Using a 2x barlow

[Graman (GR)]

Hi, I only started in this hobby late last year, so a relative newbie. However, the COVID-19 isolation and WFH means that I am getting many opportunities to scan the skies at night and I am really enjoying it!

I recently purchased a 2" Bintel/GSO 2 x Barlow to use with my SW72ED refractor scope (essentially to double the magnification of my existing EPs).

However, I can't seem to get any of my existing EPs in focus with the 2 x Barlow. They are generic 10mm, 15mm and 25mm Plossls (nothing fancy). Looks like I need more travel inwards/towards the scope to get them in focus. Does that make sense? They seem to focus fine without the Barlow.

Question - is there anyway to solve for this? Would purchasing a focal extension tube help? If so, what length do I need to get - looks like Bintel/GSO have them at 35mm, 50mm or 80mm lengths. Is is better to go longer than shorter?

Thanks in advance!

GR

[Dave88 (Dave)]

Hi GR,

I can't really offer any explanation because I'm very much a novice but I've had exactly the same issue when using my Skywatcher ED72 and GSO 2" 2xbarlow.

I found a 1.25" TS optics branded barlow in the cupboard which is 80mm long and with this barlow I can achieve focus.

I'm keen to hear the advice from others though because I'd prefer to use a 2" barlow myself. The 2" lens seems to make the eyepiece assembly a lot more rigid.

Cheers

[raymo]

I am assuming that you are inserting the diagonal into the scope, and inserting the barlow into the diagonal, and then inserting the eyepiece into the Barlow.
You can insert the Barlow between the diagonal and the scope, and insert the eyepiece into the diagonal as normal. Most refractors will allow the use of a Barlow using one or other of these two arrangements, and sometimes both.
Note that the two positions of the Barlow in the optical train give different amounts of magnification. If I remember rightly one gives the expected 2x and the other gives about 1.5x.
There are also the so called shortie Barlows that can help with this problem, being much shorter than the usual Barlow.
raymo

[Graman (GR)]

Yes, looks like we are facing similar problems. It would be great if someone more experienced can shed some more light.

Just out of curiosity, is the TS optics barlow shorter in length that the GSO Barlow? Are you using a 2 inch diagonal as well? I am using one connected directly to the focuser.

Common sense would suggest that a focal extender will not help as it increases the distance from focus point to eyepiece. What we need is something that will reduce that distance.

Perhaps some one more experienced can comment.

[Graman (GR)]

Oh, I didn’t realise. I will try the second way you describe and connect the Barlow straight to the scope and see if it makes a diff.

Thanks for the suggestion!

Originally Posted by raymo

I am assuming that you are inserting the diagonal into the scope, and inserting the barlow into the diagonal, and then inserting the eyepiece into the Barlow.
You can insert the Barlow between the diagonal and the scope, and insert the eyepiece into the diagonal as normal. Most refractors will allow the use of a Barlow using one or other of these two arrangements, and sometimes both.
Note that the two positions of the Barlow in the optical train give different amounts of magnification. If I remember rightly one gives the expected 2x and the other gives about 1.5x.
There are also the so called shortie Barlows that can help with this problem, being much shorter than the usual Barlow.
raymo

[astro744]

If you need inward travel then an extension tube will only make it worse for then you will need to add the length of the extension to the amount if inward travel that you need.

I just tested with my ED80 (not original focuser as I bought used and previous buyer replaced the focuser for better astrophotography). In any case I was using a Bintel 2x Quartz Star diagonal and Bintel 2x Barlow early non-ED model. Without Barlow I have 33mm inward travel remaining. With Barlow I have 11mm inward travel remaining. I tried with the following eyepieces:; 30mm 1rpd, 40mm Meade 5000 Plossl, (both 2") and also with 25mm 1.25" Bintel Plossl. The 2"-1.25" adapter I used was the one that came with the Barlow, recessed type with zero length added. I got the same result to within 1mm.

I put the diagonal into the telescope first and then the Barlow into the diagonal. This gave me the above result. I then tried with Barlow into telescope first and then diagonal into the Barlow. I could not get focus at all, not even close at both ends of the travel.

I just packed it away and as I'm writing this I thought I should try with a 1.25" diagonal and 1.25" Barlow as well as a 2x, 2" Powermate. I'll try this later and report back.

Additional content:
I checked with ED80 with 2" diagonal with 2"-1.25" adapter and 25mm Plossl and then the same but with Tele Vue 1.25", 2x Barlow insterted into the 2" diagonal with 2"-1.25" adapter and remaining inward travel was 33mm. The Tele Vue 1.25" 2x Barlow is parfocal and adds zero length.

Your best bet is to use a 1.25" Barlow especially if only using 1.25" eyepieces. The 2" Barlow is taking up too much focuser travel that your ED72 does not have to spare.

[doug mc]

You could try unscrewing the 2 inch GSO barlow lens cell and screwing into the front of the 2 inch diagonal. This will extend the focal plane out further, but not as much as inserting the whole barlow in first.

[Graman (GR)]

That is a great idea. I will unscrew it and try. Hopefully the weather clears up this week!