Jump to content


Photo

Dob Alt Axis Lock for EP Swaps

  • Please log in to reply
10 replies to this topic

#1 careysub

careysub

    Surveyor 1

  • *****
  • Posts: 1902
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 11 October 2013 - 08:38 AM

I have a relatively small "big" dob - an F/4.42 13.1" wood strut design - and I have a 31mm Nagler as my finder EP, which weighs almost exactly 1 kg.

Unfortunately using the Nagler to find objects has a drawback, I cannot swap it out for another EP without having the scope swing upward.

I have a variable counterweighting system that exactly balances the scope for different EP combinations, but it is impossible to simultaneously swap EPs and adjust the counterweights at the same instant to prevent scope movement (even just swapping EPs in an instant is impossible).

So a method of locking the altitude axis when changing EPs seems necessary.

I have thought of schemes to apply side pressure to the alt bearing, even using a bicycle brake but that seems like overkill and a bit clunky. One sided pressure raises issues of shifting the OTA to the side which might be a problem.

Schemes that press against or lift the Dob bearing from below seem promising. Perhaps a lift block inset in the bearing support arc.

Is there any prior art on this - simple systems that have been proven to work in practice?

#2 StarStuff1

StarStuff1

    Soyuz

  • *****
  • Posts: 3870
  • Joined: 01 Apr 2007
  • Loc: South of the Mason-Dixon Line

Posted 11 October 2013 - 11:22 AM

Springs to increase tension in alt? This works well on my 8-in f/6 dob.

#3 kfrederick

kfrederick

    Gemini

  • *****
  • Posts: 3012
  • Joined: 01 Feb 2008

Posted 11 October 2013 - 11:36 AM

31 Nangler as a finder eyepiece ? COOL

#4 careysub

careysub

    Surveyor 1

  • *****
  • Posts: 1902
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 11 October 2013 - 12:01 PM

I know of people using tension devices (elastic cords, etc.) to deal with counterweighting issues. Are you talking about something different?

It would seem that however the weight is balanced, the abrupt weight shift would still cause OTA movement.

#5 careysub

careysub

    Surveyor 1

  • *****
  • Posts: 1902
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 11 October 2013 - 12:07 PM

Since posting this, while commuting to work I came up with a scheme for a simple lift block system:

A lift block sits in a large notch at the bottom of the rocker box bearing support arc, with a hole-and-peg to keep it in place.

Below it is an eccentric axis pair of wheels (one inside the box, a thicker one outside) probably made of wood with a dowel axle. Rotating the wheel lifts the block up until it contacts the alt bearing, and lifts it slightly off contact with the teflon. A non-slip rubber surface on the lift block may help.

There must be similar systems out there in use I would think.

Any issues with this that anyone can think of?

#6 Jeff Morgan

Jeff Morgan

    Fly Me to the Moon

  • *****
  • Posts: 5686
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 11 October 2013 - 11:24 PM

:lol:

Welcome to the club. Solutions? There are several.

1) Live with it. A great many put up with the issues.

2) ServoCAT. The altitude motor laughs off these types of imbalances. Plus you get the huge benefits of tracking and GOTO. Quite possibly the best money a Dob owner will ever spend in astronomy. IMHO it far outweighs the money we spend on other conveniences, such as eyepieces. The tracking aspect alone feels like bumping your aperture to the next step.

3) Don Pensack posted an easily implemented alt clamp that appeared to have great potential. Try pm'ing him.

4) Consider a different eyepiece in the low power/finder role. It will cost you true field, but seamless operation is probably worth it. Trying to make a large scope fulfill the role of a smaller scope introduces problems. A smaller scope or binos will give you huge true fields with fewer headaches. The 35 or 27 Panoptics are great candidates, as is the 26 Nagler.

5) A rail with a sliding weight. A solution dating back to the days of the large long focus refractor on finely balanced equatorial mounts. A bit of a hassle, but it does work.

#7 careysub

careysub

    Surveyor 1

  • *****
  • Posts: 1902
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 12 October 2013 - 06:38 PM

Probably you are referring to the thread here.

The brake in question operated by a knob-operated pad of some type pressing against the side of the bearing.

Don did specifically address one of the concerns about this that I had - he emphasized that the rocker box had t be braced against flex from the lateral pressure.

Two friction pad designs are discussed. At first Don used what was basically a big washer for supplying friction, requiring the knob and axle be close to the edge. Then it was proposed that the knob compress a lever with the brake pad on the end, which is it appears what Meade does on its LightBridges.

Taking Don's advice on the rocker box, and the success that he (and Meade) has with this approach that is probably the way I will go. The lift block idea is not all that complicated, but it is more complicated than the lever side brake.

#8 derangedhermit

derangedhermit

    Apollo

  • -----
  • Posts: 1150
  • Joined: 07 Oct 2009
  • Loc: USA

Posted 12 October 2013 - 10:39 PM

If you can rebalance slightly so the scope just barely stays in place when the 31mm Nagler is in use, that may help.

If you can increase the UC weight (add that finder you wanted?) and rebalance, then the significance of the EP weight is smaller, and it may help.

If you can move the altitude bearing pads, it may help.

My favorite (due to my age): with an f/4.4 scope, the exit pupil with the 31mm ep is 7mm. Swapping the 31mm for a 26mm gives a 6mm exit pupil and saves you 10 oz.

#9 Jeff Morgan

Jeff Morgan

    Fly Me to the Moon

  • *****
  • Posts: 5686
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 12 October 2013 - 10:51 PM

Probably you are referring to the thread here.

The brake in question operated by a knob-operated pad of some type pressing against the side of the bearing.

Don did specifically address one of the concerns about this that I had - he emphasized that the rocker box had t be braced against flex from the lateral pressure.

Two friction pad designs are discussed. At first Don used what was basically a big washer for supplying friction, requiring the knob and axle be close to the edge. Then it was proposed that the knob compress a lever with the brake pad on the end, which is it appears what Meade does on its LightBridges.

Taking Don's advice on the rocker box, and the success that he (and Meade) has with this approach that is probably the way I will go. The lift block idea is not all that complicated, but it is more complicated than the lever side brake.


That was it. The bracing would be a non-issue if you used a t-nut on the inside of the rocker arm and pad on each side. For lack of a better way of describing it, think of a side-pull caliper brake on a bicycle.

#10 careysub

careysub

    Surveyor 1

  • *****
  • Posts: 1902
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 13 October 2013 - 01:36 AM

If you can rebalance slightly so the scope just barely stays in place when the 31mm Nagler is in use, that may help.

If you can increase the UC weight (add that finder you wanted?) and rebalance, then the significance of the EP weight is smaller, and it may help.

If you can move the altitude bearing pads, it may help.

My favorite (due to my age): with an f/4.4 scope, the exit pupil with the 31mm ep is 7mm. Swapping the 31mm for a 26mm gives a 6mm exit pupil and saves you 10 oz.


The scope is already finely balanced, I have an adjustable weight system - but removing 2.2 lb from the focuser in a relatively small and light telescope just creates a huge torque.

The effective focal ratio is really F/5.1 since I use a Paracorr (I) all of the time (I regard Newtonians F/4.5 and below to be intrinsically catadioptric scopes, with coma correction a normal part of the design). Thus 31 mm gives a 6.1 mm exit pupil, just about where my eye is at. This makes it the ultimate wide/rich field EP for this scope (for me).

#11 derangedhermit

derangedhermit

    Apollo

  • -----
  • Posts: 1150
  • Joined: 07 Oct 2009
  • Loc: USA

Posted 13 October 2013 - 03:21 AM

I wondered, after I posted, about a Paracorr being in use; that does change things.

I like simplicity, so I wanted to make sure no adjustments could be made to the existing setup without introducing new hardware.

Perhaps a start for experimentation, if not regular use, could be a large spring hand clamp, like a Quick Grip (maybe $7 at a big box store if you do not have one or many already). They come with rubber jaw tips.

Do you have a photo of your scope to share?






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics