Adding a lp filter/filterwheel - help and advice needed

[Thalestris24]

Hi all

I have a qhy8l ccd attached to my 150pds - works great! However, I do suffer from bad lp and I think I'd benefit from a lp filter and I'd also like to be able to do some narrowband imaging.

So first - how best to attach a lp filter to the qhy8l? Via the T2 thread on the nosepiece? 

I'm thinking in terms of a filter wheel for other filters. The qhy8l has a nosepiece/lip for attaching to a 2" focuser (but needs an extension tube) and is threaded for a T2 adapter. To complicate matters I'd like to be able to substitute a mono 1100d for the qhy8l without too much faffing around and maybe even have a go at narrowband and/or LRGB with it one day . The problem here is that even on the pds scope there isn't much focus room when the 1100d is connected at prime focus so I think a filter wheel would have to be low-profile? Any recommendations? 

At the moment, the qhy8l connects to the 2" focuser via a 40mm T2 extension. This could be shorter - I just happened to already have it. It won't achieve focus without an extension.

Now I do currently get vignetting and coma with the qyh8l. I'm not sure if vignetting is inevitable or whether using 2" adapters (rather than T2) would eliminate it?

Anyway, so whilst I'm at it, I'm thinking a coma corrector would be a good addition So Baader MPCC or SW reducer - anything to choose between? Would either improve (or worsen!) the dslr back focus?

I suppose it would have to be a 2" filter wheel and filters??

Also, with my standard 1100d I have a clip filter for lp so would be able to use that with the mono camera. 

Has anyone also added filters to a qhy and/or a dslr?

Sorry if this all seems a bit messy...

Many thanks for any advice and help

Louise

[Lonestar70]

Hi Louise,

Tricky problem to solve, especially for the 1100d; where the cameras’ FFD (Flange Focal distance) is fixed at 44mm.

This does not leave much room for a filter wheel.

I use a Nikon camera (which has a FFD of 46.5mm) and on my 150PDS this focuses with about 14 - 15mm of draw tube protruding… you may get another 2 – 3 mm.

The thinnest filter wheels I know of are those from Ian King at 19mm optical path length… so even this would be of little help.

Filter wheels: -

http://www.iankingimaging.com/show_products.php?category=140

The only other option I can think of would be: -

Filter drawer: -

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p2159_Filter-Quick-Changer-incl--1x-2--filter-drawer---low-profile.html

This is only 15mm optical path length so may just work with the DSLR… the down side is that it is a manual filter change.

I believe the 2” nosepiece for this is fitted with an M48 x 0.75mm filter thread thus allowing a LP filter to be attached in addition to the LRGB filters.

Things get a bit more complicated when you also include a ‘COMA CORRECTOR’

The problem here is that the SW version (and I believe also the MPCC) both have a fixed back focus distance between the final element of the Coma Corrector and the camera focal plane of 55mm.

This is normally accomplished by using the correct camera flange adaptor specified for the Coma Corrector, the thickness of which are matched to the cameras FFD thus giving the required spacing by default.

Net result is that the scopes focuser has even less draw tube protruding, which, in turn, will negate even the filter drawer option.

Note... I get about 10mm of draw tube protruding when using the Coma Corrector.

The SW coma corrector has a 2” nosepiece which is threaded for attaching standard 2" filters (I am led to believe this also applies to the BDCC), however, this would mean removing the camera every time you wanted to change the filter… not really recommended.

It is, however, useful for fitting a LP filter, which can remain in position.

Things are somewhat easier with the QHY8 and one of the motorised filter wheels (linked to above) can be fitted between the scope focuser and the camera… or the Coma corrector and the camera with suitable length extensions to satisfy the back focus distances required.

These would need to be fitted between the filter wheel and the focuser when not using the Coma corrector.

When using the Coma corrector it would be necessary to add suitable spacers/extensions to achieve the 55mm requirement between the Comma corrector and the camera focal plane… if any further back focus distance is then required it would have to be fitted between the Coma corrector and the focuser (possibly using one of the Baader adjustable 2” extension adaptors (available from FLO).

A very useful set of spacer rings is available here: -

http://www.iankingimaging.com/show_product.php?id=1430

Using a 2” set-up will also help a lot with Vignetting.

I hope this helps a bit.

Best regards.

Sandy.  .

[Thalestris24]

Hi Sandy

Thanks for the reply! I was beginning to think I'd just be left on my own here... :'(

I suppose in the normal way one wouldn't be using ccd -type filters with a dslr but as it's mono... (or will be - I haven't got it yet!).

Oh well, it's looking like I'll have to abandon using it with the 150pds :( I suppose it will be a lot simpler if I only concern myself with qhy8l/150pds combo. Maybe I can just use the mono 1100d with my ST80. I ought to be able to add a filter wheel to that providing I can get one that physically fits - the ST-80 is piggybacked on the 150pds and there's not a huge space between the focuser tube and 150pds tube. Or maybe I should upgrade to an 80ED... Hmm...

So, with the qhy8l I can use a coma corrector with lp filter - that's good Do you find the SW 0.9x CC ok? I suppose, in my case, anything would be an improvement   I'd need appropriate spacers between the qhy8l and the corrector. Hard to tell exactly what's required.

I don't think I'd bother with a filter wheel just for the qhy8l.

Um, I couldn't find any adjustable 2" extension adapter on FLO, like you mentioned? T-spacers would seem to negate the benefits of using 2" components? On the other hand, the camera nosepiece is only 2" externally - from an inner perspective it's a T-thread.

Here's a pic of my current setup:

Thanks lots for the help

Louise

[Lonestar70]

Hi Louise,

I must apologise for giving you a wrong bit of info... The 2" adjustable extension turns out to be the baader adjustable t- extension... I was actually thinking of the adjustable baader helical focuser adaptor, however I got my wires crossed as this is for 1 1/4"... OLD AGE  

http://www.firstlightoptics.com/adaptors/baader-varilock-46-lockable-t-2-extension-tube.html

There is also an aternative version here: -

http://www.teleskop-express.de/shop/product_info.php/info/p2641_TS-Optics---variable-T2-extension-20-5mm-to-30mm.html

I would agree that the 150PDS/1100D mono combo is a virtual no no as there just doesn't seem to be a viable filter set-up possible given the available focuser travel, the addition of a good APO 2" x 2 or x 2.5 Barlow might just buy enough space for a filter wheel as this would move the focus point a bit further out and you could still use the coma corrector with it... I do this with mine for visual use, so I don't see any reason it would not work for Photo's... never tried it... the down side would be the need for longer exposures and tighter tracking due to the extended focal length.

My Nikon is still a colour sensor camera so filters don't come in to it... which makes life a little simpler.

The SW coma corrector works extremely well giving nice round star images right accross the field.

It also gives a slightly larger field of view as it is also a x0.9 focal reducer.

You could certainly use the 1100d with the ST80 and you would have plenty of room for a filter wheel, seeing as you would normally need to use a 50mm extension to the focuser (to replace the optical length of the diagonal)

The filter wheel can be mounted with the wheel housing upwards rather than downwards which should fit in the space available between the 2 OTA's... the only issue you would then have is correcting the focus between RGB filters... more especially the blue... as the ST80 is poorly corrected for this colour.

The ED80 would also work in much the same set-up, but without the RGB focus problem.

Having said this, the ED80 really needs the field flattener/focal reducer (FF/FR) for best results and this would be just as tricky as the 150PDS.

The reason is that the ED80 FF/FR does not require the 50mm extension, it screws directly on to the draw tube, and the Camera focal plane must be 55mm from the rear element ( the same as the coma corrector)

I am now set up to use my ED80 for astrophotography... just need some spare time and clear skies... it's hard to get both at the same time...   and tend to use the 150PDS more for visual... I find camera positioning much simpler with the Frac.

Using the QHY8L with the coma corrector should be reasonable straight forward... your camera FFD is 20mm and the coma corrector is 55mm... 55 - 20 = 35mm extension length required... the FLO adjustable above will give you this.

I am currently saving penny's for a CCD camera and filter wheel so will need to do some homework on this subject myself.

Hope this helps.

Best regards.

Sandy.

[Thalestris24]

Hi Sandy

No worries! The adjustable extenders look ok providing they don't introduce the dreaded flex  and if they stay in the adjusted position.

I suppose I could ditch the 1100d + filter wheel idea and maybe just fit an Ha filter, or splash out on a ED80 but skip the coma corrector. It would be a shame to waste all those mono pixels! I'll have to give it some thought. There must be a way forward here! Tbh the ST80 isn't the best for dso's but I have some affection for it. Still, I should try and fit the mono 1100d to something decent really.

But then I'd really need another mount too - eeks! £££  I have to admit I've been toying with that idea though it's hard to justify really. I'll sleep on it for a few weeks... Or lie awake fretting whilst obsessively checking my bank balance...

Thanks for your input - it's helped me think about it all more clearly

A cooled ccd is much better than a dslr but different to work with in several ways. I definitely couldn't afford, the mono ccd equivalent of the mono 1100d so I'll have to live with the noise! And the clicking! But at least I won't have to wait for it to cool down and warm up . As I say, I don't actually have it yet but it should arrive before too long.

Good luck with your own ccd endeavors

Cheers

Louise

[sidelight]

There is a Baader Rowe coma corrector that has a longer (91.5mm) backfocus, I believe is has been designed so it can fit inside the focuser tube completely.  I did ask if anyone had used one a while back but had no takers, this is exactly what I have been considering and trying to work out if it would work with my DLSR.  As far as I can tell would allow the coma corrector to sit inside the focuser tube and then if you have 20mm or so of spare back focus that might let you have the inward travel you need to get focus with a DSLR and filter wheel.  I'm still trying to work out if this would work myself.

Cheers

Ross

[wxsatuser]

There is a Baader Rowe coma corrector that has a longer (91.5mm) backfocus, I believe is has been designed so it can fit inside the focuser tube completely.  I did ask if anyone had used one a while back but had no takers, this is exactly what I have been considering and trying to work out if it would work with my DLSR.  As far as I can tell would allow the coma corrector to sit inside the focuser tube and then if you have 20mm or so of spare back focus that might let you have the inward travel you need to get focus with a DSLR and filter wheel.  I'm still trying to work out if this would work myself.

Cheers

Ross

It could work.

There would be enough backfocus for FW etc as there is 91.5mm and for the dslr some extension like a Baader Varilock 46 would do the trick.

[Thalestris24]

There is a Baader Rowe coma corrector that has a longer (91.5mm) backfocus, I believe is has been designed so it can fit inside the focuser tube completely.  I did ask if anyone had used one a while back but had no takers, this is exactly what I have been considering and trying to work out if it would work with my DLSR.  As far as I can tell would allow the coma corrector to sit inside the focuser tube and then if you have 20mm or so of spare back focus that might let you have the inward travel you need to get focus with a DSLR and filter wheel.  I'm still trying to work out if this would work myself.

Cheers

Ross

Hi Ross

Thanks for that Might be worth considering. Mind you, I'm always a bit wary when specs say 'perfect at f4, great at fx'. I mean, what's the difference between perfect and great? Still, I expect it would improve - I'm beginning to notice the coma on my setup more and more. I guess it wouldn't be suitable for use with an ED80 though. There is an old thread here: http://stargazerslounge.com/topic/160335-baader-rcci-rowe-coma-corrector-so-far-not-impressed/ Unfortunately it was left unresolved - frustrating!

Cheers

Louise

[Lonestar70]

Hi Louise,

The coma corrector is only required for your 150PDS... for an ED80, or your ST80 for that matter, you need a field flattener.

Coma is caused by the mirrors in a newtonian scope distorting stars at the edge of the field... which appear to have short tails (like tadpoles).

Refractor scope lenses produce a curved image field with respect to the cameras sensor... i.e. the centre of this image is closer to the sensor than the outer edges of the image.

Somewhat different issues but both need correction for imaging.

It can get a bit confusing at times.

Hope you can find a solution

Best regards.

Sandy.

[Thalestris24]

Hi Sandy

Yes indeed! I'm guilty of being mentally liberal above - suffering from old brain cells too! In any case, it's only the coma on the 150pds which is of concern at the moment

Have you looked at the Rowe cc?

Cheers

Louise

[Lonestar70]

Hi Sandy

Yes indeed! I'm guilty of being mentally liberal above - suffering from old brain cells too! In any case, it's only the coma on the 150pds which is of concern at the moment

Have you looked at the Rowe cc?

Cheers

Louise

Yes, and like you I am not entirely convinced... in theory it should work, however, I have not seen any solution put forward to the issues highlighted in the link... and it's an expensive gamble I would hesitate to make.

My gut feeling is that it is critical on how far into the focuser the first element is placed... making it somewhat unpredictable from scope to scope.

There must be a solution to this somewhere.

Keep happy.

Sandy.

[Thalestris24]

^Hi

^{Just thought I'd post this.}

^{There is an example and discussion about using the Rowe cc here:}

^{http://scottastrophe.smugmug.com/Astronomy/The-Journey-Continues-2012/i-52CKf6K}

^{http://scottastrophe.smugmug.com/Astronomy/The-Journey-Continues-2012/i-HCjnXgX}

^{It looks like the positioning of the cc is quite critical, vignetting is marked, and stars are distorted (reason?). Am I put off? Maybe. Will keep an eye open for other possibilities.}

^Louise

[Lonestar70]

^Hi

^{Just thought I'd post this.}

^{There is an example and discussion about using the Rowe cc here:}

^{http://scottastrophe.smugmug.com/Astronomy/The-Journey-Continues-2012/i-52CKf6K}

^{http://scottastrophe.smugmug.com/Astronomy/The-Journey-Continues-2012/i-HCjnXgX}

^{It looks like the positioning of the cc is quite critical, vignetting is marked, and stars are distorted (reason?). Am I put off? Maybe. Will keep an eye open for other possibilities.}

^Louise

Hi Louise,

Well that seems to confirm my gut feeling.

I have thought of a possible solution that would allow you to use both the SW coma corrector and also a filter wheel.

Take a look at the set-up 2nd from left at the bottom of the attached PDF.

I use a hyperion zoom lens with the SW coma corrector on my 150pds and it works extremely well... I have also used it with the ED80 for visual astro and wildlife viewing.

This particular set-up can be used for eyepiece projection with a DLSR and of particular interest is the t2 extensions used to increase image scale.

The  40mm - 55mm or more T2 extension pieces could easily be replaced by a filter wheel  and shorter extension which would give you what you are looking for with your Mono DSLR... it would also allow some control of image scale by adjusting the length of the short extension.

I would be inclined to use a fixed focal length eyepiece, such as the 24mm which will give you the widest field of view and is a lower profile eyepiece... fit this into the SW coma corrector and add the filterwheel etc to the eyepiece.

The only objection would possibly be the overall length of the image train sticking out of the focuser, however, if the scope tube is rotated in its rings such that the image train is vertical it should not adversly effect balance.

I am tempted to try this on my 150PDS with the Nikon, just to see what length of T2 extension is needed to get the image covering the whole of the sensor.

Might be worth consideration.

Keep happy.

Best regards.

Sandy.

hyperion_overview.pdf

[Thalestris24]

Hi Sandy

Thanks for that. I'll bear it in mind but I'm not sure I want to get into eyepiece projection - I'd like to try and keep things as simple as possible! I think, for now at least, I'll just use the qhy8l with the 150pds and get a coma corrector and lp filter. As for the 1100d, I'm thinking about various options... I might just use it in widefield for now. I could just piggyback it onto the 150pds and use it with the 75-300mm lens. I could forget about the filter wheel and only use it with an Ha clip filter.  Or... get an ed80 (or another frac) and maybe use one of these field flatteners:

http://www.teleskop-express.de/shop/product_info.php/info/p1010_TS-Universal-2--Field-Flattener-for-Refractors-f-5-to-f-8---Flat-Field-for-Astroph.html

It seems to have a much bigger back focus

I'll have to see if I can find performance info/examples

Cheers

Louise

[Thalestris24]

I see there's a TSFlat2 review here:

http://stargazerslounge.com/topic/120669-ts-2-field-flattener-tsflat2-review/?hl=%2Btsflat2#entry1202547

The with-flattener images are not perfect but there could be other reasons for that. Still, the correction compared to the without-flattener image is apparent. +1 for this option at the moment

[Lonestar70]

I see there's a TSFlat2 review here:

http://stargazerslounge.com/topic/120669-ts-2-field-flattener-tsflat2-review/?hl=%2Btsflat2#entry1202547

The with-flattener images are not perfect but there could be other reasons for that. Still, the correction compared to the without-flattener image is apparent. +1 for this option at the moment

Hi Louise,

Very interesting... I have not seen this one before... thanks for the links.

It appears to be along similar lines to the Baader Rowe RCCI... with one big difference... this one works.

The test images shown in the last link clearly show how the curved field causes the star shape to get more and more oval, in a radial pattern, towards the edges of the image without a flattener.

The small remaining errors shown ,with the flattener, can probably be fixed with some small variations to spacing.

It will probably need a good deal of playing about with it to find the spacing sweet spot for the particular camera (and possible filterwheel), but once found it would stay fixed.

It does not appear to introduce much to worry about in the way of vignetting either (probably due to the much larger internal dimensions of the 2" section) and it can also take a fixed filter in the nose piece (LP).

I would imagine it would work very well with an ED80 and your 1100D (including a filter wheel).

Great find.

Best Regards.

Sandy.

[Thalestris24]

Hi Sandy

I'm busy drawing up a wish list Alas, I didn't win the lotto today Oh well and at least I know there are possibilities for the mono 1100d.

I'm quite surprised at the number of views this topic has had. I don't know how many other people with dslrs might be interested in attaching filter wheels - maybe a few with modded cameras.

Cheers

Louise

[Lonestar70]

Hi Sandy

I'm busy drawing up a wish list Alas, I didn't win the lotto today Oh well and at least I know there are possibilities for the mono 1100d.

I'm quite surprised at the number of views this topic has had. I don't know how many other people with dslrs might be interested in attaching filter wheels - maybe a few with modded cameras.

Cheers

Louise

Hi Louise,

 I gave up on wish lists... every time I did one the decimal point kept moving itself to the right as if by magic  

Yes there have been a good few viewing this thread... possibly because the problem is a common one, even for CCD users when you are restricted to the more usual 55mm back focus of coma correctors and field flatteners... as we have seen during the discussion this creates a problem in using a filterwheel regardless of camera type and a lot of people use Mono CCD's and I suspect there are also a good few with Mono DSLR's.

Hopefully it will give some other members some options to think about.

One thing is for sure though... it won't stop the decimal point moving in the wrong direction  

Keep Happy.

Best Regards.

Sandy.

[Thalestris24]

Hi

Just an update - finally got a coma corrector today I got the Baader MPCC from Flo and spacers from Teleskop-Express. Also got a 2" IDAS d1, lol. Can't believe how much dosh was wrapped up in two little packages...  Anyway, have fitted it all together with the qhy8y and did a daylight imaging check. So far, so good. Looks like I might possibly be able to test it properly tonight (forecast clear in the early hours ), and do any spacer tweaks. I got M48 spacers/extension tubes. One is adjustable so should be able to get the required spacing distances  ok. I went for 48mm to help minimize vignetting. Hope it does!

My mono 1100d is supposed to be shipping this week (fingers crossed!) but I've not decided what to do with it yet. It is possible to get an M48 T-ring which in principle would let me swap between the qhy8y and the canon - without a filter wheel, of course.

Cheers

Louise