I have an old Tasco 17TR 80mm x 900mm refractor I purchased new in 91 which has been highly modified. It has a Vixen ED lens cell (it screwed straight on after having a thread machined on the pipe), baffles, flocking and a machined alloy adapter to fit a WO Synta DDG focuser.
To obtain visual focus after fitting the adapter and focuser I had to cut the tube 50mm shorter and I used it mainly for visual.
The other night I ran out with it thinking I'd chuck it on a small Celestron mount which belongs to a 70mm reftactor I keep meaning to give to a friends kids, slapped my Niokon camera on the back and wanted to snap the moon, just rough quick shots playing. I couldn't obtain focus, I just couldn't get the sensor close enough to the focal point. The best I could get was a white blob and though some cloud had drifted over I can usually make out the silhouette of the moons darker features
I've read a lot which tends to recommend the easiest option is to add a Barlow. I didn't want to do that.
Is my only other option to further trim the tube down shorter?
It's not a major issue to cut the tube but I'd rather not remove too much, I guess I could always TIG it back on if necessary but that would be a real pain to be honest.
I also don't want to take 5mm off at a time and trial it to see if I can achieve focus because that becomes overly time consuming.
I guess 20mm off and I have extenders which the Williams Optics focuser can handle. It's obviously not a primary imaging scope but I like to play on occasion.
Please disregard this question. I'll cut the tube down again if I can be bothered, I don't believe there's any other solution or, just throw everything in the tip.
Hi Leo, as no one else has replied, if you are trying to image through the telescope with an DSLR then you need at least 50/60mm of drawtube in travel from where your eyepieces generally reach focus for the camera sensor to reach the focal plane. Do you use a diagonal for visual, if so , when trying to use the camera don't use with the diagonal, then you should be able to reach focus easily by extending the drawtube .
Thanks Jeff!
I was trying sans diagonal. As mentioned it's not a standard old refractor and while I do know I cut the tube shorter with the machined adapter for the WO focuser I also know I achieved focus visually but it's not a scope I use for imaging (it was when new with a 35mm Nikon camera) but those days are long gone. I just wanted to play the other night because it's light and easy to play with on a small mount. I'm going to try it out with a Barlow but that wasn't my original intention, I think maybe another 20-25mm off the tube may be the only way. Out focus is no problem, the focuser is strong and I have extension tubes but there's only so far in it can go without the aid of a big hammer (I have a 16 pound thing here I'm sure would shift it), lol. I wouldn't do that to a piece of scrap tube, let alone a much loved old telescope with a WO focuser on it.
It begs the question that I should have asked earlier, when using the scope for visual, without a diagonal, how much intravel is left for the drawtube, that is the relevant dimension . Obviously swapping the lens cell and focuser has altered the required length of the tube, plus the focal length of the Vixen objective may have been different to the old Tasco one.
Nothing a bit of chopping wont fix. By the way, a 16pd hammer is probably overkill, a 2 kg would do the trick if used judiciously. But there is the old fitters maxim, if it doesn't fit , get a bigger hammer.
The lens cell itself made no difference. I used it for years before I wanted a better focuser (the original wasn't great and restricted to .96" eyepieces). The lens cell was offered as a screw on replacement for the original Tasco lens when I bumped the scope packing up one night and chipped the lens. I got it through Tasco themselves, higher quality than the original and it only required an external thread on the tube which cost me $10 for a machinist to do.
I didn't pay a great deal of attention when I last checked the visual operation which was when I did the focuser mod. I have an original Megrez 80mm which is so easy to grab if I want the smaller scope and I mostly use it. I would have it written down somewhere because the focuser I use has a digital readout and I have scribbled numbers everywhere.
I was going to play last night to check visual but cloud rolled in, today it's raining. Oh well, the rain can't hang around too long (yes, I should be a comedian), I HOPE.
I have till Wednesday next week to get my old car registered and I have to put new front rotors on (Falcon EB II), not the later floating disc, full bearings and seals. I should have done it yesterday while the sun was out but I checked the weather on my phone (which I don't usually do) and it showed gale force winds so I didn't do it. I'm never looking at that app again, the wind was mild. I don't like laying under a car on stands in strong winds, safety margin for an old fool with a sloping driveway (and chocks at rear wheels, foolish not stupid).
This is the modified Tasco. The focuser is my older secondary WO focuser, not the Synta DDG unit which is currently on my 152mm Skywatcher refractor. The adapter is just to keep dust out wile it's standing in the back room, not part of the attempted imaging train.
Hand rolled tube rings because the original mount came with a clamshell unit and when I did the mods I didn't have 80mm tube rings (probably still don't come to think of it).
