Televue Powermate 2x or 4x for imaging (with 105mm scope)?

[NickK]

I wanted some feedback before splashing out on a 2" powermate.

My uncertainty comes more from the imaging side rather than the visual side so I thought I'd plonk this in this sub forum for the best input.

Visually the scope with a 13mm EP will easily show detailed bands on Jupiter, split the trapezium on M42. At 192x (3.5mm) the average local conditions make it more difficult.

The scope is a 105mm APO (670mm FL) with field flattener built in, and either a 838L (5.4um pixels) or Titan (7.4um). This means with the 383L attached I get 1.67 arcsec/pixel. I think the manual states around 1 arcsec for the maximum resolution (I take it this when the APO light convergence has issues).

2x = 1340mm FL at f/12. 0.80 or 1.14 as/px (383/Titan), 4x imaging time

4x = 2680mm FL at f/25. 0.41 or 0.57 as/px (383/Titan), 16x imaging time

I'm leaning towards the 2x for bright DSOs and planet use. In my view it would max out the optics, the EQ6 and the camera sensitivity.. but something still says - go for the 4x ..

Lastly I have permission with the express command to order it now (for SGL7)

So with the limited aperture (although excellent optics) I'd love some feedback for the 2x or 4x before I push the buy button.

[Macavity]

Wot, no answers?

But you did cast the problem so clearly and I am thinking of similar things. I have a very different setup (MAK150, Filter wheel, Watec Video camera). Working my way through... Nyquist Limits, Over/Under sampling, opimising the COMPONENT order etc.

With my larger (8.5 micron) pixels, I had imagined around f/24 ... or a bit more. (2-3x Barlow with my f12 scope!) I'd suggest you'd be reasonably safe with f/24? Ain't that why they make 5x Powermates - To match fast f/5-6 APOs? Just to stimulate discussion - Don't take *my* word for it.

Aside: Just bought myself a 2" Barlow, with ambitions to mount the bare lens element scope side of the filter wheel. But I sense that gives me at least 4x magnification. Doh! It would be neat to "lose" some of the hardware inside the focusser tho'. I note they do such things with focal reducers, but...

[NickK]

Dawes' limit is a formula to express the maximum resolving power of a microscope or telescope. It is so named for its discoverer, W. R. Dawes, although it is also credited to Lord Rayleigh.

The formula takes different forms depending on the units.

R = 4.56/D D in inches, R in arcseconds

R = 11.6/D D in centimeters, R in arcseconds

where D is the diameter of the main lens (aperture)

R is the resolving power of the instrument

So R= 11.6/10.5 = 1.10 arc seconds, so I'd be pushing the limit according to Mr Dawes! After this I'd start seeing blurring (depending on the wavelength).

Two times it is!

Edit: just reading up further:

The actual formula is R = l / D. where R = resolving power in radians, D = diameter of the objective lens, l = wavelength of the light.

So if you are narrowband imaging with a lower wavelength, l, then you could push up the resolution further..

[NickK]

Picked up the 2x 2" Powermate with a quick trip to Baker Street, so I'll give you all an update soon of my impressions (I may be back home quick enough to have a look tonight ).

First impressions - big and heavy! Think of someone has taken a 2" tube and filled it with glass..

[NickK]

Ok. Had a play from inside (through window glass), no tracking or guiding, as it's cloudy and a full moon.

Initial impression is very good although I am still waiting for a proper Powermate + 383L+ test as the moon is too bright for the 383's minimum shutter speed, I could focus on the curve of the moon. Only a week or so till SGL7

Getting stars in the 16IC I thing will be possible through the OAG too as it could pick up the curve of the moon.

The Titan is very good with it as it can cope with the brightness by using 0.01 or 0.001 sec shutter speed.

EPs - awesome, although the 3.5 EP (192x) has problems but I think 384x mag is an interesting option when the seeing is good and I'm not looking through glass. The 20x 31mm & 52x 13mm really come alive.

No blurring or aberrations for EP or either cams, focusing is really sensitive and the moon was moving at quite a rate through the 1370mm fl FOV! Different back focus points too so the pentax's tank like focuser and 2" image train doesn't budge with the cams hanging off it!

[NickK]

Crop of a single 1 second exposure using the 2x on m42 trapezium. Note the aligned was fixed after... but there's separation on E without any processing and it looks like (with good conditions+alignment) that F would pop out too!

The image was at 0.80 arc sec/pixel! Dawes Limit on the scope being 1.10!

Visually the planets were good, the 2x192x was a little burred due to the conditions. Jupiter had lovely bands!

[Catanonia]

I have always wondered whether this is possible or you might loose detail with extra sub standard optics.

I did try it with a ED120 Pro and a 2inch Skywatcher delux 2x barlow and the results were dismall with coma etc.

Now I have my SuperNewt at F2.8 and 568mm, but natively without thereducer it could run at F4 800mm, now add a barlows x2 in to the mix then I have F8 1600mm which is quite doeable really.

But problem is getting a GOOD optic 2x 2 inch barlow and getting the field flat.

[NickK]

Now I have my SuperNewt at F2.8 and 568mm, but natively without thereducer it could run at F4 800mm, now add a barlows x2 in to the mix then I have F8 1600mm which is quite doeable really.

I was quite surprised that the EQ6 did 1340mm fl for two 20min exposures of M82.. the only snag is that the OAG prism dropped into view (now repositioned and the grub screw is tightened!)

I have a sneaking suspicion that dark skies are needed to get the best out of a pseudo-F12 4" APO at that resolution. Fingers crossed for SGL7.

But problem is getting a GOOD optic 2x 2 inch barlow and getting the field flat.

That's the problem - one bad lens in the imaging train drags it down to the lowest common denominator. I'm lucky the SDP's natural field is as flat as a pancake without any coma.