16x70mm finder with Prostar 22mm Erfle with illuminated cross hair

[michael.h.f.wilkinson]

I have recently updated my home-grown 12.5 x 70mm finder to a 16 x 70mm finder, with a tad more true field of view. I previously got some mounting brackets (Robtics) which allowed me to mount a cheap 70mm F/5 achro (Eur 50) as a finder scope on my C8. I also changed its rather naf silver-grey apperance into nice black by application of some black foil. I used an old 28mm Kelner from a defunct pair of binoculars (Eur 2.50:D) and some of my own hair as cross hairs. This worked OK but the 45-50 deg AFOV of the finder was a bit cramped, and the cross hairs not that visible. Furthermore, the cheap rack and pinion focuser had more play than I wanted.

The first problem was sorted by getting a Prostar 22mm illuminated reticle Erfle EP from down-under (here: AstroShop - Product Details). It sports a 70 deg AFOV, and yields 16x magnification on my 70mm, so a reasonable 4.4 deg true FOV. Even after import duties and postage costs, it was a bit cheaper than a similar Lumicon 20mm, which would give a rather steep 17.5x magnification, and just 4 deg true FOV. It took a week or two to arrive, and I did have to open it up to remove some dirt that was on the glass onto which the cross hair has been etched. Other than that, it is a well built piece of kit, with a very decent optical quality.

The other issue was solved by disassembling the focuser, and applying the same foil to the inner draw tube, which eliminated the problem, bar a VERY minor wobble, which moves a target by about the width of the cross hair.

I have now mounted the big viewfinder closer to the center line of the declination axis, which makes the whole set-up more balanced. It also allows me to keep the old 6x30 viewfinder in place. I find this useful to get a general aim, and then use the 16x70 to star hop to the exact location. I can often spot the DSO in the finder, which makes life SO much easier.

In future, I might replace the 6x30 by an RDF, but for now I am very happy. The new finder has already bagged me M74 and M78, and I cannot wait to see how it performs when Coma and Virgo swing into view.

[AndyH]

Hi Michael !

As a fellow 'big finder' fan, they are superb pieces of kit, aren't they. Faint fuzzies just jump out at you.

You've done a great job. Looks beautiful. I use a similar illuminated eyepiece from T-S (20mm 70 degree erfle) on my 80mm. These eyepieces make a big difference in the FOV/comfort department.

Cheers,

Andy.

[michael.h.f.wilkinson]

I looked into the TS version, but that would restrict the FOV to 4 deg. so I went for the Prostar, which gives me 4.4 deg. Every little helps.

[russ]

I've decided on the Orion USA 20mm Super Wide Illuminated eyepiece for the 65mm finder. That will give a true field of 4deg at 17.5x. Been experimenting with standard non reticule eyepieces and 4deg with that power will be about right. The 22mm Prostar will workout too expensive after shipping, import duty, tax and handling.

[michael.h.f.wilkinson]

I've decided on the Orion USA 20mm Super Wide Illuminated eyepiece for the 65mm finder. That will give a true field of 4deg at 17.5x. Been experimenting with standard non reticule eyepieces and 4deg with that power will be about right. The 22mm Prostar will workout too expensive after shipping, import duty, tax and handling.

When I got the Prostar the Aussie dollar was lower than today. For me the price difference with the 20mm was about 5 euro, also because Dutch import duties on opticsare just 5%. That got me 4.4 deg FOV rather than 4.0.

[russ]

When I got the Prostar the Aussie dollar was lower than today. For me the price difference with the 20mm was about 5 euro, also because Dutch import duties on opticsare just 5%. That got me 4.4 deg FOV rather than 4.0.

It's a shame because the 22mm would have been perfect, my would have also given a 4.4deg field with that eyepiece. Nevermind, the 20mm will suffice.

[AndyH]

Russ - I reckon my TS eyepiece is in effect the same as yours. A cracking 'piece.

To my shame, I recently aquired another broken 80mm binocular. Two more 80mm finders here I come.......oh dear. Damn you astroboot.

Andy.

[michael.h.f.wilkinson]

Russ - I reckon my TS eyepiece is in effect the same as yours. A cracking 'piece.

To my shame, I recently aquired another broken 80mm binocular. Two more 80mm finders here I come.......oh dear. Damn you astroboot.

Andy.

Its somehow addictive, turning broken kit into finder scopes and other gear, isn't it

[AndyH]

Ohhhh, yes it is

I tried. Honestly I did. I waited 2 weeks, hoping someone would buy it first. I had a few pounds in my paypal a/c and my willpower deserted me.

Really can't complain though. £10 for the set.

:)Andy.

[russ]

Hard to resist when the prices are so good.

Looking at the pics, i think you're right Andy. The Orion and TS 20mm widefield are the same item. Just the job!

[michael.h.f.wilkinson]

Hard to resist when the prices are so good.

Looking at the pics, i think you're right Andy. The Orion and TS 20mm widefield are the same item. Just the job!

I looked long and hard at different options, and have seen the 20mm under different brand names (including TS, Orion and Lumicon). I have just seen the Prostar has gone up a lot in price. It cost 90 AUD, total price with shipping 117 AUD. The Aussie dollar was only 0.67 euro at the time, that meant 78 euro, adding import duties and handling 90 - 95 euro, just a bit more than the TS. Given the strength of the Aussie dollar, you have to wonder why the price has gone up so much.

[AndyH]

Luckily I'm a bit of a Tal collectoholic and have a lot of their 25mm plossls and a few of their x-hair recticles. The latter do cut down the fov, but I'm sure you'll all agree that it's impossible to keep paying out big bucks for top quality illuminated eyepieces.

A note to fellow tinkerers. Astroboot has often got £3 diagonals. Perfect for the job. ie: perfect for a low power work.

In the past, I've used Ray, of Raymac engineering, to make my adapters between the bino body and diagonal. When I can scrape together some dosh, I'll do so again.

Andy.

[russ]

Luckily I'm a bit of a Tal collectoholic and have a lot of their 25mm plossls and a few of their x-hair recticles. The latter do cut down the fov, but I'm sure you'll all agree that it's impossible to keep paying out big bucks for top quality illuminated eyepieces.

A note to fellow tinkerers. Astroboot has often got £3 diagonals. Perfect for the job. ie: perfect for a low power work.

In the past, I've used Ray, of Raymac engineering, to make my adapters between the bino body and diagonal. When I can scrape together some dosh, I'll do so again.

Andy.

Andy, is that Ray of Ray's rings fame?

I need a cheap diagonal, forgot about Astroboot (how did i manage that?). Will pop in there now.

[AndyH]

That's the very man.

Andy.

[russ]

Handy to know he does the adapters for the bino body. Have you used him before that task? Was it costly?

You have me thinking

[michael.h.f.wilkinson]

BTW, using human hair (such as I have left on my scalp ) I have made very useful reticle EPs from old binocular EPs. Many nowadays sport 65-70 deg FOV, ideal for the purpose. I have not tried illuminating them yet.

[AndyH]

Wasn't costly.

Nice work.

Something to bear in mind is the length of the adapter. Will it fit in the finder bracket length wise? The ones I have for my 60mm and 2 previous 80mm's had to be longer on the bino side of the adapter.

Here's how I did it with mine.

Sit an eyepiece on a steel rule and pull the 'piece back til it comes to focus on a very distant object or star. The measured distance back is what the adapter needs to be if used in straight thru mode. Subtract approx 80mm from this. That's what a std diagonal uses up. Okay. Now check that this length will fit between the finder brackets rings. If too short you'll need to get a flange machined in to the adapter, on the bino side to compensate. Be aware this will add to the diameter of the adapter. So check that this diameter will still fit.

If all is okay, then there still may be a problem. ie: not enough room for a plastic thumbscrew to stick out the side for the diagonal. I use a plastic grub screw. I tension it up on the diagonal before putting it in the rings, as the latter probably cover the threaded hole.

Hopefully if you have a look through the first few pages of my flickr a/c, they may better explain my fuddled words above.

Flickr: bogershowie's Photostream

Andy.

[AndyH]

BTW, using human hair (such as I have left on my scalp ) I have made very useful reticle EPs from old binocular EPs. Many nowadays sport 65-70 deg FOV, ideal for the purpose. I have not tried illuminating them yet.

Nice idea. Sadly I shave my bonce, so no joy there, for me

I've been experimenting with good old fashioned fuse wire. Just need to find some tube to stick the wire on, which I can then insert into eyepieces. Whilst the Tal reticules are great, they DO restrict the fov, considerably. ie: A 25mm Tal plossl gives me about 3.5 degrees(guesstimated) on my 80mm finders. With the reticule screwed in, it drops to 2.6 .

The work of a tinkerer is never ending

Andy.

[michael.h.f.wilkinson]

I took a cruddy moon filter, removed the glass (plastic really) and glued the hair on. I then screwed that into a 1.25" EP tube from a discarded (Christiaan would turn in his grave type) Huygens 6mm EP. I inserted the bin EP in that, and adjusted it until the cross-hairs (literally) were in focus. Still prefer the Prostar solution, but that is mainly because the EP in question was a 28mm Kelner: 50 deg AFOV and exit pupil on the high side.

[michael.h.f.wilkinson]

I have just noticed that if I get the TS 32mm 2" Erfle, and use the WO 2" 90 deg Amici prism, that would turn my 80mm APO into a 15x80mm erecting, 90 deg finder with 4.6 deg true FOV . I should not think these kinds of thoughts

[russ]

I have just noticed that if I get the TS 32mm 2" Erfle, and use the WO 2" 90 deg Amici prism, that would turn my 80mm APO into a 15x80mm erecting, 90 deg finder with 4.6 deg true FOV . I should not think these kinds of thoughts

That would be a seriously good finder!