Quote:
Originally Posted by barx1963
No, it is purely a push to with Argo Navis. Was not aware of any issues operating it with a Servo cat, I am assuming some sort of electrical interference occurs?
Malcolm
|
No, it's a lot simpler than this but far more damaging to your nights observing. It puts an end to it.
The Kendrick Digifire units have a built in "low voltage cut-out". You cannot disable the low voltage cut-out, or over ride it, or at least you couldn't on the older units. I am not sure if they have changed this, but I doubt it. I believe the low voltage cut-out kicks in at about 11.5 volts. I have no idea why they have this, but I can only assume it is to prevent people flattening their car batteries to a level which prevents them from starting their cars and getting home. It is also worth noting for scopes that have the Dew Heater controller mounted on the upper cage assembly using "powered truss poles", there will also be a slight voltage drop from the bottom end of the scope to the upper cage assembly. This might be in the order of .2 to .4 volts depending on how its all wired up. Notwithstanding your battery is still running at 11.8 volts your dew heater controller will only be receiving about 11.5 volts and the low voltage cut-out will kick in at this level.
At the beginning of an observing session all is fine for the first couple of hours. However, with scopes which have several dew heater straps, motorised go to and tracking drives and a few other electrical gadgets in use the power drain can be heavy. While Servocat tracking doesn't use much power, Servocat GOTO's certainly do. After a couple of hours the battery voltage drops to a level where the low voltage cut-out in the Digifire unit begins to work its magic. The first thing you notice is when you do a GOTO and the power drain is heavy the LEDS on the Digifire begin to flash. This means power is getting critically low. The next thing that happens after another 30 minutes or so is that the Digifire unit switches off every time you do a GOTO. After the battery recovers a little from the GOTO the Digifire unit might start up again. After another 30 minutes or so the Digifire unit will just shut down completely and not restart for the remainder of the evening because as far as it is concerned the battery is critically low. The bigger the scope the worse it gets, as a big scope chews up more power being moved around when you do GOTO's. The voltage drop of the battery does not cause problems with any of the other devices powered by the battery. They will just pull more current to work as the battery voltage drops, but they will continue to work perfectly all night. This includes, the Argo Navis unit, the Servocat, the Dew Heater Straps and the fans. This of course can depend on the size of the battery you are using and its condition, but it is a very annoying thing when it happens and IMO you are better off looking at one of the other dew heater controllers if your scope has motorised GOTO and tracking. I speak from experience. I got dewed out several times after my Digifire 7 spat the dummy. I ended up selling it to a guy whose scope had no tracking and I bought a Thousand Oaks controller.
The Thousand Oaks controller also has a low voltage warning and cut-out system which gives a flashing light at 11.5 volts and cuts out completely at 11.2 volts, but you can disable the cut-out when using a scope dedicated battery. There is a switch which enables you to disable the cut-out. The Thousand Oaks unit is about 70% of the price of the Kendrick Digifire 7 and works perfectly.
http://www.optcorp.com/th-ddhc-dew-h...ontroller.html
In addition, in all circumstances but more so in cold weather the dial knobs on the Thousand Oaks unit are much easier to adjust individually than the press panel on the Digifire units. The Thousand Oaks system like the Digifire 7 has 4 outputs individually controlled. The Digifire 7 has 2 additional non controlled outputs, but on a Newtonian with a separate secondary heater, you don't need those. I use 4 outputs on my 18" and 14" scopes (objective and eyepiece on the finder, Telrad and the eyepiece). I only use 3 of the outputs on my 10" as I only have an optical finder, not a finder + Telrad. Peter Read at SDM Telescopes used to fit the Digifire 7 units as standard on all the scopes he built. In light of the issues explained above, he now fits the Thousand Oaks controllers as standard on all scopes.
Cheers,
John B