DSLR + reducer + OAG + spacing = confused !

[matsey]

Hi all,

I just can't seem to wrap my head around this and need some advice on whether I have the correct OAG for what I want to do going forward. So confused and need some help !

Here's the equipment I currently have:

- Canon 650d DSLR

- OAG from FLO (this one ... http://www.firstligh...xis-guider.html )

- Lodestar guide camera

I'm using this at the moment with the SCT 9.25 and Celestron f6.3 focal reducer, which seems to be working OK (though I confess because of weather and other commitments, I haven't used it a great deal since getting the guide equipment end June).

Hopefully end September I'll have the funds to finish off my imaging setup, this is what I'm planning on buying:

- Skywatcher Equinox 80 Pro (http://www.firstligh...po-pro-ota.html)

- Field Flattener (http://www.firstligh...-flattener.html or http://www.firstligh...-flattener.html)

- TV TRF-2008 reducer/flattener (http://www.televue.c...?id=71&Tab=_TRF)

From speaking to one of the guys at my astro society's astrophotography group, I have just found out that spacing between the focal reducer and the camera sensor is critical - I had no idea ! According to the Televue website, the distance for the TV reducer is 55mm +/- 2mm. From what I've read, I believe the distance between the sensor and the front of the camera body is 45mm.

So... I think... the setup to get the 55mm distance should be: camera body (45mm) --> t-ring adapter (10mm) --> focal reducer. But that would mean that the OAG would need to sit in front of the reducer, giving the camera and the guide camera completely different focus points. But if I go with camera body (45mm) --> t-ring adapter (10mm) --> OAG (13mm) --> focal reducer, I've increased the distance from sensor to reducer to 67mm, which is way outside the spacing tolerance. So.. head now officially blown !

So that's my confusion.... does this mean that the OAG I have will not work with this new setup, or is there some kind of adapter or different way of setting everything up that I need to get it to work? I've tried reading through the SGL thread on this OAG, but (and I could be missing something) most of the talk I think relates to CCDs rather than DSLRs where there isn't a problem with the sensor being so far back in the body.

Thank you very much in advance, looking forward to my head being unscrambled just a little bit !!

Matsey

[lensman57]

Hi all,

I just can't seem to wrap my head around this and need some advice on whether I have the correct OAG for what I want to do going forward. So confused and need some help !

Here's the equipment I currently have:

- Canon 650d DSLR

- OAG from FLO (this one ... http://www.firstligh...xis-guider.html )

- Lodestar guide camera

I'm using this at the moment with the SCT 9.25 and Celestron f6.3 focal reducer, which seems to be working OK (though I confess because of weather and other commitments, I haven't used it a great deal since getting the guide equipment end June).

Hopefully end September I'll have the funds to finish off my imaging setup, this is what I'm planning on buying:

- Skywatcher Equinox 80 Pro (http://www.firstligh...po-pro-ota.html)

- Field Flattener (http://www.firstligh...-flattener.html or http://www.firstligh...-flattener.html)

- TV TRF-2008 reducer/flattener (http://www.televue.c...?id=71&Tab=_TRF)

From speaking to one of the guys at my astro society's astrophotography group, I have just found out that spacing between the focal reducer and the camera sensor is critical - I had no idea ! According to the Televue website, the distance for the TV reducer is 55mm +/- 2mm. From what I've read, I believe the distance between the sensor and the front of the camera body is 45mm.

So... I think... the setup to get the 55mm distance should be: camera body (45mm) --> t-ring adapter (10mm) --> focal reducer. But that would mean that the OAG would need to sit in front of the reducer, giving the camera and the guide camera completely different focus points. But if I go with camera body (45mm) --> t-ring adapter (10mm) --> OAG (13mm) --> focal reducer, I've increased the distance from sensor to reducer to 67mm, which is way outside the spacing tolerance. So.. head now officially blown !

So that's my confusion.... does this mean that the OAG I have will not work with this new setup, or is there some kind of adapter or different way of setting everything up that I need to get it to work? I've tried reading through the SGL thread on this OAG, but (and I could be missing something) most of the talk I think relates to CCDs rather than DSLRs where there isn't a problem with the sensor being so far back in the body.

Thank you very much in advance, looking forward to my head being unscrambled just a little bit !!

Matsey

Hi,

I do not have your set up as I use a SW 80 ed for imaging but I did buy a Meade f6.3 reducer to play about with a while ago and I too started to scratch my hair out as to the spacing. Why it should not be clearly stated on the barrel of the reducer is anyones guess, anyhow for a Meade which is essentially simillar to a Celestron, apparently I needed 110 mm of back focus from the flange to chip. Now for Meade things got complicated as the design was changed to shorten the bacfocus to about 45mm so the scope could clear the mount in an upright position. Now celestron makes an SCT to T adapter for mounting DSLRs which measures 2.125" or about 54 mm in length, the distance from the T adapter mounted on a Canon DSLR to the chip is roughly 55 mm, the back focus of most SW reducers for their ED range BTW. So if Celestron has kept to any logic the back focus is 54 mm + 55 mm = 109 mm. How this works out on an OAG is another matter, I would ring up FLO and ask someone if I were you though just to make sure as the distance is critical if you don't want to end up with coma fringing.

A.G

[steppenwolf]

If you are going to use a typical focal reducer or field flattener with a spacing requirement of 55mm (the 'industry standard') I think you will struggle to get the correct (critical) spacing with the excellent OVL OAG. However, you could buy a Riccardi reducer which has a more generous spacing requirement. T here may well be others that also have a more generous allowance.

[matsey]

Thanks for the replies so far

AG - apologies (my fault, rather long first post! ), it won't be the celestron reducer I'm intending on using with the Equinox 80, it's the Televue one with the 55mm spacing requirement.

Steve - that's a most excellent suggestion to look at other reducers, I completely hadn't thought about that as a solution! Other than the possible future reducer spacing issue, I agree it is an excellent OAG, I have been very happy with it on my current setup. I've had a quick look at the Ricaddi one, which is a bit eek expensive for my budget. However, as you suggested I've had a look for other reducers and have come across this WO one which looks like it should work with the Equinox 80: http://www.firstligh...attener-iv.html . According to the info, that has a spacing requirement of 66-86, so I could easily fit the OAG between the reducer and the camera body/t-ring - that would give a spacing of 68mm, which would be perfect. (And that'll please FLO as well I'm sure, as I'll no longer be going elsewhere for the reducer )

So as that is looking like a very good solution, my next question would be - where in the image train should I be fitting my 2" light pollution filter?! The filter is currently being screwed onto the nosepiece on the OAG (which is then slotted into the a baader 2" click lock adapter), but I think with this type of reducer I won't be using the nosepiece as the reducer will screw onto the focuser directly... so possibly I'll need some other kind of adapter/something to get the filter in place as well.. not sure what though?

I will in any case be sure to discuss with FLO before placing an order to make sure I order everything I need.

Thanks again, much appreciated.

Matsey

[steppenwolf]

The WO FR IV is a very good thought - I have one myself for use on my 4" refractor. The FR IV has the advantage of being adjustable so it could well be that you can find the correct setting.

Let's have a look! For example, on my WO FLT 98, the recommended setting is 76.8mm (assuming a typical DSLR camera focal plain distance of 45mm plus 10mm for the 'T' adaptor). So, with the OVL OAG in situ, you have to allow an extra 15.0mm giving 76.8 - 15.0 = 61.8 which sadly cannot be achieved with the FR IV and my telescope!

This issue of spacing is a nightmare to be honest. Astro-CCD users have an advantage here because the mounting face to sensor distance is usually much less (17.5mm for Starlight Xpress and around 13.0mm for a typical ATIK CCD camera) but it is still not all plain sailing - add a filter wheel to a mono CCD camera and it starts getting difficult again!! Field flattener/focal reducer/coma corrector manufacturers please take note!.

[lensman57]

The WO FR IV is a very good thought - I have one myself for use on my 4" refractor. The FR IV has the advantage of being adjustable so it could well be that you can find the correct setting.

Let's have a look! For example, on my WO FLT 98, the recommended setting is 76.8mm (assuming a typical DSLR camera focal plain distance of 45mm plus 10mm for the 'T' adaptor). So, with the OVL OAG in situ, you have to allow an extra 15.0mm giving 76.8 - 15.0 = 61.8 which sadly cannot be achieved with the FR IV and my telescope!

This issue of spacing is a nightmare to be honest. Astro-CCD users have an advantage here because the mounting face to sensor distance is usually much less (17.5mm for Starlight Xpress and around 13.0mm for a typical ATIK CCD camera) but it is still not all plain sailing - add a filter wheel to a mono CCD camera and it starts getting difficult again!! Field flattener/focal reducer/coma corrector manufacturers please take note!.

You are absolutely right Steve, the lack of inward focus ditance is a real problem, I have got the spacing sorted out witmy Meade reducer but I am not sure if I could achieve focus with either SW 80 ED or my old StarWave 80 ED, If I had money I'd probably buy an Apo telescope with the flattener built into the scope path.

Regards,

A.G

[matsey]

Steve - thanks for the reply and apologies for not being able to respond to the thread until now.

I think it is looking like the wo reducer is going to work, but still confused a bit by the whole spacing calculations. On your calculation you got to 61.8mm with a final -15mm, but I'm not sure what that final deduction is for, is it the spacing without the oag? Or is that to do with the maximum back focus of the scope itself? That again isn't something I've thought to consider if that is the case!

Ok, so, back focus of scope aside (will research this further if that might be an issue), let's assume that the WO reducer will work with the equinox 80 + OVL oag + dslr. So that just leaves the question of a suitable flattener, which I'd want to use if I want to image at the scope's native f6.25. Do flatteners have critical spacing requirements as well? I've had a quick search (on my phone in a coffee shop right now) and couldn't find much information about spacing for flatteners.

So if someone is able to confirm if spacing is critical for flatteners as well I can research see if I can find something suitable.

And, phew, you're right those manufacturers don't make this easy at all!!

Thanks again,

Matsey

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[steppenwolf]

On your calculation you got to 61.8mm with a final -15mm, but I'm not sure what that final deduction is for, is it the spacing without the oag?

The final 15.0mm is the depth of the OVL OAG - this pushes the CCD camera outwards by 15.0mm so you have to reduce the spacing to the FR by 15.0mm to compensate. With the FR IV you can do this by adjusting the setting on the FR dial so, you need to dial in 76.8 - 15 = 61.8. Unfortunately, you can't do that as the minimum setting on the FR IV is 66 which means that the spacing will be 66 - 61.8 = 4.2 mm too far outwards.

[Oddsocks]

I saw this 2" flattener for f5 to f8 refractors in an astronomy centre near Frankfurt this year, no idea how good the results are, I've not seen any images taken with it and don't know anyone who has one, but I remembered it had a long back-focus of around 100mm (depending on the refractor focal ratio) and at the show was being used with an off-axis guider and DSLR.

The price was around 220 euros at the show but it seems to have come down a little since then.

It might meet your needs but you would also have to allow for cost of the additional (adjustable) couplers and spacers to get the correct back-focus distance.

http://www.teleskop-...or-Astroph.html

Regards,

William

[steppenwolf]

Do flatteners have critical spacing requirements as well?

Yes, these too have specific spacing requirements and again the 'industry standard' is 55mm

my next question would be - where in the image train should I be fitting my 2" light pollution filter?!

I install mine in the nosepiece of the flattener/focal reducer.

[matsey]

The final 15.0mm is the depth of the OVL OAG - this pushes the CCD camera outwards by 15.0mm so you have to reduce the spacing to the FR by 15.0mm to compensate. With the FR IV you can do this by adjusting the setting on the FR dial so, you need to dial in 76.8 - 15 = 61.8. Unfortunately, you can't do that as the minimum setting on the FR IV is 66 which means that the spacing will be 66 - 61.8 = 4.2 mm too far outwards.

Thanks for the clarification. So for my setup I've got... camera body (45mm) -> t ring (10mm) -> oag (15mm) = 70mm. The spacing requirement of the reducer is 76.8, so could I make up the extra 6.8 with an extension tube/spacer or similar? Baader have a 7.5mm extension tube, which would be 0.7 too much - if tolerance would allow? Would that work, or have I completely misunderstood what an extension tube would be used for?

I saw this 2" flattener for f5 to f8 refractors in an astronomy centre near Frankfurt this year, no idea how good the results are, I've not seen any images taken with it and don't know anyone who has one, but I remembered it had a long back-focus of around 100mm (depending on the refractor focal ratio) and at the show was being used with an off-axis guider and DSLR.

http://www.teleskop-...or-Astroph.html

Thanks William - that definitely looks like a contender for a flattener, I'll check it out.

Hmmm... so (I think !), I do have some options for flattener/reducers with the OVL OAG + DSLR. I need to do a bit more research, but I think I'm getting close.

However !! Another thought ! Going back to my astrophotography group meeting where this all kicked off, I remember that the guy I was speaking to mentioned an OAG from Teleskop Service. This one: http://www.teleskop-...--fuer-CCD.html. From what he said, and from the look of the pictures, this OAG attaches directly to the camera body in place of the t-ring. If the reducer/filter fitted directly onto the OAG, the spacing would then be reduced to 54mm (45mm + 9mm for the OAG). Add a 1mm spacer (if needed), and I'm then bang on for the "standard" 55mm spacing requirement. So I just wonder if that would be a better option - ie, if I (reluctantly) sell my current OAG and get the TS one, I would have a lot more options for reducer/flatteners. Something else to think about I guess...

[mrflib]

Hi guys

I didn't want to start a new thread so I thought I'd drag this one back up. I too was about to purchase the OVL OAG from FLO and it occurred to me that spacing might be an issue. I have a 115mm APO with a 3" focuser and use both a field flattener and a 0.79X reducer/flattener in front of my 1100D.

Looking at the numbers, simply put, I think I may be stuffed. FLO are looking in to it for me but I thought I would ask here too. 20,000 minds are better than one n'all.

Picture 1 shows the dimension of the 3" field flattener, picture 2 is a screenshot of the info of my 0.79 reducer. They both look like roughly 55mm backfocus is required.

I found this 0mm T2 adapter at Teleskop Service which might help a little... but perhaps not enough. If I can help it I really didn't want to buy any more flatteners/reducers!

Thanks for any help

[matsey]

Hi mrflib

You've probably asked at just the right time as today I have finally decided on a resolution to my problem!

I'm afraid as hard as I tried I just couldn't find a way to make the FLO OAG work with the reducer I wanted (also with 55mm spacing requirement). Even without any kind of adapter to attach the oag to the camera you're still looking at 58mm minimum (13 oag / 45 camera body) - easy to add space, not so easy to take it away!!

The solution for me has been to go for the Teleskop Service oag + their eos adapter. OAG adds 9mm, the adapter connects the OAG flush with the camera body adding no additional space so total for oag + adapter + camera body is 54mm. Add on a 1mm spacer (FLO do a baader set for £11) and you're spot on the 55mm.

Here are the links to the oag and adapter :

http://www.teleskop-express.de/shop/product_info.php/info/p641_Off-Axis-Guider---only-9mm-length---with-T2-adaptation---Special-Offer.html

http://www.teleskop-express.de/shop/product_info.php/info/p6262_Adapter-ring-for-EOS-bayonet-at-camera-end-for-the-TS-OAG-9.html

(though you might need a different adapter for the 1100d - worth checking with TS they were very helpful with me)

So it is with some reluctance I'll be selling on the FLO oag but like you I found I'd much rather have the reducer/flatteners I want and know will work with my setup and swap out the oag instead.

Hope that helps - I have placed the order for the whole kit (thank you Mr Taxman for that refund ) so if you want to wait a bit I will report back on whether the new kit works, see no reason why it wouldn't though.

Matsey

Eek .... order placed for lots of new astro gear - I may be responsible for the end of the summer heatwave!! Sorry!! :0

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[mrflib]

Matesy - I've just found this - could this be the answer?

http://www.365astronomy.com/lacerta-skyfoto-skyphoto-offaxis-guider-for-canon-eos-p-1433.html

[matsey]

Yes that definitely looks like that will work as well, good find.

I think I'd still go for the TS one only because I know someone who already has one but to be fair the one you link too looks equally as good.

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[mrflib]

Right OK - well if I can find one in stock I'll probably give it a punt. I'm on the hunt for a review of it now but it does look pretty solid.

[steppenwolf]

Hi Matsey, sorry, I've only just noticed your last question (post 11 in this thread) but it looks like you have found a workable solution - good result!

[matsey]

Hi Matsey, sorry, I've only just noticed your last question (post 11 in this thread) but it looks like you have found a workable solution - good result!

No problem at all, I'd not been back to the thread for a while myself. But yes I think this will be the best solution long term, I know it's meant buying a second oag but that definitely feels preferable to having to compromise on the reducer/flattener part of the setup.

Just looking forward to getting it all in place and having some clear skies to use it all now :)

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