Posts Made By: Doug Scobel

Tom, I use a 7 AH 12V lead-acid gel cell on board my 13" dob. I use it to power two 12v computer fans, plus about three watts worth of dew control. It will power everything all night if I need it to.

I've seen them in hobby stores that carry R/C stuff. It's about three inches by eight inches by about 4 inches high. Only weighs a couple pounds.

One thing to be careful of with gel cells is overcharging. I burned out two with a one amp trickle charger before I switched to a charger with automatic turn-off.

Good luck,
Doug

Floyd, if your Discovery mirror is 1/10 wave *P-V*, which would be 1/5 at the wavefront, then it may be worth the money. But if it is 1/10 wave *at the wavefront*, then there's no point in having it refigured. But even at 1/5 wave wavefront error, atmospheric seeing will probably be your limiting factor nine nights out of ten. How do you "know" that the mirror as it stands is indeed is 1/10 wave, and is it P-V or at the wavefront?

Are there any noticeable defects in the diffraction pattern that you can positively attribute to the primary mirror? If not, then I would do some more evaluation (like have the primary independently tested, or do it yourself) before spending that much money. But if refiguring is warranted, then it sounds like a bargain, if they can achieve what they claim.

All IMO anyway.

Best regards,
Doug Scobel

Tom, the indoor Saturn V exhibit at the Kennedy Space Center is worth the entire price of admission, at least it was for me. The time of the Apollo launches was about the same time I got my first telescope (a 6" f/8), so my interest in the space program and astronomy are intertwined.

Have fun,
Doug

Ron, if you end up using a spider then here's a tip on delivering electricity to the fan along a spider vane. Find a place that sells doll house stuff, and get a roll of doll house wiring tape. It consists of two copper foil strips, about 1/4" wide each, side-by-side on a self-adhesive backing. Just apply a strip to one of the vanes. The "top" (non-adhesive) side is also covered, but the covering is easily peeled off. You can then clean the top of the copper conductors at each end and solder connecting wires directly on the surface. When done paint with flat black and you can hardly tell that it's there, it's so thin. Much neater and thinner than trying to route and align wires along a vane.

I did this for connecting to the dew heater behind the secondary on my 13" and it works great.

Doug

Did the pattern move any more quickly or slowly with the fan on or off? IOW, did having the fan on speed up the currents? If so, then the currents are probably in the tube and the fan is just blowing them around. If not, then it's probably the atmosphere.

Dave, parabolizing will not alter the focal ratio significantly. Once you're well into fine grinding, say at around 500 grit size, it's pretty difficult to change the FL by much. Even harder when polishing. OTOH, in the early stages of fine grinding, you may see the FL move significantly. This can happen if your figure is not spherical, which it often is not at the end of rough grinding. If the mirror is hyperbolic (typical) at that stage, you may see the FL lengthen as the surfaces come into better contact (i.e., more spherical) during fine grinding.

The bottom line is that you want to get the FL as close as possible with your most coarse grit size. The finer the grit, the harder it is to move the FL.

Happy grinding,
Doug Scobel

Tom, I've found that 1 watt on my Telrad is just right. It will stay clear on the dewiest of nights. If I neglect to turn it on and it starts to dew, it will clear off in about 5-10 minutes. I also have 1.5W behind my 2.6" secondary, but I've not needed to use it yet (I just added it this winter). My secondary is buried about 16 inches inside my 16" tube (13" mirror), so it's protected from dewing pretty well. If your's is closer to the end of the tube then it may dew more readily. But the Telrad is out in the open, so I would guess that it will dew up nearly every time. It does for me, while I'll have the secondary dew up only on the worst nights, maybe once a year.

Remember that heating the secondary may introduce convection currents that could harm the image, so I would put it on a separate switch so that you can heat it only when you need to.

I'm with you re: dew heaters for the eyepiece, I don't use one. I switch eyepieces often, and it's just a hassle to replace the heater all the time. I just keep the EP's covered when not in use.

Best regards,
Doug Scobel

Dave, how big of a secondary mirror are you using? You're likely using a 1.3" m.a., so it will be quite light and the curved spider ought to have no problems supporting it. Regarding diffraction, given vanes with the same thickness, the curved vane (single semicircle) ought to have no more total diffraction than the three vane, possibly less. But as others have mentioned if the former has thicker vanes then it may reduce the benefit. FWIW, Portaball uses Protostar curved vane spiders in their scopes.

Decisions, decisions...

Best regards,
Doug Scobel

Dave, something else to consider above all the other good advice given is difficulty in figuring. The faster the more difficult. I shouldn't really say more difficult, but the tolerances involved in figuring a fast mirror are tighter than for a slower mirror of the same diameter. Probably not a huge difference between f/6 and f/7.5, but not trivial either.

Good luck,
Doug

Mark, I recently picked up a few old UO plossls (25, 12.5, & 10mm) for exactly that purpose. I haven't had to clean any makeup off my Naglers and Radians, but I got tired of having to clean off fingerprints from kids and adults putting their fingers on the lenses saying "is this where I look?". They work OK with the 13" f/4.5 (quite soft around the edges), but perfectly fine in the 8" f/8. The 10mm has no eye relief tho.

I'm with you - I'd rather reduce the wow factor than ruin my expensive eyepieces. And with the moon and planets the wow factor is still there.

Doug Scobel