More questions

[JohnM]

OK, hate to beat a dead horse, BUT !
I picked up one of those discontinued Celestron 4 inch OTAs. I'm going to do a stage 2 mod with it, it'll put way less load on my mount than my old 6 incher.

A Baader DERF is a few months away because of cost, but I can get Wratten #29 filters at a pretty reasonable price.
This rig will be for imaging only, would it be worth trying the #29 filter?, at full aperture or could there be a chance of better results by stopping down some?

I know a lot of the guys here who have been doing and writing about this subject for a long time have said the quality of these photographic filters is just too low for good results. Then again some others have had good luck with them.

I wouldn't be out a lot of money if it's a bust.
So, what say you all?

[marktownley]

Have you a 2" nighttime Ha filter John? You could use this as an ERF half way down the tube...

[JohnM]

Really? I had never heard of that function for that filter.
Hmm, half way down the tube, I'll have to think about how I'd put it in.

[JohnM]

Wouldn't it have to be positioned in the light cone at the correct position to avoid vignetting or masking the cone?

[marktownley]

Really? I had never heard of that function for that filter.
Hmm, half way down the tube, I'll have to think about how I'd put it in.

Yes, it will work. Just pull the objective cell / focuser off, you may be lucky and there will be a baffle half way (ish) down the tube you can fix it to. Just make sure you incorporate a bit of tilt to throw the reflection.

[marktownley]

Wouldn't it have to be positioned in the light cone at the correct position to avoid vignetting or masking the cone?

Yes, but this won't be difficult to work out the position at all.

[JohnM]

Yeah, OK. This tube has a baffle about halfway down, but it's at ~64mm.
I've already made a full scale drawing of the light path so I can measure to the correct position.
I suppose I could cut a hatchway for access and fab up a door for it.
Thanks for that tip Mark, I might get more than WL out of this thing sooner than I thought.

[marktownley]

I might get more than WL out of this thing sooner than I thought.

The beauty of modding 8)

[JohnM]

If this works and produces good images I'll be thinking about going back to my 6 inch achro!

[marktownley]

If this works and produces good images I'll be thinking about going back to my 6 inch achro!

Unless your seeing is pretty darn good most of the time you'll probably find you get most use out of the 4"... I can use my 4" much more often than my 5"...

[JohnM]

True story, my thoughts exactly. That's one of the reasons I jumped at the chance to get one of those 4 inch available from OPT for so cheap now.

[swisswalter]

Hi John

great thread, good luck on the mod. A 13 nm Ha works very well ;-)

[JohnM]

I found an 82mm Ha, it should be here tomorrow. I checked with some of my other 82mm filters, it's nearly a perfect fit into the front baffle.

[Merlin66]

John,
Who's supplying an 82mm Ha filter?
Sounds more like a photographic red filter, than a multi-coated narrow bandwidth optical filter?

[JohnM]

Lumicon, I'll be finding out before too much longer if this will work.

[Merlin66]

I think you'll find the Lumicon Ha filter is just a red photographic filter...not the best solution.
http://www.lumicon.com/store/p/90-82mm- ... ilter.aspx
A multi-coated optical narrow band Ha filter reflects 99% of the energy back towards the objective, a red "coloured" filter just absorbs the energy and heats up.....

[JohnM]

If that proves to be the case then this filter will just have to go into service on a camera lens.
Then leaving no real alternative but the Baader DERF.

[JohnM]

Hmm, OK, I got that filter from Lumicon.
I bet it's a 25A, probably high quality, but a 25A just the same.
I'm going to put the tube together with the OG and filter in position and the probe from a digital pyrometer I have at the focus point and see what it reaches while tracking.

[peter drew]

I have a 80mm F10 PST mod with a 82mm 25A photographic filter as a front ERF at home for a quick grab and go before setting up anything better, it works quite well, better than a standard PST but not as well as a Baader D-ERF, as could be expected.

[JohnM]

Good to hear Peter.
I'm curious just how much heat I'll get.
I think there's going to be enough Sun today for some tests. I'll have it mounted before too long.

[JohnM]

I have the filter in position in the lightcone and only the objective installed.
The temperature sensor picks ~250°F at the focus point with the scope guided so the beam is on center.
As soon as the light beam begins to move off center the temperature immediately starts to fall off.

So there it is, if I stick my pinky in place of the probe I can feel that heat.
Is it too much?

Should I make a mask and put that filter ahead of the objective?
That would make the scope about f12.5

[marktownley]

If you have the sun and the free time then give it a go. Do you have a 2" IR/UV cut filter you could mount on one of the baffles towards the focuser? If you have it will reduce some of that heat on your pinky

[JohnM]

Out of curiosity I mounted the PST tube with only the objective.
I didn't take time to find a way to mount the temp probe, but hand holding it into the beam at about focus point I could get 120°-140°F from that little 40mm objective.

I have several different types of 82mm UV/IR filters, so I could put one of them on the Lumicon, or I could mount a 2" further down in the light cone.

I did hang all the Ha components and found the Sun with the DMK at prime.
To get the histogram back on the chart I had to cut exposure almost to minimum.

About it for the day though, PM storm clouds moving in and hiding the Sun. Time to clean up and put away tools.

[vikeljay]

Please let us know the results.
Jim

[vikeljay]

Why not just screw a nightime HA filter and a UV/IR filter on the bottom of the camera?
Jim

[JohnM]

This is an old thread. A lot of work and tests done and posted in newer threads since this one.
All the issues and problems associated with trying to use these filters have proven to be the case.