A step-up in the refractor market.

[JamesF]

Well, I'm confused. I can't find any suggestion that either the Baader or Lunt wedges contain components that are not optically flat. That should mean that the focal length of the telescope remains unaltered and that the image scale therefore also remains unaltered. The maths is relatively simple -- arcseconds per mm = 206265 / focal length, or 344 arcseconds per mm. The Sun is half a degree wide -- 32 arcminutes at the most, or 1920 arcseconds. That should cover just over 5.5mm of the focal plane. Yet somehow that gets so big that it won't even come close to fitting on your DSLR sensor which is 15mm high.

The wedge and the camera are the only components you have on the focuser of the scope when this happens?

James

[sologuitarist61]

Yes, nothing else at all, but I should have said at the outset that I haven't had much of a chance to play with it yet as I have only had the T nosepiece for a week and the weather hasn't been forthcoming, so probably got no more than 5 minutes to try it out. However, with the focuser racked right in the image of the sun was out of focus and racking the focuser out just made the out of focus image even larger, which gives me the impression (from out of focus photo lenses) that I was getting more and more away from the point of true focus. Then the sun went in and I had no opportunity to try it further, so other factors may be coming into play. Will try it again tomorrow at prime focus out the front window (I doubt the sun will shin) and will use the scope as a lens and see is I can get it to focus across the valley. Would have tried this yesterday and the day before but the windows have been pouring with rain both days!!

[JamesF]

Ahhh, so perhaps the problem could be solved if you had a scope with more in-focus. Perhaps what you need is to pick up a cheap ST120 or ST150 second hand, remove the focuser and saw 40mm off the end of the OTA

I wonder if you might get away with putting a 2x barlow in the wedge (especially one with a T-ring fitting on the back) and putting the camera on that?

James

[sologuitarist61]

That is some thing I could certainly try and will do so when conditions allow, many thanks for the tip (about the Barlow that is, don't think I will go for the sawn off ST120 just yet!!). However I think I need to get it set up again and see if I can see what the problem is as there is no reason why the DSLR should not come into focus in the prime position (or indeed through the wedge). Thanks you for your time an effort James in trying to put me on the right track - I do appreciate it greatly

[JamesF]

When you put the camera on the scope without the wedge, how do you attach it? Onto the T-thread on the outside of the 1.25" eyepiece holder? I can give it a go with mine to see if it will reach focus for a comparison. If that's the case it looks like a T to 2" adaptor might mean you could drop the 1.25" bit and gain you another 12 or 13mm of in focus. I think I can try that as well.

I can't find useful specs for the Lunt wedge, but the Baader version apparently adds 128mm to the optical path which is a good 50 to 60mm more than a standard diagonal. That's a lot of focuser travel to have to give up. If the Lunt is similar I'm not entirely surprised that you struggle to reach focus. The barlow might help by moving the image plane further out, but there's only one way to find out It would be a nice bonus if it worked though as you should get a very nicely sized image for the DSLR, occupying well over two thirds of the height of the frame.

James

[sologuitarist61]

When you put the camera on the scope without the wedge, how do you attach it? Onto the T-thread on the outside of the 1.25" eyepiece holder? I can give it a go with mine to see if it will reach focus for a comparison. If that's the case it looks like a T to 2" adaptor might mean you could drop the 1.25" bit and gain you another 12 or 13mm of in focus. I think I can try that as well.

James

I wonder if that is the problem James - stupidly I had forgotten that I could attach direct to the 1.25" focuser and, as a result have been using a separate 1.25" to T thread adapter. That will certainly add the the length of the optical train, if only by ten to 15mm's, and that may be what is making the difference at prime focus . Doh!!!!! Stupid, stupid, stupid ....

[JamesF]

I was looking at my ST120 just now Richard, and noticed that the focuser body actually fits in a flange that fits into the back of the main tube. The flange must add about 20mm to the overall length of the OTA. If you could get a ring made up with inner and outer diameters to match the focuser body and tube then you could effectively shorten the tube length by 20mm.

I also reckon an inventive chap could probably buy a second hand standard Skywatcher R&P focuser for not much from someone who has replaced one, move the rack down the focuser tube a little and shave about 15mm off the back end of the focuser body with a hacksaw for even more in-focus.

It may also be possible to get replacement a replacement focuser with a shorter body, or even get a bracket made up that would allow a newt-style focuser to be fitted.

Other options do seem a bit limited if you want to use the wedge. I've read that ideally you need 120mm+ of aperture to be able to resolve the finer detail in white light images so the smaller scopes that I know of with short tubes probably aren't options. I know Alexandra uses a wedge with a TEC APO140ED, but I promise the price will make your eyes water...

James

[sologuitarist61]

Right, I am in a position to update now that we have had some good weather and with the ST120 the fault was mine. I had forgotten that I still had the original 1.25" focuser nosepiece with the T thread on it and was using another that I had but into the existing nosepiece (hope this makes sense). I removed the new one and put the old one back in and the DSLR straight onto that and yippee I had focus, so that was OK. In my stupidity I was putting a nosepiece into another nosepiece and so the sensor was moved that much further back in the image train and hence the focusing problem

Then it was the turn of the SW 102 guiderscope, but it didn't work on that one and focus cannot be achieved though doing rough measurements shows the DSLR is sitting in much the same position as on the 120, but I have a Linear Bar Crayford on that one and so the focuser itself appears to be longer and therefore the culprit.

Anyway, as long as I can get prime focus on one of them then I am happy. Many thanks for your assistant with this James as you pointed me in the right direction. I appreciate it!

[JamesF]

I shall expect images to be posted next time it's sunny then, Richard

James

[sologuitarist61]

No Luck for a day or two I fear James, but here are a couple through the front window (hence two panes of nor very clean glass ). Not brilliant but at least it shows that it works!!! Yippee

About a mile away as the crow flies (or indeed, light travels!)

Just across the valley, so about 300 yrds

Looking at the forecast there should be sun here tomorrow and Tuesday, but I won't hold my breath!!