Takahashi FC100-DL help... pt.2

[Mr Jones]

Righto, people. Finally treated to clear skies last night, and managed to get a couple of shivery -2 degree hours under stars.

Putting the issues I have with my iOptron AZPro to one side for now, I could do with some of your expert wisdom.

So after the kind help of a couple of SGL members here, I managed to get the extender tube required to have all my eyepieces come into focus mid way through the available travel (was a 40mm Baader item).

However, I could not get my 2x Tak barlow to come into focus when mounted pre diagonal (as most sources state is required to reach 2x). 

Running it post diagonal does make focus possible, but certainly doesn't look like 2x to me. Plus, I am not confident about inserting it fully as I feel the end of the barrel is going to contact the prism.

So why would the above be happening? What am I not considering here?

[John]

Is this the barlow ?:

https://www.rothervalleyoptics.co.uk/takahashi-2x-barlow-lens-125.html

It does say that the amplification if used after the diagonal will be diminished.

It seems an odd design to me - a "shorty" body and 1.25 inch barrel but a rather long optical element section with the potential, as you say, to impact a diagonal prism / mirror surface

There is a very recent thread on another forum where folks discuss similar issues that they have had with this barlow:

https://www.cloudynights.com/topic/749134-takahashi-2x-barlow-vs-others/

Edited January 24, 2021 by John

[Mr Jones]

Thanks, John.

Yes; thats the one.

I struggle to understand why I can pull focus post diagonal but not pre diagonal. 

If it means having to pull the back end apart to fit extra spacers mid session, I think I might have to send it back and invest in a TOE to get that extra mag.

I'd certainly not be happy shoving it into my prism. I fear that's asking for trouble.

[John]

The only barlow that I use with my FC100-DL is the Baader 2.25x Q-Turret which is used after the diagonal and, while eating up a bit of inward travel, comes to focus OK.

If I put that before the diagonal it gives more amplification (3.5x I'd guess) but also needs a lot more inwards travel so a similar situation. The Baader has the optical elements within the 1.25 inch barrel though so the risk of it impacting the prism is much lower.

The Baader is probably not as good optically as the Tak of course. I'm fortunate to have a good range of short / very short focal length eyepieces so I tend to opt for those more often than using a barlow.

It seems to be quite complex to assess the impact that a barlow will have on the focal point when used in different configurations. I guess someone with optical design expertise could work it out if the focal length of the barlow is known 

Is the focal length of the Tak 2x barlow given anywhere ?

 

[Mr Jones]

Have not found it yet, but will search...

[GazOC]

The distance from the Barlow to the eyepiece gives the magnification by increasing the focal length of the system. The further away, the greater the magnication. So although you should get x2 before the diagonal, by putting the Barlow after the diagonal (nearer the eyepiece) you'll get a lot less.

This also moves the focal point differing amounts. With the Barlow before the diagonal the focal point will be further out than with the Barlow after the diagonal 

 

[John]

49 minutes ago, GazOC said:

The distance from the Barlow to the eyepiece gives the magnification by increasing the focal length of the system. The further away, the greater the magnication. So although you should get x2 before the diagonal, by putting the Barlow after the diagonal (nearer the eyepiece) you'll get a lot less.

This also moves the focal point differing amounts. With the Barlow before the diagonal the focal point will be further out than with the Barlow after the diagonal 

 

I've just been doing some experiments with my Vixen ED102SS (focal length 663mm), the Baader 2.25x barlow and the Baader T2 prism and zoom eyepiece at 21.5mm. Target was the top of a conifier about 200 metres away. Interesting results:

- No barlow = focuser drawtube 55mm extended at focus.

- Barlow in the diagonal (so 2.25x amplification) = focuser drawtube 49mm extended at focus so a manageable amount of inward focuser travel.

- Barlow before the diagonal (much higher amplification - maybe 4x-5x ?) = ran out of OUTWARD travel of focuser. With focuser drawtube at max extension and holding the barlow / diagonal / eyepiece unit at sharp focus I reckoned the extension would be around 115mm so much, much further out. Matches @GazOC's prediction but more so than I expected.

How relevant to your situation this is Mr Jones, I don't know and the proximity of the target might have made a difference as well but it did demonstrate that the difference in focus point, depending on where you place the barlow, can be very significant. I expect the optical element of the Baader 2.25x has a shorter focal length than that of the Tak 2x barlow.

Kept me busy on a boring Sunday morning anyway

 

Edited January 24, 2021 by John

[Mr Jones]

Thanks for the investigative work. Very interesting. 

I'm still not quite sure I truly understand why mag is affected so much dependant on pre or post diagonal. But at least now I have a clear choice to make...

Purchase another Baader 40mm extension (I'm guessing that would be enough?) and cope with the strip down and rebuild in the dark scenario; or ditch the barlow and invest in TOE or similar hi po eyepiece/s.

[GazOC]

It's easy to spend other people's money but my choice would definitely be to bite the bullet and buy the dedicated high power eyepieces you need 

There's only so much faffing around with parts you want to do on a cold night and the Tak would look a lot nicer without 100mm of plastic hanging off the back end 😉

[mikeDnight]

I felt my Tak Barlow was more trouble than it was worth and sold it not long after buying it. It did come to focus on my DC with Tak prism though. In side by side comparisons I found the Skywatcher Delux barlow gave an image that was just as good, and that fit post diagonal. However, the nose of the Barlow can touch the prism surface, so a spacer ring or a couple of rubber O rings can be fit over the barlow's nose to prevent contact.

[Paz]

I have always preferred high power eyepieces to using barlows but since I've got into binoviewing this issue of varying the distance between a barlow and the eyepiece has become something I use pro actively to get the right amount of infocus outfocus as required. It seems to be as much an art as a science!

[JeremyS]

I'm not a Barlow sort of guy, but I do have a Tak 2 x Barlow and it is indeed designed to go after the diagonal.

[Mr Jones]

So; the Tak barlow is winging its way back to the retailer. I'm not content to run it post prism and cause damage while fumbling in the dark.

So now the burning 🔥 question... which TOE would be best at delivering high mag views on the DL?

I hear you can push these scopes hard, but will the 3.7mm exit pupil of the 3.3mm be going too far?

Would it be a LOT better (more pleasant to use, and get to use more often) reducing to the 4mm?

All thoughts appreciated... the TOE are expensive, and I'd rather not blindly make a mistake. 

[John]

I don't have a TOE but I do have Pentax XW's and a Nagler 2mm-4mm zoom. When observing targets which benefit from high powers 4 mm seems to be my starting point with the DL and I'm often using 3.5mm, 3mm and even 2.5mm now and then. At 225x to 300x the scope seems quite comfortable. 

I have considered the Tak TOE's and, if I ever do go that way, I would actually want to own both the 4mm and the 3.3mm for sure and would be highly tempted by the 2.5mm to compete the set.

 

[andrew s]

On 24/01/2021 at 10:05, Mr Jones said:

Thanks, John.

Yes; thats the one.

I struggle to understand why I can pull focus post diagonal but not pre diagonal. 

If it means having to pull the back end apart to fit extra spacers mid session, I think I might have to send it back and invest in a TOE to get that extra mag.

I'd certainly not be happy shoving it into my prism. I fear that's asking for trouble.

If the Barlow is placed too far in (more than its focal length inside the native focal point) then the telescope plus Barlow will not produce a real image. Regards Andrew 

Edited January 26, 2021 by andrew s

[Mr Jones]

15 hours ago, John said:

I don't have a TOE but I do have Pentax XW's and a Nagler 2mm-4mm zoom. When observing targets which benefit from high powers 4 mm seems to be my starting point with the DL and I'm often using 3.5mm, 3mm and even 2.5mm now and then. At 225x to 300x the scope seems quite comfortable. 

I have considered the Tak TOE's and, if I ever do go that way, I would actually want to own both the 4mm and the 3.3mm for sure and would be highly tempted by the 2.5mm to compete the set.

 

Pretty exciting to hear the DL can handle those sort of mags. 

I think I'll plump for the 4mm as a stage one, and if my vision isn't like looking through a petra dish full of microbes, I'll get the 3.3mm too.

Thanks again for all your help, John.