RC300 - Piggyback ED80 or ED 120?

[bloodhound31]

I'm going to buy the GSO RCA-300 Ritchey-Chrétien (http://www.bintel.com.au/Telescopes/Cassegrain/Bintel-GSO-RCA-300-f/8-Astrograph--Steel-/604/productview.aspx) and put it on the new EQ8 (http://www.bintel.com.au/Mounts---Tripods/EQ-Mounts/SkyWatcher-EQ8/1445/productview.aspx) with either my ED80 or ED120 on top as a guidescope. This will also enable me to keep the wide-field capability when I reverse the roles of the scopes.

I've checked with Bintel the weight load of either combination on the mount and even with cameras, they both come in under limits so that's all fine.

My question is:

Is the 120 or the 80 best for guiding on this big scope and mount? Why? I'd like to hear your thoughts.

Baz.

1. My current telescopes ED80 and ED120 on an EQ6Pro GEM.

2. Proposed GSO RC 300.

3. Proposed EQ8 GEM.

[Logieberra]

Baz, wow! Can't wait to see this setup!

If I may comment, with gear this serious, I'd go off axis guiding. Sure, a refractor above will guide it, but why not go the whole hog and do it right?

MMOAG.

[bloodhound31]

I guess because I am really familiar with piggyback guiding and find it very easy to locate a guide star with my custom-built, independently adjustable guide-scope mount.

From what I understand about off-axis when I last attempted it, sometimes it can be VERY hard to even find a guide star in the FOV, depending on what you are imaging.

[g__day]

Second the above comment, for several reasons.

Unless your SCT has a fixed mirror - that can't shift, sway or flop even even a miniscule amount you may find guiding from a refractor whilst you image from a longer focal length SCT really challenging - I did! Differential flexure is a killer between scopes, but I still think many SCTs have slight mirror shifts that vary with elevation - which means you're only guiding option is on or off axis at primary focal length).

With a OAG I can easily do 30 minute subs on my C9.25 (I am sure the same would be true for an ONAG). Guiding at full focal length is brilliant! I tried both side by side and piggy backed quiding with a 80mm refractor the results were horrible - I couldn't even get 5 minute subs I'd be happy with some days.

I saw the new EQ 8 at Bintel a few days ago - looks great and has a really solid feel to it Baz - I think you'll be pleased with it.

If it were my option I'd go the 12" GSO RC and the 120mm ED - I like light grasp! I'd place an OAG on the GSO - and probably add a focal reducer (consider maybe the Lumicon models which have both). I would also invest in PC controllable focusers for both these scopes over time - having experienced now the Moonlight and Meade PC (+ JMI focus) controllable focusers - I really like the Moonlight - but both work well!

As for always getting a guide star, consider guiding with a sensitive enough camera - like a Lodestar, or go ON Axis Guiding with ForSight Innovations ONAG (Cold mirror) and be spoilt for choice.

Cheers,

Matthew

[naskies]

That's some nice kit, Baz!

Have you considered what imaging camera / focuser / filters / etc you'll be using? It might worth planning it out ahead of time to double check that everything is suitably matched and compatible.



I'm only using an RC8 and have yet to find a patch of sky that doesn't have a suitable guide star for my Lodestar using 2 sec exposures in 2x2 binning. Most areas of the sky have pretty bright guide stars (0.05 to 0.5 sec exposures).

If your mount has a smooth PE curve (and the EQ8 looks like it does from what people have posted), then you can use even longer exposures without a worry.

An RC12 has over twice the light gathering ability as an RC8 so you'll be fine :)

[jase]

Agree with what others have said. At a native 2432mm focal length, I'd be shying away from the guidescope idea. An off axis guider is the way to go. Save yourself from the headaches.

[LewisM]

I use a long FL guidescope, and a Lodestar. Never once have I been unable to find a guide star in light polluted suburbia (but failed several times with the Orion SSAG).

I am going OAG (again - the SSAG was not good enough for the OAG I tried), but need to find one that will fit in with the imaging train I have (all screwed together - no compression rings etc). Teleskop-Service have one that should suit, and it is one of the THIN ones too.

[bloodhound31]

Cheers guys. Man it's good to have you all as a resource. Sounds like OAG is the way to go then.

I tried Googling lodestar, but I don't think the search results are reflecting what I think you guys are referring to.

Can anyone give me a plain-speak soldiers-five on what it all looks like, how it's set up and how it works? Photos would be great too.

Baz.

[clive milne]

Baz,
Persevere with the oag and invest in a loadstar, the improved sensitivity should cure your guide star availability problems. The guide scope route will be problematic irrespective of the aperture you throw at it..

-c

[DavidTrap]

Agree with the comment about a Lodestar vs a SSAG for OAG at long focal lengths.

Claude at AEC can supply a Lodestar. Made by Starlight Express. Link (http://members.ozemail.com.au/~aecclaudio/starlight_lodestar.htm)

DT

[bloodhound31]

Righto then. Thanks for the link David, that looks indeed like the way to go.

My proposed RC has the following specs:

• 3" Dual Speed Linear Crayford focuser with 2" and 1.25" fittings

• Two 2" focuser extension rings for fine-tuning the back focus

So perhaps an Orion Deluxe Off Axis Guider (http://www.bintel.com.au/Astrophotography/Astrophotography-Accessories/Orion-Deluxe-Off-Axis-Guider/726/productview.aspx)

Could you recommend any other OAG to go with the Lodestar? Oh, and I'm guessing the mono Lodestar is better than the CMOS version?

Should I get a focal reducer/field flattener with it too? How does this affect the operation of the telescope and results?

I'm sorry, I used to have some of this stuff on my old C11, but I never mastered it and I've pretty much forgotten everything about it.

[DavidTrap]

Definitely the mono one.

DT

[clive milne]

Perhaps the best slim line off axis guider is this one:

http://www.optcorp.com/starlight-xpress-slimline-off-axis-guider.html

the best price I found was from opt.


The best guider I have found is the monochrome loadstar, the best price was from telescope express in Germany.

There are a couple of exceptional focusers you can put on your RC and on that list would be:
FLI Atlas (not to be confused with the pdf) almost the default standard.
Reginato (Italy) ... it looks good on paper anyway.
Clement compliant (USA)
Feathertouch (USA)
And last but not least, Planewave, (arguably the pick of the bunch)

[naskies]

Baz, what camera are you planning on using with this set up? If the sensor size is large then you'll definitely need a field flattener, in which case you'll need to also ensure that your camera + filters + OAG + anything else will fit within the back focus distance of the flattener, and that the focuser is up to scratch. A motorised focuser might also be very handy - especially given the steel tube.

[bloodhound31]

Dave, my current cameras are an Orion Starshoot Deep Space colour CCD camera, DBK21AU618.AS planetary camera and a 5DMK II DSLR.

[Logieberra]

Baz, we can talk Tuesday on all this.

I'll also introduce you to Phil. He's a local guy, using a similar sized scope (12"), off axis guiding with arguably the best of the OAGs - the Astrodon MMOAG, and using a very sensitive guide camera to suit - the SBIG STi. I have one of these too, and chose it over the Lodestar as it has a mechanical shutter. Both are superb guiders, but costly.

P.S. I previously owned a Orion Deluxe OAG. You sure get what you pay for in this hobby...

[g__day]

I would highly recommend Lumicon for OAG - http://www.telescope-service.com/Lumicon/start/Lumicon.html

Not only consider a motorised focuser, consider a motorised, PC controllable one with a temperature sensor. My logic here is:

1. A steel tube will shrink as the temperature falls, so you will need to compensate to maintain focus as temperature varies (which is why I love Carbon Fibre tubes - almost no thermal expansion / contraction)

2. Focusing with say a Bathinov mask - or any focusing - I don't like to touch the tube at all - I want it rock steady and for folk to not even walk by it when I am focusing or imaging! So PC controlled is great!

3. If the tube has a read out of both its position and temperature then you should be able to chart and match temperatures to position.

4. If you use eventually use a mono camera and filters - then (not sure is this was refractors only or SCTs too) each filter will focus to a different position - so its handy tracking where each colour focuses at any given temperature.

Quite a few folk here use Moonlite focusers - they are exceptional quality and look great too! http://www.focuser.com/storefront.shtml

The Moonlite focusers with High resolution Stepper motors, temperature sensing and Variable speed mini controllers (hand operated or USB controller v2) work very well!http://www.focuser.com/cgi-bin/dman.cgi?page=category&plugin=dstore.cgi&category=5

PS

The Astrodon MMOAG plus fittings is around the same price as the ONAG http://www.innovationsforesight.com/store/index.php?app=ecom&ns=prodshow&ref=On_axis_guider review here http://www.cloudynights.com/item.php?item_id=2706

[bloodhound31]

Looks like I'll have to take out a second mortgage!

[clive milne]

You would think that the following statement would be so bleeding obvious as to be unnecessary; the place for the oag is ahead of the focal plane of the main instrument (not the piggyback scope)
It beggars belief that there is at least one RC12 gathering dust (that I know of) because the owner is obtuse and deaf to reason specifically on this point.

[bloodhound31]

Huh?

[g__day]

One other thought - piggy back or side saddle?

I have tried both options. First I mounted a 80mm WO refractor on top of the C9.25 and had a 127mm MAK side by side. Now I have the C9.25 on the left and a WO 110mm on the right with a WO 88 on top of it. All mounting by the way is using Losmandy side saddle gear - very solid stuff!

I think the side by side is a better option for my needs. My logic is:

1. It lowers the turning moment - that's three heavy scope I have on my mount - plus all the focusers, OAGs, lens, and camera's - lets make the mount's job as easy as possible

2. It places less stress on the SCT outer tube and there its front corrector plate - less distortion means less differential flexure which is a very good thing

I always think the more time and effort planning how all your gear will integrate and inter-operate before you purchase is a great investment of time. The mechanical construction of your observatory itself is a example of this - I really am in awe of what you have achieved!

A few things I would like to know:

1. What is the light pollution like where you will be observing?
2. Do you plan to do strictly narrowband imaging if its poor?
3. What imaging camera/s and/or filters do you plan to match to your OTAs?
4. Are you forward planning peltier or water cooled gear out of interest?

You have build a great observatory that will allow you to do many things over time.

PS

To Clive's post - you put the OAG on your scope with the longest focal length, ahead of the point where it focuses! My PHD guiding graphs can jump one or two bars vertically at a time - which may raise eyebrows until one realises that's still 0.8 of an arc second - so its most likely seeing. On a shorter focal length scope you aren't getting the guiding sensitivity that a long focal length SCT requires - in my opinion / experience!

[clive milne]

Incidentally...

I had a couple of illuminating conversations a month or so ago.
First one was to Andrews' coms. I was basically trying to sort the fact from fiction wrt the 16" GSO RC which is ostensibly in the pipeline. The price they were quoting was $4500. I asked about optical quality, they said 1/20th of a wave p-v. I responded something along the lines of; look, I would be satisfied if you could guarantee a good honest 1/4 wave.... I have the means to verify this interferometrically via a (professional) third party, would you be prepared to stand by the quality of this product?... all I'm asking for is a genuine 1/4 wave. Their response was jaw dropping... quote: Look, we would not be interested in selling this telescope to you on those terms, I suggest you call Bintel, they are the people you should deal with!
I was speechless.

Next conversation was to someone I have known personally for decades and has been in the business for almost as long. I wont say who, but I am sure a lot of you know him as well. He said that GSO (as of 6 weeks ago) had no intention whatsoever of producing a complete 16" RC OTA... period. And that was direct from the horses mouth. He went on to say that A coms sell 'grey imports' that are not backed by a factory warrantee. Which explains a bit.

Anyway.... keep that in mind when you go to buy your RC12.

hope that helps,
-c

[Logieberra]

Bazmo, before you drop deposits and such, a CF 12" model may be in the pipeline. I'd hold out for the CF, personally.

[bloodhound31]

Holding out for a while now mate.... :sadeyes:

[Paul Haese]

Sounds like you know all about it Clive. As usual you have all the facts correct.:P I take it your talking about my system here.

Just to put things straight here. I have been automating a wide field unit in anticipation of automating the RC12. ie a full robotic internet remote system. Not a trivial task. I did one image with a guide scope to test it. I am not nor did I ever intend to use a guide scope to image with that focal length. I am using a MOAG next to my STL11. When I produced the image of NGC253 I was still testing the scope. It is not gathering dust. Automation is difficult to achieve and requires a great deal of time to integrate all the systems thoroughly. In other words just for you Clive it takes many hours to get it working properly. I know the RC12 is good and can produce good images even with a guide scope (which has large amounts of flexure). The MOAG does not have that same problem.

Best of luck Barry with your equipment choices.

[gregbradley]

Just for the sake of accuracy I have guide scope guided my CDK 17 with a reducer at 1761mm focal length (the reducer bars the use of anything but a self guiding camera for guiding due to the lack of back focus) with good success.

So it can be done and I could easily tweak from what I had with a much larger T-point model, a more refined polar aligment using the Sky X and a supermodel.

But yes its a bit fussy for routine imaging and MMOAG leaves it for dead of course.

As far as getting a guide star with a MMOAG is concerned I routinely image with one on my CDK17 at nearly 3 metres and Sbig STi guider (lovely unit) and I usually have no trouble. It has happened a few times I had to reframe the image to find a guide star but usually the object was still not far from centre and plenty of chip real estate left to capture it.

If you setup a MMOAG on the main scope you also use that to guide the refractor when you use it. You simply leave the guide camera on the MMOAG and plug off the end opening where the CCD cam goes on the MMOAG and put your imaging camera on the refractor.

With a 300mm RC you will probably want to use Tpoint to get the required accuracy of polar alignment and a well done PEC curve to settle it down even more. Drift alignment takes you a long way but there is still some accuracy left behind that t-point can detect and refine the polar alignment even more.

Also I agree with getting the carbon fibre version. I think expansion becomes more critical with longer focal length scopes and especially RCs which are sensitive to the spacing between mirrors.

Greg.

[g__day]

I prefer Carbon Fibre - the alternate to me would be a really, really good - temperature compensating focuser. However I'd rather simply avoid the issue. It's amazing how well CF holds focus with temperature variances.

[Paul Haese]

Baz, just some inside experience of the RC12 from GSO.

Focusor, through that away. It is ok but cannot haul a decent load up hill. Start looking at focusors with 6-9kg capacity and those that allow automated focusing (you will want to use this later so think ahead now). Some starting points are Moonlite focusors, Feather Touch focusors and FLI Atlas. Each are progressively more expensive, but each are strong. Look at rack and pinion on the Feather Touch. I am not sure if Moonlite make a rack and pinion but take a look there too. Each focusor has to be 3" to protect the light path from the mirrors. You should not use a 2" on these scopes.

For OAG go with the MMOAG from Astrodon. Best OAG in the business and built like a tank. If you can get hold of an STL11000 camera then the MOAG is the going to work better than the MMOAG. If you want to use the stand alone OAG the MMOAG will work for you. It is pricey but worth the money. Good fittings and places that make adapters like Precise Parts will know exactly how to fit that together with other components.

The Loadstar is a good camera but also consider the ST-I from SBIG. It has its own shutter and that means a lot of if you plan on doing a lot of unattended imaging. Don't get me wrong the Loadstar has great credentials but just take a look around. Buy the best guide camera once. The SSAG works too but it was a temporary measure from my own experience. It served me well but in the end died and many months before that it was act up and not calibrate properly.

The scopes you have will be fine for wide field imaging on their own but I would just pick one and use that one on top of the mount. Work on the premise that you want about 3/4 weight capacity of the mount for total setup. With that you need to consider camera, focusors, OAG's and cabling. It starts to add up after a while and this will get you quickly close to the 3/4 weight capacity of the mount.

Personally if you consider that the gear you are going to buy is high end, then think of the RC12 as a temporary resident of the system, you may decide later to buy a bigger scope and or a more expensive scope altogether. Buying the expensive peripheral components means you don't have to upgrade again and that will save you money. Don't balk at the costs as reliability is precious in my opinion. Stuff breaking down late at night means lost time.

The RC12 is a good scope but you need to tame a few issues. Focusor is one and the other is dew protection of the secondary. These scopes dew up on the secondary, so consider putting in a dew strap by Kendrick. I found it works a treat and then keep the fans running all night on the primary. The metal tube is another but refocusing every half and hour keeps that under control.

Buy yourself a Tak collimation scope too. You will need it and it is a good tool investment for all future scopes. I found my RC came a little out of collimation in the primary and I needed to sort that out. The only way you can do that is by using a Tak collimation scope. You can use Ken Crawford's avi (on his site) which shows how to collimate the rear cell on a RC scope.

If you think of anything else you want to know by all means ask. I am more than happy to help with frank and correct knowledge from my own use, not some supposition posited. These scopes have good optics but you need to work a little to get them working well. Don't believe half the crap that you can read about the GSO RC scopes or the innuendo that appears on this site from certain individuals. Those people have their own agendum to get people to buy some other telescope design. Their comments constantly appear in the GSO RC threads and each time their comments deride the scope and yet I have never seen them actually say they have one or tested one or even imaged with one.;)

Let me know if I can help further.:)

[bloodhound31]

I'll never be able to afford any of this stuff. Way over my budget unless some Philanthropist decides to help me out.

Couple of things.. What's MOAG and MMOAG? What's the difference between off axis guiding and off axis guiding? Three inch focussers? I can only find 2 and a half max at Moonlight.

[Paul Haese]

MOAG = Manual Off Axis Guider. MMOAG = Monster Manual Off Axis Guider.

http://www.astrodon.com/products/hardware/moag_off-axis_guider/

http://www.astrodon.com/products/hardware/monstermoag_off-axis_guider/

The monster MOAG is bigger and designed for bigger sensors. I always think to the future now for purchases. Is this purchase going to be transferrable to another system of better quality? If yes then you are on the right path.

A 2.5" focusor would be ok, but not ideal for that scope. It would work fine with smaller sensors but would be too narrow for larger sensors. In that case you would need to go to a larger focusor.

Depending on budget, you might have to compromise on camera but your focusor and OAG guider are holding things up in the air. You need to have a beefy focusor with autofocus ability. You could use a QHY larger format camera and save some money there too.

Bottom line is if you scrimp on these components you will be buying problems for yourself.

[Peter.M]

a good set of flats will get rid of the dust.

[bloodhound31]

I've been reading all the info you guys have sent in and researching on the net to school myself up on all of these components.

One camera I have been trying to understand is the SBIG series.

THese three caught my eye, but I'm trying to understand which is best and why I would want them on the RC12.

STF-8300 Mono and Color (https://www.sbig.com/products/cameras/stf-series/stf/)

STT-8300 (https://www.sbig.com/products/cameras/stt/)

And this - the STF 8300 package (https://www.sbig.com/~sbig/site/assets/files/1367/astronomy_ad_8300fast_working4.pdf)

Finding out the price is tricky too, being in Australia.

[renormalised]

I like that idea :) :P

Megan Gale or Jodie Kidd would do. Christie Brinkley (in her day) would also suffice, I reckon :) :P

[renormalised]

Ask Peter Ward about them. He's the dealer for the SBIG cameras that's closest to you and will know about each of the cameras.

http://www.atscope.com.au/

[bloodhound31]

Thanks Carl. Contacting him now.

[gregbradley]

Without a doubt you want the SBIG model with the built in off axis guider. I think its the STF (not sure of the model names).

If you don't have either self guiding, self guiding in the filter wheel or an off axis guider you are asking for trouble big time and will end up throwing the scope into the scrap heap it will be that annoying!

Guide scopes at that imaging focal length are going to be very tricky and an ED80 whilst a lovely APO scope has a weak, semi loose focuser which will not be up to the job.

I just installed an SBIG STi guider guide kit on my CDK17. I intend testing it first clear night to see if it will guide accurately at 3 metres focal length. it is very rigid even though its only a 100mm lens guide "scope".
I can let you know how it goes.

I have imaged semi successfully using a Vixen VMC 95 guide scope (over 1000mm focal length). I would sometimes get slightly eggy stars in some angles. With an offaxis guider I almost never do. And that was on a PME mount with PEC on.

Greg.

[bloodhound31]

Thanks Greg. I take it with self guiding you don't need the separate guide-camera like a Lodestar or something because there's a chip in the main camera that does the job?

What about the MOAG or MMOAG? Do you still need that? I wouldn't think so if the camera is self-guiding.... Trying to make sense of it having never seen one set up and had a play with it.

I just sent an email away to Peter looking for a total package to go on the back of the RC300. Let's see where that goes.

Hey you guys I much appreciate all this. Once it all comes together I am sure I will be far better armed to pull the trigger because of all of your advice/experience.

Cheers!

Baz.

[DavidTrap]

Baz,

You might be getting yourself into a whole world of pain if you're trying to use a very long focal length scope with an 8300 chip. The pixels are very small, so your arcsec/pixel will be very small as a consequence. You could bin it at 2x2, but there are problems with the 8300 chip when binning.

I understand budget is a significant factor for yourself. Why are you after a 12inch RC?? Aperture fever??? What sort of imaging do you want to do?? I've heard you mention community outreach stuff quite a lot - would you do better with a large newtonian for a brighter image through the eyepiece??

I've recently gone through the mental exercise of planning a school observatory (not sure if it will get the go-ahead or not) - and after getting all excited about putting in a large OTA, I realised most of the objects they'd be interested in would be more than adequately imaged by a 4 inch refractor. Much lower costs and much less headaches when it comes to guiding & focusing.

I'm not trying to be rude, but you don't seem to be abreast of the issues with heavy-duty deep sky imaging (based on some of your questions). Are you trying to bite off more than you can chew by buying a 12 inch RC??? Would you do better using your existing 120mm refractor and learning how to do monochrome imaging first, before taking the leap to a longer focal length?? All of the expensive upgrades that people are suggesting are correct - you'll have a lot of headaches if you try to cut costs.

Happy to chat to you by PM if you prefer.

DT

[bloodhound31]

No that's fine David, happy to air it out here mate.

I intend to keep the smaller refractors I have along with a range of cheap scopes I already have to pop out on the lawn for community outreach viewing nights.

I used to own a 12 inch SCT which was way ahead of me at the time, but having images widefield for a bit I must confess I desire to go deeper. I've imaged a few distant galaxies with the Starshoot on the 120mm but I have to crop the heck out of them to fill the frame at the end of the day.

I may have bitten off more than I can chew, but I like to buy things I can grow into.
If the 8300 chip is too large for the RC300, then perhaps you guys can educate me on the optimal camera for the job.

I eventually want to get into narrowband imaging down the track and I don't want to have to keep buying more and more equipment. I'd rather get the lot in one hit then go to work on the steep learning curve. I understand it will be frustrating, but you can probably guess from my past and present projects that I have a great deal of patience.

So please, if you guys have suggestions for a total package to go on the RC300 and EQ8 I would love to hear what, why, how much and how. :D

Budget is a factor for now but it may not be that way for long... ;)

Cheers!! :thumbsup:

Baz.

[DavidTrap]

Ok Baz,

The 8300 chip is too small. You'll do better with an 11 or 16mp camera - larger and deeper pixels. But at a much higher cost...

DT

[bloodhound31]

For instance? :)

[gregbradley]

The only cameras that are self guiding are SBIG cameras as its an SBIG invention that is patented. Self guiding cameras are typically the ST series and STL.

The ST8300 does not have any guiding at all. I think its the STF and the STT that do. The STF has the OAG built in. The STT has the updated electronics and I think is compatible with the new filter wheels - Peter would know and you can also check the SBIG website.

MMOAG or MOAG are mega monster off axis guider and mega off axis guider respectively. They have a little prism that sticks down into the light path and reflects a guide star up into a guide camera like a Lodestar. This is what I use except I use an SBIG STi guide camera (same shape as Lodestar).

You would only need that if you had an ST8300 (the cheapest in the 8300 range of SBIG cameras) as it has no guide capability without it.

With an STF you could install your Lodestar guide camera in the built in off axis guider and have a guide cable going from the Lodestar to the mount.

The STT has better electronics and is compatible with the filter wheel with the guide chip in it. These are better solutions than self guiding inside the camera as that means the guider guides through the filters. Guiding through the filters is a problem imaging with narrowband filters are they require long guide exposures to capture the guide star like 30 seconds or more. That is too long for autoguiding. Hence the models above either have a built in offaxis guider (requires a Lodestar or SBIG STi guide camera) or the filter wheel has the guide chip in it and does not require a separate guide camera (nice and neat, one less USB cable and power lead).

If you price an ST8300 + MOAG plus filter wheel and 36mm filters as one option. Then STF8300 with OAG and filter wheel plus 36mm filters then STT8300 plus the guiding filter wheel plus 36mm filters you can compare costs.

Alternative would be FLI Microline 8300 + filter wheel and 36mm filters and MOAG. Or QSI583 or 683 WSG +25mm filters which has built in filter wheel and off axis guider.

There are other 8300 cameras on the market, Apogee Alta, Apogee Ascent, Atik, QHY, Starlight Express.

Just as a thought the FLI Microline 8300 is upgradeable to another KAF series chip later if you want. I was quoted $3750 by FLI to swap out my KAF8300 for a KAF6303E if I wanted. Not bad if you check the prices for KAF6303 cameras.

Greg.

[bloodhound31]

Thanks Greg, that is excellent information mate. The more I read back through all these posts the more it is all making sense.

Different folks do have different opinions and suggestions though. What are your thoughts on the 8300 chip being too small for the RC300, as David has mentioned?

Baz.

[renormalised]

http://www.bintel.com.au/Astrophotography/Finger-Lakes-Intrumentation/FLI-Proline-16803-w/CenterLine-CFW/1410/productview.aspx

http://www.bintel.com.au/Astrophotography/Finger-Lakes-Intrumentation/FLI-ProLine-11002M-0/731/productview.aspx

both with one of these...

http://www.bintel.com.au/Astrophotography/Finger-Lakes-Intrumentation/FLI-Atlas-Focuser/402/productview.aspx

[gregbradley]

Yes KAF8300 would not be an ideal match. They all work, its a matter of which works best. KAF8300 has 5.4 micron pixels.

Theory is sampling theory where you want around 3X to get a good sample of sound or light. Too small a sample and it means you are getting too small a slice of the available light to fall on each pixel. Then if that slice is below the seeing level then it will be more blurred as well.

So the target is 1/3 of your typical seeing. So if your seeing is around 2-3 arc seconds you get .66 to 1 arc seconds per pixel for ideal sampling. My CDK17 and Proline 16803 for example is at .66 arc seconds. That seems to work well. The 8300 though is more like .4 and I know from trying it that it loses sensitivity (due to too small a slice of the light falling on each pixel not because its less sensitive - its not).

If seeing is good then the 8300 would work. But you'd have to get seeing around 1.5 arc secs which is not many places in Australia.

Ron Wodaskis free ccd calculator shows all this. For 2.5 metres focal length you are going to want 9 micron pixels and this will give you about .77 arc seconds/pixel which is in the sweet spot. 5.4micron pixels will give you .45 which is too low.

I am about to try a Trius 694 camera with 4.54 micron pixels and high QE on my CDK17. I'll have results after the first clear night.
I'll probably end up using it on my refractors but who knows.

9 micron pixel sensors are the KAF6303E, the KAI 110002 and the KAF16803.

There's a reason the KAI11002 is still popular after all these years.
Its a 35mm sized sensor so you'll need a corrected circle that will illuminate a chip this large. From memory its around a 44mm diagonal of corrected imaging circle from your scope for it to work without vignetting and coma.

Greg.

[bloodhound31]

Looking at the MMOAG. It looks like you can't rotate the prism around the FOV as you can with say - the Orion Deluxe OAG.

What is the method then, for finding a guide star? Do you have to rotate the whole unit on the back of the focus tube?

Baz.

[Paul Haese]

You use a rotator Barry. Either motorized or manual.

[bloodhound31]

Right. Ta.

[gregbradley]

I have only occassionally had trouble getting a guide star using a MMOAG at 3 metre focal length. Sometimes it has meant moving the object from centre of the image off to one side. I don't rotate anything, simply use the joystick to move the scope a bit to one side or the other too look for a guide star. You can also increase the guide exposure length to bring up dim stars if your mount can handle it.

Greg.