Does Mono DSI need a UV filter for guiding?

[mldee (Mike)]

After some very generous help yesterday from Peter_4059 to cut a suitable thread on an old 2"-1.25 adaptor, I now have all the bits to mount my DSI Pro II mono on the back of a generic SW 8x50 finder for guiding purposes, as has been discussed here in previous threads. The concept looks good and I have been able to check focus range using handheld approach today.

My main question: Do I need to use a UV/IR reject filter in front of the sensor to reduce excess sensor "light overload" or doesn't it matter?

Secondly, how much "movement" does the generic dovetail+finder-mount arrangement cause when using to hold the guider with a ~200 gram DSI on the back?

I have a couple of small guide rings I could use, but prefer to keep things as simple as possible.

[peter_4059 (Peter)]

Mike,

I've tried guiding with and without the UV/IR and it doesn't seem to make much difference and in fact I have heard some people recommend slightly defocusing the guide scope for better guiding and I understand the main reason to use the UV/IR filter is that these wavelengths don't come to focus at the same point as the visible spectrum.

Another advantage of using the UV/IR filter is that it will prevent dust getting on the CCD surface. You should be able to screw one on the front of your 1 1/4" DSI nosepiece.

Peter

[mldee (Mike)]

Thanks for the quick response, Peter. At this stage, it looks like I'm going to go without a nosepiece, which also makes the focus point easier. I could squeeze on a nosepiece + T adaptor, etc, but gets a little complicated, and I have to get the special DSI T adaptor from Scopestuff.

I noticed after removing the DSI filter slide box that the 2" adaptor flange sits just inside the ring of four screw holes that held the filter box. I am going to make up four little clips and basically clamp the flange directly to the face of the DSI, then I can screw it into the back of the scope tube quite easily (your thread works great).

If my approach works, I'll then dab some epoxy as well on the tabs where they contact the flange, so they will become firmly affixed in case I ever want to remove it from the DSI.

I have the flat UV filter from my DSI III OSC which I was thinking of using, as I use a regular glass one with it now.

I'll post some pics when it's done, probably mid week.

[mldee (Mike)]

Well, got the gadget put together today, had to go to a hobby shop to find four very small replacement screws (2mm?) to hold the threaded 2" adaptor on the front of the DSI. I'm using a couple of steel washers under the screws to clip over the flange on the adaptor, and it's surprisingly sturdy.

I put it on the C8 top vixen rail tonight with an ADM adaptor, and achieved good focus on Canopus, which was about all I could see through the clouds. Smaller stars were coming and going, but not dependable enough to try and calibrate for guiding. Maybe later or tomorrow. Then some pics for others who may want to try it.

So far, all has turned out great. Not using a UV/IR filter yet, want to see some smaller stars in an A/B comparison first.

If all goes well, I think I'll follow AlexN's advice and go for "Elegant simplicity"; Leave the ADM SBS plate on the EQ6, but just mount one scope with the guider on the LHS, keeping the RHS for my USB hub, qHY supply, etc. This will make balancing far easier and probably remove the need for any top weights. It is easy to swap scopes on the LHS dovetail and if I do want to pop a second scope on, the saddle is always ready.

That's the theory at this stage, anyway. Once again, thanks to Peter_4059 for his invaluable assistance in doing the adaptor thread. Without that, the project would have been impossible.

[peter_4059 (Peter)]

Great to hear you are able to achieve focus Mike. Look forward to seeing some images if the clouds ever part again.

[mldee (Mike)]

Had an hour or so's cloud-free time after sunset, and was able to set the little guider up and calibrate in PHD on a smallish star with .5S exposure. I then let it guide for about 45 minutes solid as a rock, until the clouds rolled in.

Based on that, I'll take the WO 80 and main guide scopes off the RHS of the mount tomorrow and do a rebalance, with just the C8 and the little guider on the LHS. Then it's time to try a few happy snaps.

I also had the QHY8 with a junk zoom lens piggy backed as well on the C8, but couldn't quite get to focus. I'd like to do some really widefield stuff with a good 50mm Pentax lens on the front, hope to have that set up by the weekend.

As I keep confessing, I lerv to tinker. Anyone for an EQ6 hypertune?......

[Tandum (Robin)]

If I had a 1.25" uv filter I'd use it but if I had to buy one I wouldn't