Omegon manual 9-position filter wheel review

[Martin Meredith]

I recently bought a manual filter wheel in order to pursue multi-spectral near-real time viewing in the forthcoming version of LodestarLive and thought it might be of interest to others to see how it all fits together.

The model I opted for was the Omegon 1.25" wheel with space for 9 filters from astroshop mainly as it holds all the filters I'm ever likely to need and also acts as a convenient dust-free storage for them all. The all-metal construction feels very robust.

Here are the components I needed to use it with the Lodestar. (It also shows an approach if using just a single filter via a C-thread nosepiece from FLO i.e. without the filter wheel.)

The filterwheel has a 1.25" barrel to attach to the focuser. I use an Antares twist lock, but the barrel can be inserted into any standard focuser tube. On the camera side, the wheel has both a T2 and M48 thread via the supplied adaptor, but to use it with the Lodestar I needed a T2 to 1.25" adaptor. The one shown is the TS standard profile model (from Rother Valley Optics).

The filter wheel is opened via 4 Allen bolts with the supplied Allen key. These shots show the various components.

and reversed

and here with LRGBC and HOS filters, plus an empty slot for straight thru viewing.

I found it more economical to buy the colour and narrowband filters as a package from Baader. The advantage is that they are parfocal -- important for their intented use in near-real time viewing. The clear [C] filter is used for ease of focusing.

Here's the kit mounted on the scope.

Manual filter changing is done by turning the large serrated outer edge of the filter holder at the horizontal cutout (shown at the bottom in this shot). When the bolts are tightened sufficiently to eliminate light leaks the filter wheel is quite stiff. While this is reassuring for precise filter placement with no slippage, I was concerned that at the scope the movement might cause focus change or even scope movement, but so far (after one session) this has not been an issue. In any case, if necessary it would be possible to loosen the bolts a little and perhaps use a thin strip of rubber or foam to guard against light leaks -- time will tell if this is necessary.

A minor irritation is that the numbers on the filter wheel face towards the scope itself, so with the apparatus mounted on my Newtonian it was tricky to see them. Next time I'll try orienting the wheel differently. But this seems to be a design issue that could be resolved by having the cutout (and numbers) on the camera side of the wheel.

So far, after one 80 minute session in which I changed filters about a dozen times, using C to focus and both RGB and narrowband sets to image/observe, I'm very happy with the system. I normally use an extender with the Quattro to reach focus, but with the new kit I didn't need this: the Lodestar came to focus easily and I didn't need to refocus after locking down the initial focus. 

I'll update this if and when my experience changes.

Martin

[HiloDon]

Hi Martin,

Looks like a nice setup. I'm not sure I'll get into the multi-spectral yet, but I would like to have a system where I have three filters. One straight through, one blocked for darks, and one NB Ha. Does Baader make a full spectrum filter that can be used, so refocus isn't necessary with the straight through position?

Thanks,

Don

P.S. Maybe their UV/IR filter is parfocal. That would work, too.

[Martin Meredith]

Hi Don

The clear filter [C in the set LRGBC] is what seems to be required to avoid refocusing. I checked straight through vs clear and the difference is quite large (sharp vs donuts). Knowing little or nothing about filters and optics in general it surprised me a little that a 2mm piece of glass has a big effect on focus… but I guess it makes sense. I'm not sure about the UV/IR filter but it wouldn't surprise me if it were parfocal. I sure hope so to avoid paying for a piece of clear glass! This thread has some useful information on parfocality (is that a word?).

The dark blocking is a nice idea I hadn't thought of. So maybe I need 10 positions... Originally I was planning to purchase a narrowband set but I managed to find a good deal from Baader on Amazon including the LRGBC set for not a lot more, and having RGB does seem a good way to get more authentic colour into open and globular clusters, for instance. Later I'll post some of my initial results on a couple of open clusters but I was pretty happy with the blue/yellow distinctions I was capturing. Using the filter wheel is very fast and the whole process in LL is much less hassle than I anticipated due to the ease with which Paul has integrated multi-spectral captures.

cheers

Martin

[aparker]

Martin,

Thanks for this helpful review.  I'll be interested to hear how this works out for you long-term.  I have recently started to think about some way to swap out filters not necessarily for multi-color "real-time" viewing but for doing viewing with eg Ha or OIII and possibly getting into some scientific data gathering on variable stars using standard photometric filters.  Does this company also offer a motorized wheel, or a way to add automation to this manual product?

Alex

[laudropb]

Dry informative review. Thanks for posting.

[Paul81]

Nice review Martin, looking forward to seeing what the new kit can do!

[Martin Meredith]

Thanks all. Alex, I've not come across a motorised offering from Omegon but there are others out there that are USB-powered (and pricey). 

I've had the wheel out again and found that it needs to be gripped with the free hand while swapping filters to prevent field movement. This can be alleviated by loosening the 4 bolts, but this has the effect of letting some light in and also leaves too much play in the filter position. The tolerances are really fine though and a slight loosening produces negligible light leakage (measured with the camera compared to capping it) and does ease movement a bit. No doubt it will loosen up more with time but so far that is my only gripe. 

Don, I've taped a piece of flocking fabric across the spare hole to create a dark frame slide. Yet to see how well it works.

cheers

Martin

[Martin Meredith]

A quick update. After 4 nights' usage the wheel is loosening up a little (a good thing). Taking darks is now very easy with the cheap flocking solution. Finally, I discovered that the wheel can be mounted to face either way, since the adaptors on each side both unscrew and can be added to the other side of the wheel.

Martin

[Steve789]

Thanks for this review, I've just been looking at buying this filter wheel myself. There doesn't seem to be much choice out there for manual filter wheels with more than 5 positions.

Do you know what the flange to flange distance is of the filter wheel i.e. the back focus required to accommodate the filter wheel?

[Martin Meredith]

The casing itself is 17mm wide; to that you have to add on another 4mm for the flange on the scope side. On the camera side, in principle, if you have a camera that slides in a 1.25" barrel you can get the chip arbitrarily close to the filter (actually inside the filter wheel) so the back focus in this case is less than 17+4. If you're using the supplied adaptor on the camera side then that takes up another 6mm or so, not including the thread. So the worst case seems to be about 27mm taken up by the filter wheel.

i.e. scope + focuser tube + 4mm flange + 17mm wheel + 6mm camera adaptor

Hope that makes sense!

Martin

[Steve789]

Thanks Martin, that's very helpful. I think I'll go ahead and "add to cart".

I'm currently experimenting with an old 200mm M42 camera lens for my atik 314L+ to try and increase the field of view. My current filter wheel has a 22mm wide body which leaves me 3.5mm over the required 45mm back focus (plus it doesn't have enough room for all my filters) so the Omegon should work perfectly.

I just need to work out if I need to get any extensions to get the correct spacing for my coma corrector when I use it with my reflector.

It seems weird but enjoy this planning/purchasing phase of astrophotography almost more than actually going out there and doing it!