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Old 01-03-2020, 02:43 PM
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mental4astro (Alexander)

mental4astro is offline
Join Date: Jun 2008
Location: sydney, australia
Posts: 4,933
Hi Damien,

I'm going to suggest something different.

Have you cleaned the alt and az bearings of your dob? This is one of the most basic things that most dob owners forget about, or doesn't occur to them that needs doing.

If you don't, you will find that you need to use more force to move the scope, the action becomes jerky, gittery and vibrates, and it will display a whole lot of backlash because the mount is also flexing because of the additional force.

I have 5 dobs, from 114mm to 17.5", and I clean their bearings once a year and apply wax to the bearing surfaces (not the Teflon!). I have no problem following the planets at 400X, and the eyepieces I use don't have an exceptionally wide apparent field of view. The action of my dobs is silky smooth and I have no backlash with them.

None of my dobs are motorized either, and for a very specific reason - you actually SEE MORE in DSO's with a push-pull dob. It all has to do with the way our eyes work at low levels of light. In low light, the rods and cones in our eyes quickly saturate, and their sensitivity quickly drops. It is by moving the scope that the moving image triggers the refresh mechanism of our eyes, and their maximum sensitivity is restored, and all of a sudden you are once again catching a glimpse of that faint nebulosity that at first you "thought" you saw, but lost with a static image. Moving your head or eyes is not enough - the image itself needs to move. With a go-to scope, you can achieve the same by giving the scope a good tap to induce a vibration, but this is not something people with a go-to scope do with any regularity. You may think the constant nudge-nudge-nudging is a pain, but it works in your favour.

If you post a picture or two of your dob, we will be best able to advise you on how to go about cleaning the bearings of your dob to get it working to its very best again

Another factor that can be working against you is where you set up your scope. Grass, and especially lush turf is the very worst place to set up on. Mass production dobs from SkyWatcher and GSO have stupidly little "feet" made out rubber stops. These are fine if you set up on concrete, compacted earth or some other hard surface. But set up on grass and those ridiculous "feet" cannot reach down through the stuff, and instead the ground board is resting on the grass itself. This then makes the turf a mechanical part of the mount. So when you want to move the scope you need to use greater force because the sponginess of the grass needs to be overcome before the scope will begin to move. And when you release the scope, that sponginess is released too and is seen as backlash. This alone will make trying to follow the planets at high magnification an impossible task. If you can only set up on turf (either because of where you live or because of where you find yourself setting up), there are ways to easily modify the ground board so that turf is no longer a factor

Setting up on turf will also affect the pointing accuracy and mechanical functioning of Go-To dobs (commercially made and custom) if the same necessary steps are not taken.

Grass is also the very worst place to set up a mass production dob on as the evening dew will seep in through the seams of the laminated particleboard mount, causing this cheap stuff to puff up and ruin the mount.

If your dob is one that makes use of a friction clamping device on the altitude bearing, having this wound too tight will also significantly impact on how silky smooth the action is of your dob. These friction devices are a way of overcoming the unbalanced design and manufacture of these dobs. They DO NOT restore balance. But by loosening up these clamps too much, it may leave the scope either top heavy or bum heavy, and it tips over easily. An easy way to help overcome this is by using BIG magnets wrapped in a sock. Depending on what eyepiece you are using, the magnet is moved up or down the length of the OTA to actually restore the balance of the OTA, and the action of the scope is kept both light, smooth and consistent.

There is one other way to extend the drift period of objects in the eyepiece - using EPs with a wider Apparent Field Of View (AFOV). Plossls typically have an AFOV of 52. Plossls are also a very old eyepiece design (over 150 years actually). Contemporary EP designs have anything from 58 to 100. Some of these cost more than your dob, but excellent EPs can also be had without killing the piggy bank.

An alternative to Go-To systems for existing dobs

If you want a pointing system to help make your time more productive, instead of an go-to, you might consider a push-to set of digital setting circles, like the Nexus from Astro Devices. These will not only work with your phone, but they won't add any mechanical impedence on the quality of action of the scope so you can retain its full visual impact for your eyes. I can fit 3 of my dobs with the Nexus DSC when I am chasing particularly difficult targets. These Nexus devices can be easily installed onto mass production scopes and also custom made dobs. The Nexus DSC has a catalogue of some 2.5million individual objects. And there is the option of fitting all the Nexus devices with Extreme Encoders. These have a resolution starting from 350,000 steps and up to 1.2million steps. Even at their lower resolution it is possible to identify individual objects within the Small and Large Magellanic Clouds - something that no commercially made goto mount can do. These digital setting circles are also easily fitted to commercially made dobs and also custom made dobs.


Last edited by mental4astro; 01-03-2020 at 05:00 PM.
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