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Diagonal and filters.

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Am close to getting a 'scope now (likely a ST120, I'm pretty sure I can work out carrying one on a motorbike). :)

So I'm thinking of a 2" quartz dielectric diagonal to keep the amount of light up (pre a massive hit to it with filtering) and with a 1/10th wave to help with the quality (might this help cut reflections as well? or would I need to spend some time making sure the tube is as dark internally as possible, stuff like that?).

The main issues I have, apart from the sheer intensity of light, are UV, glare, and reflections (possibly IR too).

So I was thinking of a 2" UV & IR blocking filter fitted permanently in the nose of the diagonal. Are some noticeably better than others with these types of light? My glasses for example, have 100% UV filtering, but earlier standards were 92%.

Then, to tame that glare and light intensity, get a variable polarizing filter to fit the eyepiece (thankfully polarizing really gets glare under control).

Now I've seen variable polarizing filters that reduce the light throughput in a range from 40% down to 1%. Are there others with a wider range of usefulness available? I think a 60% light reduction would bring the performance down to lower than that of a ST102? Would the UV & IR give that a further hit?

If the performance comes back to somewhere between that of an ST80 and that of an ST102, and usable (though ideally the closer to an ST102 the better), then I'll be a happy camper. ;)

Now would there be a way of fitting another filter if it should prove necessary? Can one be fitted to the bottom of a Barlow for example?

I spotted someone somewhere discussing stacked Barlows - using a 2x and a 3x together. Now this apparently reduced brightness a lot (normally not desirable I suppose), but this could prove a useful avenue for me to pursue later, if I still can't get things sufficiently under control - any issues with doing this I should be aware of?

I don't suppose I'll be having any particular issues with CA with such heavy filtering, but if there should be, would there be a filter that would best complement a permanent install of UV & IR plus a variable Polarizing (given that it would even be possible to use such an extra filter)? For example, would a semi APO filter make a useful enough contribution say?

Any ideas and guesstimates more than welcome, and in addition, a heartfelt 'THANK YOU!' for all the insights and help I have had so far from you all. :D

Edited by Ogri
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If we're talking Diagonals, I assume we're also talking about visual.

The minus UV filter will give slightly "tighter" star images but our eyes are not that sensitive at night to UV-IR ( it's CCD's you have to worry about) I agree a variable filter will help on the Moon, but for everything else you want as much light as you can get. Depending on your location you might need a Light Pollution filter (LPR) and THAT does cut down on visible light!!

So unless you're going to image ( when you won't use the diagonal but probably only a 50mm spacer) keep it simple.

Just my 0.97 cents (after tax)


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"but our eyes are not that sensitive at night to UV-IR "

Unfortunately mine seem to be chronically sensitive to everything Ken, after an eye operation that went badly wrong. As the eye had the pupil dilated for the operation, and the laser surgery ended up welding the inner mechanism in place, the pupils in both can't shut down (as they are interlinked).

Hence just to be able to go outside, I need special daytime and night time glasses.

For example setting up dads Heritage 130p just before Christmas, with far from a full moon, and with a Meade moonfilter, I did about 5 minutes tops looking at the moon, and about 20 minutes looking at other things, and I ended up with arc eye and on painkillers for a week.

This might sound utterly nuts, but for the first 6 - 8 months after the operation, I sat in a room with lights off, heavy curtains closed, and sunglasses on, and if I picked up say a piece of blank A4 paper, it was dazzlingly bright to me. It took a heck of a long time just working out what was going on, and trying to learn to cope with it (from early days with a largely useless eye patch, large progress has been made, I'm pleased to say), and for many years, the only safe time I could even go out of doors was if it was heavily overcast (raining, ideally) and not anywhere near a road at night time, due to the lights from cars (I really, really hate those cars with the ultra bright bluish headlights - you won't believe the pain they have given me). If the sun came out while I was out, it would often put me in bed for over a week on strong painkillers.

Strangely enough, flourescent lights really give me a hard time too, and as the doctors surgery is full of them, I've had to do a rapid exit stage left if the waiting time goes beyond a certain point.

Hence the size of the scope (the ST120) should allow enough light gathering, for me to scrub enough of 'the bad stuff' off and out of it, so I can use it, plus, I can use masks on the objective lens to give 110/100/90 etc., to help fine tune things as well.

It's a challenge, and I'd like to get it beat. :)

Eventually, I'd like to get CCD viewing set up (which was a good recommendation here), which will sort a lot of issues, and the insights into filtering, etc I learn might help that a bit when I get to it as well.

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Hmmm.... as you say .... a bit of a challenge!!

One of the up sides of your situation would be, having wide pupils, so the low magnification, wide view observing of low surface brightness objects may be "niche" market for you.

The variable polarising filters should handle the objectional bright light with no additional filters....

Bright objects may still be problematic at times, but I'm sure having come this far, you'll be able to work something out.

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