EdgeHD 8" ... autoguider & off-axis guider?

[souls33k3r]

Hi All,

I need a bit of help in deciding the best combo for AP.

I am looking to make a purchase of Starlight Xpress Lodestar X2 Autoguider but do i need an off-axis guider to compliment it? Maybe i'm reading this all wrong but off-axis guider helps reduce the mirror flops in SCT's which EdgeHD has these manual knobs to control them.

Maybe i'm just off the whole idea so much that i have no idea what i'm talking about.

So basically i am looking to purchase a 0.7x focal reducer and the Starlight Xpress Lodestar X2 Autoguider (or do you have any other preference on an autoguider for DSO AP), do i really really need an off-axis guider?

Thank you

[souls33k3r]

Is this the wrong section to ask this? Mods please move this post accordingly if required. Thank you

[ollypenrice]

At long focal lengths and with reflectors (because of mirror movement) you should really use an OAG. You might have to watch the back focus though, if using the reducer. I think you should be OK while using a DSLR. It's more the longer backfocus of some CCD/filterwheels which can be problematic. The reducer has a quoted 'chip distance' and into that distance you have to fit the OAG, any filterwheel and the camera. Just check it out before writing any cheques!

While some people have said they manage to guide even with finder guiders on this scope they may be basing their satisfaction on getting round stars. However, you can have round stars and considerable guiding error if the errors are random. I would only ever guide an SCT via OAG myself.

Olly

[Oddsocks]

For the EdgeHD, or any other long focal length SCT or Mak, an OAG is best but it has it's limitations.

Mirror flop is only one cause of guiding errors, differential flexure can also be a problem where you use a separate guide-scope since the axis of the guide-scope can be pulled away from the main axis of the SCT as the weight shifts around when the telescope points to different parts of the sky or reacts to acceleration changes imposed by the guiding software.

The problem with relying on a OAG is that for many deep space objects, particularly those that are outside our galactic plane, i.e. other galaxies,  it can be difficult to find a suitable guide star that is in the same field of view as the object you want to image, this is down to the small field of view that you have with a SCT or Mak' and the fact that the OAG prism only picks up a fraction of the available field, even when using a reducer you can still struggle to find a suitable star to guide on. In this case you have to use a separate guide scope attached to the main body of the SCT that has a shorter focal length, so the guide camera sees a bigger field or the guide scope can be pointed off axis a little to find a nearby guide star that is suitable.

The guiding software will still make a pretty good job of keeping the target object to a small percentage of guiding error even though the focal lengths of the SCT/Mak' and the guide scope are very different but in this set up it is inevitable that some subs will be lost to differential flexure and the FWHM will be larger making the stars appear slightly bloated together with a slightly lower definition overall.

If you do have a guide star close by to the target object then of course an OAG is the best solution since it will be rigidly aligned to the camera axis so that if there is any mirror movement due to flop or any other cause then the guiding corrections to the scope will always result in the object being placed on the same pixels for the duration of the exposure.

When imaging objects within our own galaxy guide stars are much more plentiful and in most cases a suitable star will appear in the OAG's field of view.

So you see, separate guide scope or OAG is not really the question, both have there uses, the choice often comes down to what the target is and whether a suitable guide star can be found close by.

The Lodestar can be swapped between guide scope or OAG as necessary, when doing this you just need to replace the cap over the OAG exit port to stop external light getting in to the system.

It is a long time since I last used a long focal length SCT for imaging, over fifteen years, but I did have both an external guide scope and an OAG and swapped between them fairly often. For a while I also used an active optics unit, which includes an OAG, but at that time the guide cameras available were not very sensitive and the active optics unit was only used a half dozen times as I just could never find a suitable guide star within the field of view.

Things have changed in recent years and the current Lodestar is much more sensitive than the camera I used so perhaps you may find it easier to find suitable guide stars with an OAG in all situations.

I suspect though you will still need both approaches, stick a suitable short guide scope on adjustable rings on the body of the SCT and for all objects you will find a guide star but be prepared to drop a few subs due to flexure. For those objects with a suitable guide star in the field then swap the camera to the OAG and enjoy improved guiding and less dropped subs.

[ollypenrice]

While it's true that an OAG won't pick up lots of guide stars I found that I never failed to find one when using a Lodestar. This was at a hefty 2.4 metre FL. A sensitive camera is essential though.

Olly

[souls33k3r]

Thanks for the replies guys, much appreciated. I'm not well versed with the terminologies as such and getting my head around what you fine folks have advised will be a bit of a challenge for me but I am willing to dig more in to this since i am only just stepping in to this minefield. So I will read these posts over and over again until I am familiar with all this. Maybe you guys have put this in layman's terms but my old head is struggling to understand the first time around

[souls33k3r]

You might have to watch the back focus though, if using the reducer. I think you should be OK while using a DSLR. It's more the longer backfocus of some CCD/filterwheels which can be problematic. The reducer has a quoted 'chip distance' and into that distance you have to fit the OAG, any filterwheel and the camera. Just check it out before writing any cheques!

Olly

 

Sorry Olly, though I sincerely appreciate your knowledge in this hobby and wish that some day I could could even half of what you know already but you just went too technical for me here. Not what what you mean by "Back focus" and basically the complete sentence which I've quoted here.

For the EdgeHD, or any other long focal length SCT or Mak, an OAG is best but it has it's limitations.

Mirror flop is only one cause of guiding errors, differential flexure can also be a problem where you use a separate guide-scope since the axis of the guide-scope can be pulled away from the main axis of the SCT as the weight shifts around when the telescope points to different parts of the sky or reacts to acceleration changes imposed by the guiding software.

The problem with relying on a OAG is that for many deep space objects, particularly those that are outside our galactic plane, i.e. other galaxies,  it can be difficult to find a suitable guide star that is in the same field of view as the object you want to image, this is down to the small field of view that you have with a SCT or Mak' and the fact that the OAG prism only picks up a fraction of the available field, even when using a reducer you can still struggle to find a suitable star to guide on. In this case you have to use a separate guide scope attached to the main body of the SCT that has a shorter focal length, so the guide camera sees a bigger field or the guide scope can be pointed off axis a little to find a nearby guide star that is suitable.

The guiding software will still make a pretty good job of keeping the target object to a small percentage of guiding error even though the focal lengths of the SCT/Mak' and the guide scope are very different but in this set up it is inevitable that some subs will be lost to differential flexure and the FWHM will be larger making the stars appear slightly bloated together with a slightly lower definition overall.

If you do have a guide star close by to the target object then of course an OAG is the best solution since it will be rigidly aligned to the camera axis so that if there is any mirror movement due to flop or any other cause then the guiding corrections to the scope will always result in the object being placed on the same pixels for the duration of the exposure.

When imaging objects within our own galaxy guide stars are much more plentiful and in most cases a suitable star will appear in the OAG's field of view.

So you see, separate guide scope or OAG is not really the question, both have there uses, the choice often comes down to what the target is and whether a suitable guide star can be found close by.

The Lodestar can be swapped between guide scope or OAG as necessary, when doing this you just need to replace the cap over the OAG exit port to stop external light getting in to the system.

It is a long time since I last used a long focal length SCT for imaging, over fifteen years, but I did have both an external guide scope and an OAG and swapped between them fairly often. For a while I also used an active optics unit, which includes an OAG, but at that time the guide cameras available were not very sensitive and the active optics unit was only used a half dozen times as I just could never find a suitable guide star within the field of view.

Things have changed in recent years and the current Lodestar is much more sensitive than the camera I used so perhaps you may find it easier to find suitable guide stars with an OAG in all situations.

I suspect though you will still need both approaches, stick a suitable short guide scope on adjustable rings on the body of the SCT and for all objects you will find a guide star but be prepared to drop a few subs due to flexure. For those objects with a suitable guide star in the field then swap the camera to the OAG and enjoy improved guiding and less dropped subs.

Cheers Oddsocks, absolutely understood everything apart from the "Flexure" which I'm sure wither you or someone else will be able to help me understand .

My SCT came with a standard 9x50 finder scope, i don't suppose that would be enough? What would you suggest?

But like what Olly said, lodestars are supposed to be a beast for finding guide stars because of their sensitivity which is what I would then initially give it a go and if that doesn't work then a separate guide scope will be the way to go forward.

So far we've been talking about the normal SCT, what about the EdgeHD? Would I need an off-axis guider for that?

[souls33k3r]

Olly, focusing on what you said I came across this thread which is what now i understand about what you meant by Back Focus

http://stargazerslounge.com/topic/227820-celestron-edge-800hd-7x-focal-reducer-celestron-93648-off-axis-guider-incompatability-when-used-with-canon-650d-dslr/

Really??? Im so frustrated right now. Though I haven't bought any of these equipment yet but I feel robbed by my purchase. I certainly do not want to go the CCD route yet but if this is what I'm in for then im so not impressed by what I've been sold.

[ncjunk]

Cheers Oddsocks, absolutely understood everything apart from the "Flexure" which I'm sure wither you or someone else will be able to help me understand .

My SCT came with a standard 9x50 finder scope, i don't suppose that would be enough? What would you suggest?

But like what Olly said, lodestars are supposed to be a beast for finding guide stars because of their sensitivity which is what I would then initially give it a go and if that doesn't work then a separate guide scope will be the way to go forward.

So far we've been talking about the normal SCT, what about the EdgeHD? Would I need an off-axis guider for that?

Flexure badly explained: when you add a separate guide scope in an ideal world they will be aligned perfectly and 100% rigid with no possibly way for them to move. Unfortunately life's not perfect so one scope, lets say the guide scope, may move slightly which misaligns it with the main scope [the guide image moves but the main sct doesn't] this movement can be caused by the mounting you are using to fix the guide scope not being tight enough or the metal contracting/expanding causing the scope to move. Basically when you use a separate guide scope you also end up guiding any small movements, errors, in that telescope which the main scope doesn't have.

conversly the guide scope won't see the errors due to mirror shift on the sct - i use an oag on my sct so that everything is seeing the same errors and fighting for me to produce the best images they can, which aren't very good in my case, bless their little cotton socks, not that I dress my camera in cotton socks.

I hate my mirror flop with a passion, i just installed 3 screws to lock the mirror down and they are too short - bum.

I do like the previous replIes, very good.

Sent from my SM-N9005 using Tapatalk

[ncjunk]

Olly, focusing on what you said I came across this thread which is what now i understand about what you meant by Back Focus

http://stargazerslounge.com/topic/227820-celestron-edge-800hd-7x-focal-reducer-celestron-93648-off-axis-guider-incompatability-when-used-with-canon-650d-dslr/

Really??? Im so frustrated right now. Though I haven't bought any of these equipment yet but I feel robbed by my purchase. I certainly do not want to go the CCD route yet but if this is what I'm in for then im so not impressed by what I've been sold.

The joys of backfocus! My reducer needs 105mm from it to the camera chip. First I spent 2 hours trying every combination of extension tube i had only to come up 20mm too short, then i bought an adjustable one which when it arrived was the wrong diameter and finally i got the right size but installed it incorrectly, 8mm too long, gave up and used it like that.

i also put a crayford focuser on the sct which is a must have for motorised focusinf=g.

Sent from my SM-N9005 using Tapatalk

[ollypenrice]

Sorry I wasn't clear.

A reducer is a lens which screws onto the back of the SCT and reduces the focal length and the focal ratio.

Such a lens has a chip distance which must be respected. That is to say, the camera chip must be fixed at a specified number of mm behnd the lens. (This has nothing to do with focus, which is done using the focuser as usual after the chip distance has been set up correctly.)

Let's say the chip distance specified for your reducer is 55mm. We now start to work out whether can be acheived. We add up the length of each component in the imaging train. I'm inventing these values but they might be...

OAG                       22mm

Filterwheel             2Omm

Camera chip distance (distance from front of camera to chip) 18mm

Add these together and you get 60mm. This is not going to work. The chip will be 5mm too far from the reducer.

If the chip distance of the reducer were 100mm then you would do the same sum and find that you were 40mm too close this time. Not a problem. You source a 40mm extension tube and put it on the back of the reducer or between the OAG and the filterwheel. The filterwheel should always be adjacent to the camera.

The 'Edge' aspect of the scope describes its naturally flat field, meaning you get undistorted stars to the edge of the chip. (The reducer reduces the size of this flat field, or certainly in the cae of the Lepus reducer. The Celestron one gives a bigger circle, I believe. None of this has any bearing whatever on the need for guiding.

How good your guiding needs to be depends also on the pixel size of your camera. The smaller the pixels the better you need to guide. (Simply because the finer pixels can resolve finer details which will be lost if the guiding is not correspondingly finer.)

Olly

[souls33k3r]

Sorry for replying very late on this thread (been feeling unwell).

Olly no need to be sorry mate, not your fault that i don't understand these terminologies that well. I'm being a noob eating your brains here

Now i'm seriously concerned and confused. Has Celestron been able to fix this back focus issue yet or not? Will i be good using the Celestron 0.7x focal refucer, Celestron off-axis guider and a Nikon D3100 DSLR together? I do not want to go down the CCD route (yet) because that's too much money and i'm not loaded by any means.

Guide scope is another way to go with this as suggested by Oddsocks but he mentions that too has it's fair share of issues.

[Oddsocks]

I would say it is not possible to use the combination of OAG, reducer and Nikon DSLR that you specify but you really need to confirm this with whoever supplied the items to you.

As a non-expert , and never having owned or used a Celestron product, but just looking at the Celestron web pages and data sheets I can see that:

In the Celestron data sheets for the x0.7 Edge HD reducer with the part No. #94242 and the OAG with the part No. #93648 the following back focus distances are quoted:

x0.7 reducer to image plane (CCD chip) = 105mm

Nikon DSLR "T" mount to image plane = 55mm

Subtract the two leaves you with 50mm in which to fit the OAG body, the connector rings between the "T" mount on the camera and SCT thread on the reducer body and the spacers needed to ensure the DSLR sensor and the guide camera both meet focus at the same distance.

From the OAG data sheet for the 8" Edge HD option:

OAG body back focus = 29mm

SCT thread adaptor between the OAG and the reducer back focus = 25.3mm

"T" thread adaptor between the OAG and the Nikon "T" mount back focus = 12.5mm

Medium "T" thread spacer to allow guide camera to reach same focus as DSLR sensor = 11.5mm

Add these up = 25.3mm + 29mm + 12.5mm + 11.5mm = 78.3mm

Now you only had 50mm back focus available and 50mm - 78.3mm puts you 28.3mm outside the critical focus point and according to the data sheet for the reducer the tolerance on back focus is only +/- 3mm and this itself depends on the size of the image sensor with the tolerance becoming smaller as the sensor size increases.

So just going by this published information you can not use the x0.7 reducer with the OAG, there is simply not enough back focus available.

Being practical, the Edge HD produces a fairly large and flat image that will cover the DSLR sensor and many objects will fill the frame comfortably without the need for the reducer. In these circumstances it makes sense to use just the OAG and the Nikon, you will get the best guiding possible and drop the fewest subs to guiding errors, with patience you could image larger objects than fit on the camera by building up mosaics much as Olly does and this would be a good starting point. Imaging from London you will have to contend with quite severe gradients due to light pollution and these will be less intrusive and easier to post process away with the smaller field of view of the EdgE HD than would be the case when using the reducer.

Even so, the native focal length of the Edge will make guiding, even with the OAG quite "interesting" and any small residual errors that the guiding software cannot correct will make the stars in the final image appear "x" pixels bigger in diameter than they otherwise would be. If then you wanted to use the reducer and swapped to using an external guide scope, I would suspect, given that the stars would now be smaller on the sensor because of the reducer but the guiding errors larger, because of the inherent "flexure" between guide scope and Edge HD then in the final image the stars diameter would be very similar to the OAG non-reduced version.

Many SCT and Mak owners use separate guide scopes, even though far from ideal they do achieve good results and would be best met by adding an Edge HD dovetail / accessory rail with strong rings or clamps to fix the guide scope rigidly and as close to the Edge HD body as possible. I can't suggest a particular rail/ring combination though as I have never seen an EdgeHD other than in pictures, you would need to talk to a good retailer to identify the correct rail and ring system for the Edge HD. The stock finder scope and mount that is supplied with the Edge HD is only good enough for visual alignment, it would be much too "sloppy" to provide a good fixture for a guide scope and camera plus I don't think the eyepiece of the finder scope can be removed to allow a guide camera be fitted in it's place but I'm not certain about that.

So you have some options to consider and the difficulty you are having at the moment is par for the course I'm afraid so don't be disheartened, nothing is easy once you start attaching camera's to telescopes but very satisfying once everything is resolved and you begin producing images for the first time.

[souls33k3r]

Cheers Oddsocks mate, much appreciated for your reply.

You mentioned whether the eye piece of the finderscope supplied with my scope can come out to have the autoguider slotted in, well i have seen at least one video on Youtube where this guy used the SSAG like that and made the 9x50 finderscope in to a guidescope.

I spoke with the supplier from where i bought my scope from yesterday and also someone at David Hinds support. They were not aware of such issue. I was asked to go down to my supplier of telescope tomorrow and they will find a solution for this issue for me but i already know what solutions they are going to provide which is to buy a guide scope or do not use the focal reducer when using OAG.

I have thought about this long and hard, (please do correct me if i'm wrong) but the number of objects that i can quite possibly photograph using the focal reducers are going to be limited as opposed to the distant galaxies/nebulae. Either way, it would be something good to have or unless i move to the dark side CCD.

[ollypenrice]

OK, that scope-reducer-camera combination will image at about 0.73 arcseconds per pixel. That means that each pixel covers just 73% of an arcsecond on the sky. In amateur AP terms that is a very high level of resolution to aim for and would need extremely good seeing and extremely good guiding to make possible. (I have imaged at slightly higher resolution at one time but now work at 1.8 and 3.5 arcsecs P/P. (This is just to give you a point of reference.)

My honest opinion is that you are probably not going to be able to avoid losing some of your potential resolution. This means that, under no circumstances, should you take any guiding shortcuts. You are going to need all the help you can get, so go for an OAG and sensitive camera. This will get you as close as possible to your challenging target guiding accuracy.

My calculation of your pixel scale was done on this free calculator. http://www.12dstring.me.uk/fov.htm FLO have another one.

Now, I had a quick look at the specs.  According to FLO's site the Celestron OAG is not compatible for DSLR imaging with the Edge focal reducers. Uh-oh. Your best bet would to ask a competent retailer like FLO or Ian King Imaging if there is an OAG which can allow DSRLs to work with this reducer. If there isn't then this is very bad news for the 8 inch Edge. I'd be interested to hear what happens.

If you really cannot use an OAG in the end then I would, at least, steer clear of finder guiders and go for a guidescope of longer focal length. I would not consider putting it in guide rings. I would want it bolted down hard to avoid flexure. A decent guidesscope-camera will find stars every time.

Olly

[Oddsocks]

Olly's advice above is spot-on.

Best of luck with your trip to the dealer, hope you get a resolution tomorrow, not something I would look forward to on Christmas eve!

In case you are not aware there is an excellent free program you can download that will show you what your camera will image and help you to pick suitable targets.

Stellarium:  http://www.stellarium.org/en_GB/

You just need to add your telescope and camera details to the set up page and then enable the sensor frame view and display the target.

( The Edge 8' HD is 2032mm focal length and 203mm diameter, the Nikon D3100 has a sensor 23.1mm x 15.4mm with 4608x3072 pixels 5.0um x 5.0um square)

Below are some Stellarium screen shots of common targets with the sensor frame shown for the 8" Edge HD and Nikon D3100 without the reducer.

Leo triplet group

 

M81

 

M42

 

Rosette

 

[souls33k3r]

Cheers Olly and Oddsocks, much appreciated for you both taking the time out and replying.

I spoken to David Hinds again this morning and they confirmed with their technical colleagues and indeed this is an issue which has been raised by one of their resellers and this technical person knowing it from one or two forums. Before that, they had no clue which really sounds like Celestron deliberately deceiving their potential customers, should they have told this to their resellers from the word go, none of us would be talking about this then.

Any way, i'm a mixed bag of emotions right now because all these extra bits are coming to my head. Hyperstar, OAG, Focal Reducer, Guide Scope, Dovetail Bars.

Pardon the order of what i'm going to jot down because this is what's going on in my head.

1) I might skip the reducer for now and buy the OAG any way and use it with DSLR because the worlds (nebulae/galaxies) far far away wouldn't really require much use of a reducer ... Do correct me if i'm wrong with this ... get started and master on how to do mosaics.

2) Buy a guidescope or another scope (ED80) and mount it on the dovetail and use the Lodestar X2 and skip the OAG which to be quiet honest i don't want to. (Please suggest a guidescope if possible)

3) Find another OAG which is thinner and will hopefully incorporate the reducer as suggested by you fine folks. (Going by Oddsocks calculations which is 28mm thinner)

4) Buy a hyperstar but that won't handle DLSR on 8" EdgeHD model, so will have to go down the CCD route which might happen in the future

If i was to shrug off the Celestron OAG, the other two that i can think of which are thinner would be the OVL (15mm body and 48.0mm thread (female) to telescope, ‘T’ thread (male) to both cameras) but that is still 13mm too much or the Orion thin OAG which i really can't find the specs for.

Can you please assist me with that please?

I'm a man on a mission i am!

[ollypenrice]

You are not going to like what I have to say, I don't think, but your problem is the Edge HD. It has been marketed as an astrograph but it isn't sorted out as such. F10 is too slow. It has a reducer to F7, which is better, but this seems to be incompatible with OAGs which many consider more or less essential for guiding SCTs. Even at F7 the focal length is too long to give a sensible pixel scale with a DSLR and trying to image at 0.51 ''P/P is bordering on crazy. (These are relatively insensitive cameras, uncoooled and so exposure time limitied, and won't get enough light per pixel to work at a tolerable speed.) On top of that, since you won't actually be able to resolve at this level, the focal length will reduce your field of view without adding to your resolution. Hyperstar? Well, in my view F10 is too slow and F2 is too fast and brings with it a host of difficulties. I'll take a nip of a stiff drink and say it: are you wedded to this scope? It risks sending you into a lot of spending with a risk of little success.

Alternatively, consider this. An Atik 4000 mono would, with reducer, be working at 1.07 ''P/P and that is a sight more like it, though still a challenge. You could, however, bin 2X2 and still have a similar resolution to that of my TEC140/Atik 11000. And I presume you could use an OAG. This would be a classic case of taking unbinned lum and binned colour, for instance. Don't be worried by mono and filters. If I can do it...

The problem is that the Edge HD risks bullying you into decisions you don't want to make. I've been through comparable experiences with my own 10 inch Meade SCT. I wasted lots of time and cash on trying to do DS imaging with it as a beginner, never succeeded and switched to small refractors to cut my teeth.

Olly

[souls33k3r]

You're right Olly, i don't like you at all now 

I got the first paragraph but you lost me from the starting of the second lol sorry but i'm just not well versed ... maybe in time

I have spoken to owner of the shop where i bought my scope from and told him about my frustrations and he will be right on this case and find me a solution. If he does, i'll be more than happy to name him and give him the credit for it. For now, let's call him Mr.X

So i've spoken to Mr.X in quite detail about this issue and he agrees with me that Celestron has certainly overlooked at this issue. I did mention i do not want to go the guide scope route (he knows i'm a blumming perfectionist lol) so have told him about the three thin OAGs i have come across.

I know i've seen someone on Astrobin use the OVL Thin OAG without the reducer and i'm happy with the results, i'm sure even if i go for the celestron OAG, i'd still be happy using it without the reducer but here's a curve ball.

1) How much i am limiting myself if i don't use a focal reducer? I know i'll then be imaging at F10 which really isn't realistic but has been done with longer subs.

2) How much bad guiding/imaging (flexture) if i screw the guidescope on the mount using a dovetail (or whatever you may suggest) i am looking at if i use a dedicated guide scope with a lodestar x2

[Oddsocks]

A possible solution to the Celestron OAG problem is to return it and obtain a refund then try the TS (Telescope Service, Germany) short OAG.

I have bought many items from them over the years, including expensive OTA's and they are a dependable company to deal with.

I will list the items you would need from their catalogue....BUT....I can not guarantee my calculations are correct so if you choose to follow this line of thought then you should contact them by e-mail or phone before committing yourself to order the items listed and verify the part numbers are correct and that they will assemble to fill the back focus space you have available.

You can find the TS website here: http://www.teleskop-express.de/shop/index.php at the top of each page is a translation switch between English and German language.

From Post #13 the free space you have between the SCT thread on the reducer and the camera mount "T" adaptor was 50mm

The following sequence of parts will put the TS 16mm OAG in that space.

#TSSC-M48  Adaptor, SCT thread to M48 male (connects to the reducer SCT thread)        Back focus used + 10mm

#TSVF220     20mm spacer M48 male to M48 female                                                             Back focus used + 20mm

#TSVF203     3mm spacer M48 male to M48 female                                                               Back focus used + 3mm

#TSOAG16   TS 16mm OAG body                                                                                           Back focus used + 16mm

#TSOAG-T2  Internal adaptor, (TS OAG to T2 camera mount)                                                Back focus used + 1mm

#T2i-125       1.25" eyepiece/autoguider barrel to T2 female thread for the prism stalk          Back focus not applicable

                                                                                                                                       Total back focus used = 50mm

[Thalestris24]

There's this one too, though I can't verify the spacing will still be ok with the reducer. Tbh, I think the best way forward would be to ditch the dslr and move over to a nice cooled ccd - much less noise and typically higher qe than a dslr

Louise

[ollypenrice]

There's this one too, though I can't verify the spacing will still be ok with the reducer. Tbh, I think the best way forward would be to ditch the dslr and move over to a nice cooled ccd - much less noise and typically higher qe than a dslr

Louise

As any SGL user knows, I'm always going to agree with that!! I'm more than impressed by Maurice Toet's DSLR images but they are taken at ultra fast F ratios and sensible pixel scales. Maurice does not use his DSLR at long FL/slow F ratio/Fine pixel scales.

This much I will say. Any retailer who put up a warning pointing out that the SCT and the DSLR are not compatible with present technology would earn my respect.

Olly

[souls33k3r]

A possible solution to the Celestron OAG problem is to return it and obtain a refund then try the TS (Telescope Service, Germany) short OAG.

I have bought many items from them over the years, including expensive OTA's and they are a dependable company to deal with.

I will list the items you would need from their catalogue....BUT....I can not guarantee my calculations are correct so if you choose to follow this line of thought then you should contact them by e-mail or phone before committing yourself to order the items listed and verify the part numbers are correct and that they will assemble to fill the back focus space you have available.

You can find the TS website here: http://www.teleskop-express.de/shop/index.php at the top of each page is a translation switch between English and German language.

From Post #13 the free space you have between the SCT thread on the reducer and the camera mount "T" adaptor was 50mm

The following sequence of parts will put the TS 16mm OAG in that space.

#TSSC-M48  Adaptor, SCT thread to M48 male (connects to the reducer SCT thread)        Back focus used + 10mm

#TSVF220     20mm spacer M48 male to M48 female                                                             Back focus used + 20mm

#TSVF203     3mm spacer M48 male to M48 female                                                               Back focus used + 3mm

#TSOAG16   TS 16mm OAG body                                                                                           Back focus used + 16mm

#TSOAG-T2  Internal adaptor, (TS OAG to T2 camera mount)                                                Back focus used + 1mm

#T2i-125       1.25" eyepiece/autoguider barrel to T2 female thread for the prism stalk          Back focus not applicable

                                                                                                                                       Total back focus used = 50mm

Cheers Oddsocks mate, you have no idea how appreciative i am for you finding this information out not only just for me but anyone who might stumble across this post in the future. I will most certainly keep this piece of kit in mind if any of the UK suppliers are unable to find the solution for me. Spoken to Zoltran yesterday and he too is going to see if he can figure something out, Mr.X is on the case too, got to speak to FLO legends now and see what they suggest. I'm not giving up any hope and leaving no stone un-turned.

There's this one too, though I can't verify the spacing will still be ok with the reducer. Tbh, I think the best way forward would be to ditch the dslr and move over to a nice cooled ccd - much less noise and typically higher qe than a dslr

Louise

Hahahaha Louise as much as i want buy a CCD (i've seen Sara Wagers Flikr posts using QSI and i would really want to end up with something like that) i just can not afford to buy a cooled CCD. I to be honest am not looking for super sharp and smooth images, but decent ones ... something such as my untrained eye would appreciate by the end result.

[souls33k3r]

I hope you all had a wonderful Xmas with your friends and family

Ok I think I have almost made up my mind on this now but before I spit out more cash, I wanted to find out about these thin versions of OAG ... Are they on par with say the celestron one? Or are they worse ? Fiddly?

Besides what is there to check when buying any of these OAG's apart from how much back focus will they eat up?

[ollypenrice]

TS generally know what they're doing. The only OAG problems I've heard of (and I encountered this one myself) concerned an earlier Starlight Xpress model which allowed the camera to rock on the lockscrew which held the periscope in place once the right depth had been found. I think this was sorted long ago. I fixed the one I was using with an external strap.

An OAG is a bit of a fiddle at first but, once set, never needs to be touched again. Usually the recommended approach is to focus the imaging camera on a very distant terrestrial source first. You then lower the OAG periscope (AKA turret) down into the light path till its shadow appears as a bite out of one edge of the image from the DSLR. Back it out till this bite just disappears. (You want the periscope to be lowered as far as possible into the light cone without encountering the part used by the imaging camera.) It is logical to orientate the camera so that the OAG periscope enters the the image on the long side since this will let the periscope get further into the light cone before intruding on the image. (To put that another way, if the camera is in portrait format the periscope should appear either from below the ground or from above the sky.) 

Once the periscope depth has been set you normally slide the guide camera up and down the periscope to focus it. Again you can get this more or less right on a terrestrial target and then perfect it on the stars later. Once working on the stars, again focus the imaging camera first and then the guide camera. (In a rare exception from the need for perfection in all things AP, the fine focus of the guide camera is not critical! This comes from on high in the voice of Craig Stark, author of PHD.)

Olly