warmer outside, do you still need to allow time for the mirror to acclimate or does it not matter if the mirror is colder than the ambient temperate?

 

2.  Do you guys put some sort of cover over the secondary mirror to prevent dust when not in use?  The person I bought mine from just had a zip-lock bag around it.  Any issues with continuing to do that?

 

3.  I have a SIPS coma corrector.  I know the regular Paracorr has a 1.15x barlow effect.  Does the SIPS do this as well? 

 

4.  Do you put the SIPS in before or after collimating?  I've noticed that the Glatter collimator can hit the SIPS if I don't move the focuser out a bit, which got me wondering if I am even doing this correctly.

 

4.  The other day when collimating the primary, I had to turn one of the screws all the way in.  It brought it right to where it needed to be, but if I had to go any further, I wouldn't have been able to.  This is the first time this has happened.  Does this mean the mirror has somehow shifted out of position?  How do I get this back to normal?

 

5.  Are the Catseye tools considered more thorough collimation than a Glatter and Tublug, or would it be superfluous to buy them since I already have the Glatter & Tublug?

 

6.  When using ServoCat, it usually gets me in the neighborhood but I still have to search around a little.  Is this normal or am I doing something wrong?  I made sure to set up my location on Argo Navis.  Does doing the alignment with two stars that are far away make goto more accurate?

 

7.  I'm sure 16" of aperture is overkill for solar observing, but is there any reason not to make an off-axis solar filter for this other than storage space?

 

8.  When I'm done and bring everything inside, the mirrors and eyepieces fog up, so I leave them out and wait for the glass to clear up before putting the cover back over the mirror and the eyepieces in their case.  I don't have a garage or any other room where I can make the warming up more gradual, so is there a better way to do this than what I am currently doing?

 

9.  When tracking with ServoCat, I sometimes find that at really high magnifications, the scope moves at a slightly different speed than the object I am looking at.  Is there a way to make this more accurate or is this just the nature of the beast when using high magnification?

 

10.  I am planning on taking the scope on a road trip from NJ to New Mexico in September during which I will camp in a pickup bed on the way there and back.  The scope doesn't fit through my truck's doors, so I bought a padded, weatherproof bag to transport it in the bed.  Is it a bad idea to keep the scope outside (but in the bag) for a week or two at a time?  I'm sure keeping the bag zipped up will prevent dew and slow down temperature fluctuations, but I'm sure there will still be SOME temperature change taking place inside the bag, especially in the desert.  Should I be concerned about this or am I overthinking it?

 

Thanks for any help, I'm still learning but have already learned a ton from reading this forum!



#2 Markovich

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Posted 25 January 2021 - 08:46 AM

Hello, I'll start:

 

Nice scope!!

 

Do NOT use this scope for solar viewing...just dont..get a smaller scope for that. Too big of a risk to both you and the scope should something go wrong.

 

For secondary covers, the "official solution" is a velvet Crown Royal bag..kind of a badge of honor:) Ive see some people use styrofoam cups over the mirror..I wouldn't use a plastic bag- as there is no breathing..

 

Basically any time the outdoor air is different that what the scope came from, I would run the fan for at least while . Temp delta affects the image no matter what .

 

When I bring the scope home after observing and if there is dew on it ( usually is) will leave everything out to dry off.. usually left in the van and parked in the garage. If Im observing at home and bring it it damp, I just quickly set up back up inside and let it dry overnight. I have no cats, so no need to worry about that..just in case you do.

 

I prefer the Glatter collimator tools..but having both is excellent!

 

Other than helping a friend install it, I have no first hand experience with Servo Cat.


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#3 eyeoftexas

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Posted 25 January 2021 - 09:20 AM

I picked up a 16" New Moon a few months ago and am really enjoying it so far but there are a few things I am wondering about.

 

 

2.  Do you guys put some sort of cover over the secondary mirror to prevent dust when not in use?  The person I bought mine from just had a zip-lock bag around it.  Any issues with continuing to do that?

 

 

5.  Are the Catseye tools considered more thorough collimation than a Glatter and Tublug, or would it be superfluous to buy them since I already have the Glatter & Tublug?

 

 

10.  I am planning on taking the scope on a road trip from NJ to New Mexico in September during which I will camp in a pickup bed on the way there and back.  The scope doesn't fit through my truck's doors, so I bought a padded, weatherproof bag to transport it in the bed.  Is it a bad idea to keep the scope outside (but in the bag) for a week or two at a time?  I'm sure keeping the bag zipped up will prevent dew and slow down temperature fluctuations, but I'm sure there will still be SOME temperature change taking place inside the bag, especially in the desert.  Should I be concerned about this or am I overthinking it?

 

 

Congratulations!  I bet it is nice.

 

2) yes, a cloth bag.

 

5) the barlowed laser technique is fast and easy.  The Catseye tools compliment it, centering the secondary mirror placement and final adjustments with the autocollimator.

 

10) rent a small U-Haul trailer for the scope and all of the extras doodads that go with it.  It will prevent it from getting hit by rocks kicked up by other cars, and allow you easy access to the truck bed at night during the trip.


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#4 Napp

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Posted 25 January 2021 - 09:34 AM

I’ll add a bit.

 

1.  Yes, the mirrors must be allowed to acclimate to air temperature.  
 

2. +1 for the velvet Crown Royal bag

 

second 4.  You can back all three screws out and collimate

 

7.  As posted above, DON’T use for solar. 
 

8.  Wherever you bring the scope into to store it, leave everything uncased and uncapped until thoroughly dry.

 

Additional:  You did not state a location.  If you observe in a humid area I recommend running the primary mirror fans continuously.  I observe in Florida.  Dew is heavy and often.  I have actually had condensation form on my 16 inch primary mirror.  Yes the scope has a shroud.  Running the primary mirror fans continuously prevents this.  Also, if the location is subject to rapid temperature changes at night run the fan continuously.


Edited by Napp, 25 January 2021 - 09:35 AM.


#5 Kyphoron

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Posted 25 January 2021 - 09:44 AM

Hi, 

 

  I own a 16" Nightsky Dob and I wish you luck with your new scope.

 

Question 1# The longer you let your scope adjust to the ambient temp the better your observing will be both ways as far as heat and cold. I normally uncover my mirrors several hours before observing to make sure they have reached ambient temp.

 

Question 2# Any type of cloth bag will do just fine. I use eyeglass cases with draw strings to keep it in place and not have to worry about it falling off while transporting it.

 

Question 3# Not sure on the SIPS but I have and do use a Paracorr. Only drawback is having to add weight to the back to keep it balanced.

 

Question 4# Mirrors usually don't shift, could be that your secondary isn't centered. Also you can try backing all the screws out and slowing adjusting each one a little bit at a time. If you are finding the same issue then I would think your secondary is not centered with the focuser.

 

Question 5# Any good laser collimator will do the job. I personally use either my Catseye kit, Astrosystems 2" laser or my Hotech collimator. All do the same job, just depends on how much you want to spend on collimation.

 

Question 6# Get yourself an illuminated reticle eyepiece. This way when you target a star to align with you are sure it is centered in the eyepiece. Also with a 2 star alignment the further away the two stars are apart the more accurate you will be.

 

Question 7# Solar observing. DONT!!!!!!! 

 

Question 8# You can do a couple of things actually. You can do what you are doing now and wait for the temp to equalize which at that point no more fog. You can also run your fans in the house to help it reach the temp faster or you can take a hair dryer and hold it away from the mirror and let the warm air, (not hot) equalize your mirror.

 

Question 9# Again the more accurate you are aligned the better it will track. Using a 12mm reticle eyepiece will definitely help with this. 

 

I move mine in the back of my Pathfinder. with seats reclined and it securely held in place if I have to stop fast, etc. I would NOT recommend putting it in the truck bed. It is just asking for trouble. Rent yourself a Uhaul, this is a much safer option for transporting your scope. 

 

Best of luck to you and enjoy the scope.



#6 TOMDEY

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Posted 25 January 2021 - 09:50 AM

Your #4B there --- What has happened is that you've been (on average) preferentially twisting the screws clockwise inward, and have run out of range. Just back off on all three a full turn or two, and that will recover range.

 

Explanation: The common whiffle mount has three adjustment screws with equal symmetric influence. WLOG, call them A, B, and C. That's three knobs, which allows you to adjust 2 + 1 things >>> Tilt X, Tilt Y and Z. Nearly all ~alignment instructions~ found here on CN and in the literature only implicitly allude to the first two, and ignore the 3rd influence, which is pistoning the PM fore-aft. And that contributor is obvious. The net piston is (delA + delB + DelC)/3. If you ignore that, your PM will eventually creep forward of back, depending on how you trim the tip-tilts. It's psychologically biased for us homo sapiens to twist screws clockwise when adjusting, because we subconsciously associate that with attaching things or closing a faucet. It comes from human's genetic 90% right-handed bias, and the consequent standardized parity of manually-adjusted helical screws. Our right wrist is stronger clockwise (and the left counterclockwise). Hence, "PM Creep" inward. [I was on the ergonomics committee for one of our aerospace contracts. Physiology and psychology are always percolating in the background, most always entirely subconsciously. When operatives in the field can think more about the mission and less about equipment vagaries... our side wins!].

 

Me, I'm ambidextrous, of course. Saw, hammer, screwdriver, pitching, catching, one-arm pushups, writing, shooting, petting, chain saw, axe, maul, painting, directing the orchestra, saluting. Consequently, my PMs don't drift in.    Tom

Attached Thumbnails

  • 11 screwdriver pliers hand.jpg
  • 10 Toms Resume.jpg

Edited by TOMDEY, 25 January 2021 - 10:07 AM.

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#7 MitchAlsup

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Posted 25 January 2021 - 01:40 PM

I think the right side of the photo above should be labeled:: "Ready for delicate work" !!


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#8 Don H

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Posted 25 January 2021 - 02:00 PM

I think getting a fiberglass or aluminum cap for your pickup bed would make a lot of sense. It will protect your scope from storms, sun and other exposure, as well as secure it when you eat, shop or are away from it on the road. It will also give you a nice place to sleep if you pack it right...


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#9 Jeff Morgan

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Posted 25 January 2021 - 02:37 PM

#1) Glass can hold a lot more heat than air. Meaning, if the air temps drop throughout the night, you mirror will never catch up. With that in mind, I tried pre-cooling my mirror with some success. A colder-than-ambient mirror also causes problems, but things get close quicker, and the temperature delta is less.

 

Note that one needs to be very cognizant of the local dewpoint. Cool the mirror below that and you will be a very unhappy astronomer!

 

#2) I used to use a tube sock as a secondary mirror cover. Bad move. The elastic made the sock conform and drag whatever dust was present over the mirror, resulting in many tiny sleeks. Hurts ego more than image, but still, you don't want them.

 

Then I had a realization. Dust settles down, not up. By storing my secondary uncovered and pointing down, almost zero dust accumulation - even in my arid climate!

 

Collimation tools - a true laser is handy, as is the TuBlug. Especially after dark. But I would not be without a good set of passive tools. In the daylight and early twilight, easier to use than lasers. And they never let you down or come out of adjustment. The well-equipped astronomer has both.

 

ServoCAT. Take time in DSC alignment, and use a good illuminated cross-hair. Worth the extra five minutes. You could be suffering mount errors or flexure. Your gear ratios may not be set. But those are guesses. In all ServoCAT issues, Gary Myers is the man to call. A very thorough troubleshooter, great service!


Edited by Jeff Morgan, 25 January 2021 - 06:21 PM.


#10 Vic Menard

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Posted 25 January 2021 - 04:12 PM

1.) Any large temperature differential is not good. It's common to chase the dropping ambient temperature using cooling fan(s). If a cold primary mirror is brought into contact with warmer air, and there's even a little humidity in the air, the moisture will condense on the mirror.

2.) I also cover my secondary mirror with a Crown Royal bag. I store my UTA with the secondary mirror pointing toward the ground (and it travels in my van the same way).

3.) SIPS, Paracorr 1 and Paracorr 2 are all 1.15X.

4.) I would recommend removing the SIPS coma corrector before normal, routine, or initial collimation. If you find that you can use your laser after dark with the coma corrector installed, go for it. I always remove my Paracorr 2 before collimation.

5.) I use a Glatter and Catseye tools. I particularly like my Glatter configured with the 1mm aperture stop for after dark axial alignment assessment and correction. And I prefer my TeleCat combo tool for secondary mirror placement. The Glatter self-Barlow attachment provides almost the same precision as the calibrated Catseye BlackCat Cheshire. The Catseye Infinity autocollimators will teach you how to use your other tools to get the best precision possible.

6.) I think this is more likely an issue with your Argo Navis and/or your secondary mirror placement. The ServoCAT uses your Argo Navis to find objects and then track them. I like to use two stars that are close to 180-degrees apart in azimuth and at least 30-degrees apart in altitude (ideally 15-degrees or more above the horizon and 15-degrees or more away from the zenith). Use the Quick Start Guide: http://www.stellarca...cs/Quick_V3.pdf  and if you're using the latest firmware:

http://www.stellarca...hics/Rel5_3.pdf

7.) Just don't do it.

8.) You can use a hair dryer to quickly remove condensation, but prevention is better (especially for eyepieces). A heated eyepiece case (or a heater for your eyepiece case, or warm pockets for the two or three eyepieces you'll be using) is a good fix. Heat strips for your focuser, secondary mirror and Telrad are also helpful.

9.) If a reticle doesn't work, you could try TPAS...

10.) I'd find some way to lock the covered scope in place when you're traveling--maybe a Topper? If you're in the wilderness by yourself, you should consider security (for yourself and the scope). If you're going to a campground or a dark sky site with other people, there's security in numbers. For daytime heat, a good heavy duty canopy provides real shade and a significant temperature differential--good for you and your scope (even in the insulated box). Be prepared for blowing desert sand, which will get into everything. And watch out for dust devils, micro bursts and sudden rain storms.


Edited by Vic Menard, 25 January 2021 - 04:12 PM.


#11 shark-bait

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Posted 25 January 2021 - 08:01 PM

Congratulations!

I know that you have all of the answers you need now but want to make sure you know to NOT use a paracorr (or similar) when collimating. The Glatter laser and TuBlug are perfect for your scope and I highly recommend them.

Another trick to increase contrast when using the TuBlug is to unscrew the aperture stop on the laser before you place the TuBlug in the back of the focuser. Just make sure you thread it back on the laser for secondary alignment.

#12 JamesDuffey

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Posted 25 January 2021 - 08:09 PM

"I am planning on taking the scope on a road trip from NJ to New Mexico in September"

 

You won't have to worry about dew in New Mexico...



#13 Lentini

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Posted 25 January 2021 - 08:20 PM

Congratulations!

I know that you have all of the answers you need now but want to make sure you know to NOT use a paracorr (or similar) when collimating. The Glatter laser and TuBlug are perfect for your scope and I highly recommend them.

Another trick to increase contrast when using the TuBlug is to unscrew the aperture stop on the laser before you place the TuBlug in the back of the focuser. Just make sure you thread it back on the laser for secondary alignment.

If you’re using a laser collimator (I use a tublug), why take the SIPS out during collimating? The SIPS makes the laser more diffuse, but you can still easily center it. You obviously have to take it out to use a Cheshire. 


Edited by Lentini, 25 January 2021 - 08:21 PM.


#14 shark-bait

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Posted 25 January 2021 - 08:34 PM

If you’re using a laser collimator (I use a tublug), why take the SIPS out during collimating? The SIPS makes the laser more diffuse, but you can still easily center it. You obviously have to take it out to use a Cheshire.


I actually mis-spoke and use a BLUG with my 635nm Glatter laser. You would not want a diffuse beam when adjusting the secondary. This is why a 1mm aperture stop is recommended. I can’t speak for the SIPS, but my Glatter laser makes contact with the optics in my paracorr and the BLUG diffuses the laser plenty. If the beam is diffused too much the contrast degrades when aligning the primary.

#15 Lentini

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Posted 25 January 2021 - 08:51 PM

The beam is diffuse, but the circle from the primary mirror sticker is still round and easily centered. Being diffuse or sharp doesn’t seem important for centering a circle on the tublug center spot. 



#16 shark-bait

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Posted 25 January 2021 - 09:03 PM

The beam is diffuse, but the circle from the primary mirror sticker is still round and easily centered. Being diffuse or sharp doesn’t seem important for centering a circle on the tublug center spot.


Correct. I am referencing using the laser to properly align the secondary before using the Barlow.

Always make sure the laser (without the BLUG) strikes the center spot on the secondary, then adjust the secondary to direct the beam to strike the center of the primary. After you have completed the secondary alignment portion of the collimation process, pop in the BLUG for the primary alignment. You are correct in that the diffuse beam reflects the donut shadow back to the BLUG but only to align the primary.
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#17 Lentini

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Posted 25 January 2021 - 09:59 PM

Ahh, I don’t have a secondary center spot. But I wish I did. But I did notice I had a dirty secondary when looking for it. So it’s cleaner!


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#18 shark-bait

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Posted 26 January 2021 - 07:48 AM

Ahh, I don’t have a secondary center spot. But I wish I did. But I did notice I had a dirty secondary when looking for it. So it’s cleaner!


A Cheshire/sightube combo tool or a simple homemade collimation cap works great for centering the secondary. I still would recommend “eyeballing” the laser on the center of the secondary and then making secondary adjustments before using the TuBlug. If you are only using the TuBlug, your collimation can get much better as you aren’t fully collimated.

Give it a try!


Edited by shark-bait, 26 January 2021 - 10:36 AM.


#19 eyeoftexas

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Posted 26 January 2021 - 10:23 AM

Why would you center spot the secondary mirror?  The spot would be in the middle of your views if you did.  You can center spot the primary mirror because that part of the primary is not part of the view (it's obstructed by the secondary).



#20 shark-bait

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Posted 26 January 2021 - 10:29 AM

Why would you center spot the secondary mirror?  The spot would be in the middle of your views if you did.  You can center spot the primary mirror because that part of the primary is not part of the view (it's obstructed by the secondary).

Thank you for clarifying that it is not a "center spot" like the primary  It is merely a tiny etched + that came factory with my OMI mirror from Obsession.  Most just use a cheshire and or cap for centering the secondary. There is no optical degradation.  


Edited by shark-bait, 26 January 2021 - 10:35 AM.

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#21 Vic Menard

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Posted 26 January 2021 - 12:10 PM

A "center" marker (spot/donut) on the secondary mirror (hopefully offset the correct amount if you're using an offset alignment procedure) will generally fall in the shadow of the propagated secondary mirror shadow--assuming a normally sized secondary mirror, a "small" spot/donut, and moderate to high magnification (which usually means a smaller field stop). Even if the spot becomes visible beyond the shadow of the secondary mirror, it's unlikely you'll notice it. See here:   https://www.cloudyni...obstruct-image/   and here:   http://web.telia.com...misc/secspt.htm

 

Just remember that a secondary mirror center marker alignment should not override the critical axial alignments that deliver actual image performance. A secondary mirror center marker is at best a reference for the optimal placement of the secondary mirror (which delivers centered/balanced field illumination).


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#22 Jeff Morgan

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Posted 26 January 2021 - 02:47 PM

Just remember that a secondary mirror center marker alignment should not override the critical axial alignments that deliver actual image performance. A secondary mirror center marker is at best a reference for the optimal placement of the secondary mirror (which delivers centered/balanced field illumination).

 

Can you also accomplish the same thing by aligning the edges of the secondary with the bottom edge of the sight tube?



#23 Vic Menard

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Posted 26 January 2021 - 04:37 PM

Can you also accomplish the same thing by aligning the edges of the secondary with the bottom edge of the sight tube?

You will likely accomplish a better alignment. waytogo.gif  

 

A "precise" secondary mirror center mark is dependent on a host of other geometries that may, or may not, meet that precision criteria. This is why I use three circles to achieve optimal secondary mirror placement: the bottom edge of the focuser drawtube (or the bottom edge of the sight tube), the actual edge of the secondary mirror, and the reflected edge of the primary mirror (or the primary mirror center mark is the reflected edge is not visible).

 

(Edit: You should use all three circles (including the primary mirror reflection) as this ensures that the focuser axial alignment (primary mirror center marker is centered under the focuser) is also correct, a necessary condition for assessing and correcting the secondary mirror placement.)


Edited by Vic Menard, 26 January 2021 - 04:39 PM.


#24 shark-bait

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Posted 26 January 2021 - 08:26 PM

You will likely accomplish a better alignment. waytogo.gif

A "precise" secondary mirror center mark is dependent on a host of other geometries that may, or may not, meet that precision criteria. This is why I use three circles to achieve optimal secondary mirror placement: the bottom edge of the focuser drawtube (or the bottom edge of the sight tube), the actual edge of the secondary mirror, and the reflected edge of the primary mirror (or the primary mirror center mark is the reflected edge is not visible).

(Edit: You should use all three circles (including the primary mirror reflection) as this ensures that the focuser axial alignment (primary mirror center marker is centered under the focuser) is also correct, a necessary condition for assessing and correcting the secondary mirror placement.)


I 100% agree with all of this as a sight tube will never let you down. The only scope I use a laser and BLUG on is my Obsession. While it does not need the secondary marked I sure do like it and collimation is super quick with it.

I didn’t mean to get this post off topic. Just noticed that the OP was using a laser and TuBlug and had a question as to using them in conjunction with the SIPS.

Thank you for sharing those links!

#25 Keith Rivich

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Posted 26 January 2021 - 09:36 PM



Your #4B there --- What has happened is that you've been (on average) preferentially twisting the screws clockwise inward, and have run out of range. Just back off on all three a full turn or two, and that will recover range.

 

Explanation: The common whiffle mount has three adjustment screws with equal symmetric influence. WLOG, call them A, B, and C. That's three knobs, which allows you to adjust 2 + 1 things >>> Tilt X, Tilt Y and Z. Nearly all ~alignment instructions~ found here on CN and in the literature only implicitly allude to the first two, and ignore the 3rd influence, which is pistoning the PM fore-aft. And that contributor is obvious. The net piston is (delA + delB + DelC)/3. If you ignore that, your PM will eventually creep forward of back, depending on how you trim the tip-tilts. It's psychologically biased for us homo sapiens to twist screws clockwise when adjusting, because we subconsciously associate that with attaching things or closing a faucet. It comes from human's genetic 90% right-handed bias, and the consequent standardized parity of manually-adjusted helical screws. Our right wrist is stronger clockwise (and the left counterclockwise). Hence, "PM Creep" inward. [I was on the ergonomics committee for one of our aerospace contracts. Physiology and psychology are always percolating in the background, most always entirely subconsciously. When operatives in the field can think more about the mission and less about equipment vagaries... our side wins!].

 

Me, I'm ambidextrous, of course. Saw, hammer, screwdriver, pitching, catching, one-arm pushups, writing, shooting, petting, chain saw, axe, maul, painting, directing the orchestra, saluting. Consequently, my PMs don't drift in.    Tom

Hey, I have a collimating hammer to! Darned big scopes...




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