Rotator or Flattener; which should I use

[Paul Haese]

I have come to an impasse with the GSO RC12. The back focus currently allows for a 3" rotator and an Atlas focusor with 52mm of spare backfocus. Original back focus is around 280mm. The STXL with integrated filter wheel takes up 61.87mm of that original figure.

Now to the core of the problem. If i buy a 4" flattener from ASA the adapters and flattener take up 108mm after the metal back distance is taken into account and adapter depths allowed for in the equation. So that means i cannot have both a flattener and rotator within the system.

So my question is, is it better to have the rotator (bearing in mind I am doing automation) or is it just better to crop out the frame for each image? Both seems problematic to me.

Guys doing automation and with similar imaging systems will most likely have an idea of what will work better.

[pvelez]

Paul

I use a rotator with my CDK 12.5 - its an Optec Pyxis 3", a very nice piece of kit.

I need it for my set up. Without it, I can't be guaranteed to get a guide star. I image with a STX16803. It took me a while to sort out framing - my FOV in TSX did not match my set up - once I fixed that up with a custom FOV indicator it works like a charm.

I use CCD Navigator to select targets and set goals. It also frames the images so I have a suitable guidestar. That integrates nicely with CCDAP so I can prepare an imaging run over several targets for a series of nights. I then let CCDAP start itself, frame and commence imaging. Without a rotator I'd be fiddling with framing every night - and not getting the real benefit of automation.

I've not used a flattener so I can't comment on that.

My 2 cents worth

Pete

[Paul Haese]

The STX would have that problem for sure and I see why you need the rotator for that very reason. I never quite understood the reason for guiding through filters. That is a pain.

You don't need a flattener for obvious reasons with the CDK. Nice flat field to start with. Starting to think it might be easier to have gone down the CDK route myself. ;)

[PRejto]

Hi Paul,

Seems to be quite an issue. I don't know anything about your system but these questions come to mind.

1. Is the ASA flattener the only option? Maybe there is a different option consuming less backfocus.*

2. Is there a thinner option for the rotator?

3. Can you contact the scope manufacturer for their take on your situation?

Best of luck with this!

Peter

*edit: actually can you explain the 108mm figure? Is that the required distance from the flange of the flattener to the CCD chip? If so then what you are saying is that the camera will consume ca 62 mm of that distance leaving only 46 mm for the Atlas focuser (which wouldn't fit). Guess I'm confused.

Edit #2: What about this? I think it claims to be .85" back focus 2.9" aperture. http://www.observatory.org/rotator.htm

[cfranks]

Hi Paul,

I don't know any of the measurements of your system and the proposed flattener but is there a way for a machinist to mount the flattener inside the focuser? That would recover, or share, some of the back focus. I did a similar thing when I was building a manual rotator and I mounted my WO flattener inside the rotator.

Charles

[marc4darkskies]

Paul,

Can you explain how the Atlas plus Rotator takes up nearly 230mm of backfocus (if I'm reading your post correctly)? Mine uses only 125mm (including the mounting flange to the backplane). In any case, a 108mm flattener wouldn't work so not a nice choice to have to make. Being a remote scope, I'd choose the rotator if I was forced to make a choice.

Cheers, Marcus

[ericwbenson]

Marcus, I think what Paul means:
camera/FW + adapter + pyxis + adapter + atlas + 52mm = 280mm

Since I know from my previous calculations the Pyxis 3" took up 61 mm with no adapters (but I don't know the adapter or Atlas thicknesses), taking a punt:
62 + 5? + 61 + 5? + 95? + 52 = 280

The 95mm for the Atlas seems a bit big to me since my Bellerophon 3.5" focuser only uses 41mm, and I thought the Atlas was skinnier.

Paul, that ASA corrector has 108mm for CCD to back flange separation. The mechanical thickness of the unit is 55 mm (= OAL, whatever that means, on OPTs website)

See this PDF drawing:
http://www.astrosysteme.at/images/Corr_4Zoll_Ritchey-Chretien-Field-Flattener.pdf

So the FFC might fit since looking at the PDF drawing what you have is:
108 - 5 + 45 + 5 + 61 + 5 + 61 = 280
I am ignoring the cam/fw backfocus since it is less than the FFC backfocus. You would need an adapter that uses up the space between the camera and FFC, about (108-62) = 46mm thick.
The -5 is beacuse the FFC moves the focal plane back
The 45 is the mechanical distance from the 108 number to the front of the FFC, see the drawing for it to make sense!

Surely the Atlas is less than 61mm thick?

But now you have a new problem, the FFC will rotate with the camera, and no rotator is perfect, there is always a bit of cone on rotation and sag on meridian flip that happens (it's a bearing of course). Will this affect the collimation? maybe...I dunno, expensive to find out!

Note that a camera spinning slightly eccentric on a rotator by itself just moves around on the image plane, so no effect on collimation, just sampling a slightly bigger zone back there.

I did some math the other day (limiting mag/worst case star density etc) to see if I could ditch my rotator, since I could do 10-30 sec guide exposures on the A200HR (although standard dithering then becomes a major pain). It's still a judgement call but I think I would need a guide chip at least 4x bigger than the lodestar to have a chance. That's possible to do (e.g. SX Trius) but the pickoff prism in the MMOAG wouldn't be big enough....I'm keeping the rotator for now...

Customization ain't cheap nor easy!

EB

[Paul Haese]

Thanks guys for the responses.

Ok so here is the measurements as I remember them. Coming off the back of the scope there is an adapter from the rear to the rotator. It has a 15mm back focus. Then the adapter from the rotator to the focusor is 10mm. These were the smallest adapters I could get made by Precise Parts.

From the back plate itself the mirror assembly sticks out 25mm. Then there is a GSO adapter that is 25mm connected to that, which can come out of the equation.

The Camera itself with integrated filter wheel takes up 61.84mm. I don't know if that includes the front nose piece on the filter wheel that came off the camera. I suspect that is included.

I then have a 27mm adapter from the focusor to the camera.

The focusor in the centre position takes up 36mm. The Rotator is 61mm.

So that makes 15 + 10 + 25 +25 + 62 +27 + 36 + 61 = 261. I suspect that the noise piece of the camera takes up the last 19mm.

So that makes up the current back focus.

The flattener I chose because either you get a 2" version which is not really big enough or a 4" version. The ASA flattener needs to be set at 108mm from the CCD. So we take off the 62mm from the camera and that leaves 46mm as that adapter. Next the front side which connects to the focusor has to be 45mm long as a minimum according to Precise parts and that is because a portion of this flattener sleeves into the adapter and the adapter cannot have the sleeved bit going through the focusor because of diameter. Now maybe I can change the configuration around and put the focusor first off the scope and then the rotator next and that might allow the sleeved section to go through the rotator. However I am still 20mm short of back focus.

Does that make sense? Marcus how have you got your focusor and rotator configured?

[clive milne]

Paul,
there are a couple of integrated focuser /rotators on the market which may be worth considering..
A 3" with a 74mm total height:
http://www.reginato.it/accessory.html

Planewave also do a beast of a 90mm focuser / rotator that can lift almost the same weight as Mike. http://planewave.com/products-page/general-accessories/irf90-integrated-rotating-focuser/#.U3asf1AZ7qA

[marc4darkskies]

See http://www.pbase.com/gailmarc/image/153405059

The flattener is inside the scope near the back plane behind the primary baffle. See http://www.pbase.com/gailmarc/image/155106230

236mm optimum backfocus. This is somewhat variable (+-30mm), since I have a motorized secondary, but I'd always want to stay as close as possible to optimal mirror separation consequently I would ever use all of that range.

Cheers, Marcus

[Paul Haese]

Down at the house now and just checked a while ago. From the back plate of the scope to the filter wheel is 210mm. Add the 61.84mm to the camera sensor makes it 271.84mm. It is supposed to be 288mm total. GSO numbers do not seem to work.

Marcus the rotator and focusor with adapter to the scope measures 122mm. So that is close to what your numbers show.

I don't want to buy another focusor or combined focusor and rotator. Thanks for the suggestions, but not an option really.

So it is looking like either I have a flattener or a rotator. I cannot have both it seems.

[gregbradley]

Paul I would experiment with flatteners. They are not like reducers which are much more critical and fussy for distances. I find a good flattener works on several scopes perfectly well.

For example I have a 4 inch Tak FS152 flattener I got on Astromart ages ago for US$800 or less. It works perfectly on my AP140 and my TEC180.

I used to use a 2.7 inch Tak reducer with an STL11 on a 12 inch RCOS. It had some soft stars in the corners.

I bet APM has something that may work otherwise put an ad on Astromart for a 4 inch Tak flattener. It'd be a punt but its likely to work.

$3000 euro for a flattener is nuts. AP flatteners which are probably the best in the business are a lot at about US$2200.

Greg.

[DavidTrap]

I always thought reducers and flatteners that weren't specifically designed for a particular scope were a bit like having children - you never know what your going to get?

DT

[sharpiel]

Logan has a 4" Tak flattener for sale (sort of) in the Classifieds.

[Paul Haese]

Just chasing that up now. Thank for the tip. I am thinking though this will still be too long.

regards
Paul

[clive milne]

Paul,
Just a few things to consider. ..
You can increase the back focal length of the ota by repositioning the secondary mirror. If you move it towards the primary by 18mm you will get an extra 48mm of bfl. (A positioning error out of the box might explain why your bfd is different to factory specs. There is of course a practical limit to how far you can go before you experience vignetting and field aberrations .... I'm sure Bratislav could give you an idea.

Also,
I would be reluctant to use an AO at f8 without a rotator, guide star selection will be greatly impaired.

Thinking outside the box (as is my inclination) you could put the ota in a set of rotating rings and drive the entire telescope instead of just the imaging equipment. This is entirely possible, albeit non-trivial.

[clive milne]

It's probably not relevant to your situation, but it occurred to me that if someone had self guided camera with a filter wheel in there chewing up back focus it might be better to dedicate the main ota to the L channel only and piggy back an 8 or 10 inch newtonian for rgb duties.

[Paul Haese]

Now that is something I had not considered. I am 1mm different from specs, but could move back 5mm.

Not worried about the AO issue as the guide camera is in the filter wheel and as yet not seen an instance where it does not find a guide star. So a rotator is just a luxury.

On another matter today I bought a flattener (see link below). Cheapish and large diameter which is what I wanted.

http://www.teleskop-express.de/shop/product_info.php?products_id=6972#e bericht

Now to get it and then I can work out the relevant backfocus and adapters.

[KenGee]

That is interesting I have one of those flatterers and wondered if it would work on the RC16. I was going suggest it to you.

[Paul Haese]

Ken do you know what is the actual adapter length? That is the first drawing I have seen of it but note that there is no length provided.

[clive milne]

Yes, I agree.
To be fair though, the benefit afforded by fr is in the order of a 10x increase in the number guide stars to choose from. One of these stars will be on average 10x brighter so you are more often than not able to run the AO at 10hz.
You can claw back a little bit of this advantage by sacrificing perfect framing of the imaging chip, or completely sidestep it by only imaging the 10% of dso's with the optimum guide star located at the field angle matching your imaging train. You could always periodically (manually) reposition the field angle of the guide camera to image a new batch of dso's.

[KenGee]

Hi Paul, The unit is 25mm.

[Paul Haese]

Thanks Ken.

[Paul Haese]

Ok so it looks like I might be able to do this.

271mm to work with, more if I move the secondary closer to the primary.

camera and AOX = 91.84 + 12.2 for adapter to TS flattener + 25mm for flattener + 10.3 for adapter to Atlas + 122 from atlas to rear adapter of rotator. Finally 15mm to mirror cell. That makes 276.43mm. 5mm could be gained from the secondary position.

[Astromelb]

For a rotator a Pyxis is the best solution from a quality and cost effective basis that matches your GS unit, IMHO do not consider anything else as they are nowhere near the capability.
I have a Pyxis that came on an Officina Stellare RiLA600 that I bought recently - it did not achieve focus with the CCD I have as my CCD was not your standard CCD design, so I upgraded to an Officina Stellare Rotofocuser (this is not what you want as it is EUR 3,500) for the system, and the Pyxis whilst being used (tested) once and thereby being basically perfect - is superfluous - it will not be used. If you are interested.
ASA make the best flatteners that exist globally. But you will need to give the Aust agent the system spec to get a correct selection, be prepared for a larger bill than the usual Yank or Asian stuff. Do this if you want something that is specifically engineered to your exact needs and also at the best quality level available globally. It is unapologetically expensive, but at their quality level you expect this.
There are other options that you could consider. These are grand master custom hand craftsmen in Japan (the guys that actually make this and other gear for Tak). They do not however speak or read English, but they are very capable as they have made gear for me for years, they do a really great job.
ASA on the other hand are a no risk solution, they will do exactly what you ask, to a sensational capability.
Good luck.

[Peter.M]

This all seems to be ok to me, but you might want to check that the flattener does not move the focus position. I know that with my mpcc for instance it does not effect the focal length, but the focus position does move.

[Paul Haese]

Yeah I was wondering if the focus position moved and you know the last time I went through the adapter roulette. The ASA one moves the focus back 5mm but the TS one does not say anything about focus shift. It could well be that is does move.

[Paul Haese]

Yes I already have the pyxis rotator. FLI focusor but am looking to fit in the flattener and AOX if I can.

[Paul Haese]

Clive the guide star selection is automatic at present via CCDAP's selection criteria. Never missed a guide star yet and the stars are always just at the very outer edge of the field. The pick off prism is just a fraction out side the imaging field. There is a slight shadow of the prism on my flats, but there is always plenty of stars for CCDAP to select from.

If that makes sense.

Latest notion is that I can fit all of it in. I am about 5mm short but either flattener might push focus back a bit or I can push the secondary back a little like you suggested.

[clive milne]

Paul, I'm sure you know this
but for the benefit of anyone who isn't aware;
RC's typically have an f3 primary and an f8 final focal ratio.
It follows then that any movement in the secondary position relative to the primary will be amplified at the ratio of 8/3.
Therefore, to gain an extra 5mm of back focal length, the secondary should be moved towards the primary by 1.875mm.

[bert]

Can't say I would agree with this advice. Mirror spacing is critical with an rc in preventing spherical aberration.

[Astromelb]

Dear Paul,
How are you focusing ?
If you are using an electric focus process and also a Pyxis rotator you may end up in exactly the same place I did with my RiLA600, not enough back focus.
So I changed to an Officina Stellare Rotofocuser - please see - http://www.astronomyalive.com.au/officina-stellare-telescopes/apochromatic-refractors/rotofocuser-electric-focuser-rotator-aseembly.html
It works perfectly.
Clear skies.

[bratislav]

Actually, factor is (8/3)^2 + 1 or 8.1x .
That is, for every one millimeter change in spacing, focus moves by more than 8mm. To move final focus by 5mm you need to change spacing by mere 0.6mm.
Which won't change much spherical correction, btw.

[clive milne]

Thanks for the correction,
much appreciated.
c

[Paul Haese]

Thanks for the clarification. I will keep this in mind. 5mm should do the trick but if I moved 1mm back that would make it 8mm back and that is probably more than enough to accommodate everything. Interesting times. Back end of the scope is worth ten times more than the front end. :lol:

[Paul Haese]

An update.

I received the TS Optical flattener today. See images below (not great but at least you can see what I am on about).

The flattener is rather large really. Very solid construction and has that sturdy feel to it. It comes with an adapter for DSLRs but I don't have it attached.

I am awaiting adapters being made by Precise Parts which should be shipped on Thursday.

[KenGee]

mmm that looks different to my tmb flatener.