Focal length/point problem with new crayford focuser

[Leo.A]

I'm having problems with my new crayford focuser I fitted to my modified Tasco 17TR.

The scope is an early 90's 80mm refractor with a focal length of 900mm.

The problem isn't apparent when viewing planets, etc, it's when I try to use my camera (Nikon D80) or web cam (SPC880 modified).

With the original focuser, the focal range (focuser movement) was from 840mm (measured from the lens) through to 1040mm. With the new crayford and adaptor the focal range is from 910mm through to 990mm.

SInce fitting the focuser, I get beautiful, clear views of Saturn and the rings, Mars, basically everything I've turned it towards but, I can't get the focal range for photography. The focal point is shorter than what I can obtain as it is. Last night, I removed the 1.25" adaptor and held the web cam head inside of the 2" eyepiece holder and could obtain focus (albeit wobbly) so I'm thinking I need to cut my tube down.

Any advice??

Before you ask why I'd bother, this telescope has a previous mod done in 97, the fitting of a Vixen ED series lens and the quality is stunning, now if I can get my focal point correct for photography I'll be very happy.

I can't modify the adaptor (a cost of $150 to get it machined and that's with me providing the stock and design) but I'm not overly keen on trimming the tube down. I can't see any other options though.

If I cut the tube down and it's short for visual purposes I can run a 2" or 1.25" focal extender tube (80mm length seem common and readily available).

Thanks!

Tasco 17TR with Vixen ED lens, Williams Optics Crayford focuser, home made tube rings mounted on Skywatcher EQ3 Pro GOTO mount

[andrew s]

Cutting it down is one option the other, if you are doing solar system observing with a web cam, is to use a good quality barlow or powermate. This should allow you to reach focus but with a smaller field and higher magnification not a bad idea for the moon, planets or sun (with solar filter). If it is deep sky then cutting it down seems the only real option.

Regards Andrew

[Leo.A]

The Barlow doesn't allow focus either, I tried both with and without a 2X Vixen Barlow with camera adaptor (inc on Barlow). The problem is I can't get the in focus far enough, as in get the camera focusing within the focus tube itself, not with the focuser drawn outward from the tube. When I tried the Barlow, I tried it with and without the attached lens and still could not get the camera close enough to the objective lens on the telescope to achieve any sort of focus.

[andrew s]

Looks like cutting is your only option. However, I am surprised that if you can get an eye piece to focus you can't get a Barlow to work.

The eye piece will have an aperture stop (normally a ring just inside the 11/4 or 2" fitting) and when in focus this will be about at the focal plane of the objective. Your Barlow needs to be well inside the focal plane to work. Is this the case?

Can you focus with the Barlow and eye piece? If you can then you probably need to get the Barlow further inside focus to bring its focus further out for the web cam.

Hope this helps if not it's probably the cutting route!

Regards Andrew

[Leo.A]

I have no problem obtaining clear focus with the Barlow and an 8mm eyepiece. Both are Vixen and I normally wouldn't consider a 4mm eyepiece (8mm, 2X Barlow) to be usable on such a small aperture scope. If I understand the equation correctly, that combination give me 225 times magnification and, from what I understand it shouldn't be as clear as I appear to get.

Source: Astronomics.com

More than 60x per inch of aperture is sometimes possible for planetary observing with small aperture telescopes (under 4"-5"), since a small scope looks through less of our turbulent atmosphere than a large one and therefore is less affected by unsteady seeing. Unfortunately, the exceptionally good seeing that allows such high magnifications is very rare.

During average seeing conditions (the kind you find nine nights out of ten), 25x to 30x per inch of aperture is a more sensible power for binary star and planetary observing. It is at this power that the resolution of a scope matches the resolution of your eye and images are sharpest. This gives you a highest useful power of about 200x to 240x with an 8" scope on an average night, 100x to 120x with a 4" scope,

I think between the Vixen ED lens, reasonable quality eyepieces (Vixen NPL series Plossls), the extra baffles I recently fitted plus, the respraying (non reflective matt black) of the tube internal, I got lucky. The planets look even more spectacular with my Vixen 10mm Lanthanum and 2X Barlow.

I should however mention my eyesight is near non existant without glasses but I don't / can't use glasses to observe or for photography. Should anyone else want to look through the scope they have to do some serious refocusing to obtain a clear, sharp view.

Perhaps what my son believes is in focus isn't as clear as my poor sight allows me to obtain, I can't say.

[Leo.A]

I'm taking the plunge, trimming 30mm off my tube in about 5 minutes. I re-marked all hole positions in line with the original mount holes for focuser and finderscope.

Why 30mm?

Because of where the original finderscope mounts, 30mm is the minimum to clear the original mount holes on the tube and I don't want old holes in the tube, too hard to patch even though I've discovered some wonderful low temp alloy welding rods used with a gas flame, which I haven't tried yet, just watched the video.

I'm just hoping I don't need to spend money on a focal extender tube for visual purposes but I have done the maths numerous times over and I'm sure I'm right.

I usually am one out of every one hundred attempts........

Check thrice, cut once, as per my fabrication engineering (glorifeid welder with drawing and structural analysis thrown in) training at college.

If I need an 80mm focal extender tube they are quite common (until I actually need one as per Murphy's law).

Hopefully, I'll be able to post some nice Planetary images here soon. Nice for a novice with limited finances and resources anyway!

Pray for me please?

[knobby]

Thinking of you!

Good luck ... positive vibes coming your way

[Leo.A]

Thanks Knobby!

After going over my measurements again I decided to start with 20mm off, which has now been done. I have to wait till an engineer supply shop opens on Monday to purchase a 3/16" tap so I can mount my finderscope again (I have 2 dead holes but they're covered internally by the focuser adaptor).

My moment of insanity has passed, the tube is 20mm shorter and reassembled and if it wasn't for the fact my back yard is a swamp after heavy rain yesterday and my son and his laptop are at his mothers place for the weekend, I'd be out there now trying it out.

My son is the computer controller and more knowledgeable than me with the computer control side of things (despite my IT qualifications) and we use his laptop for the web cam images. Plus, it finally got my son interested in astronomy since purchasing the computer control GOTO mount and web cam.

[nightfisher]

What an interesting scope, i hope all is well with the tube reduction

[knobby]

Thanks Knobby!

After going over my measurements again I decided to start with 20mm off, which has now been done. I have to wait till an engineer supply shop opens on Monday to purchase a 3/16" tap so I can mount my finderscope again (I have 2 dead holes but they're covered internally by the focuser adaptor).

My moment of insanity has passed, the tube is 20mm shorter and reassembled and if it wasn't for the fact my back yard is a swamp after heavy rain yesterday and my son and his laptop are at his mothers place for the weekend, I'd be out there now trying it out.

My son is the computer controller and more knowledgeable than me with the computer control side of things (despite my IT qualifications) and we use his laptop for the web cam images. Plus, it finally got my son interested in astronomy since purchasing the computer control GOTO mount and web cam.

Its nice when you involve the kids... nice bit of bonding

[Leo.A]

My son is really special, he turns 18 in August, has no friends and has been in my full time custody for 4 years. He is severely hearing impaired and got bullied so much at school (a private school) I withdrew him from the education process before his 15th birthday. He spends all day with me and it's nice to get him interested in something other than the computer programming he taught himself. He is currently writing his own operating system and search engine, not bad for a kid with no training, and despite me having IT qualifications (I have qualifications in varying fields) his knowledge has outstripped mine 1000 fold.

[knobby]

My son is really special, he turns 18 in August, has no friends and has been in my full time custody for 4 years. He is severely hearing impaired and got bullied so much at school (a private school) I withdrew him from the education process before his 15th birthday. He spends all day with me and it's nice to get him interested in something other than the computer programming he taught himself. He is currently writing his own operating system and search engine, not bad for a kid with no training, and despite me having IT qualifications (I have qualifications in varying fields) his knowledge has outstripped mine 1000 fold.

Sounds like a great kid ... if only their were more like him!