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Using an OAG with SW CC andDSLR

SnakeyJ

By SnakeyJ
in Discussions - Cameras

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SnakeyJ

SnakeyJ

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I've recently lashed out on a cheap 10mm OAG package, which I would like to trial as an alternative to my 70mm Travelmate guide scope to reduce weight and flexure.

I'm imaging with a modded EOS 1100D, standard Skywatcher CC/FR and my Explorer 200P at 900mm/F4.5.  The CC is currently fitted with an M48 to EOS adaptor and 2" nosepiece.

Presumably if I try and squeeze the CC in between the CC and DSLR, I will push the spacing out to 63mm and the CC will not work effectively.

Should I fit the OAG between the CC and nosepiece?   And has anyone tried and got any idea what extension I would need between the OAG and guide camera to reach focus with the camera?

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SnakeyJ

SnakeyJ

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Hi

I think your best bet is likely to be to fit a different cc with more backfocus. I think Teleskop Service do one. But maybe somebody else has a better idea..

Louise

Thanks Louise - I've been having a look around and the Baader RCCI Rowe CC looks to be the job with 93.5mm recommended spacing.   Just seems a shame not to be able to use the SW CC I already have as the 0.9FR gives a bit of a bonus in wider field and imaging speed.

Though the RCCI seems to support a much wider range of ratios from F3.5-6.     The MPCC is quoted at 55mm as is the SW Aplanatic.

I think I will have to try and suck it and see if I can find focus and measure the distance.

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Thalestris24

Thalestris24

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Hi Jake

I seem to recall reading bad things about Rowe Correctors a while ago so I'd check on that. I'm sure I've come across another cc which could do the job - I'll have to look through the archives, lol. Of course changing to a ccd is an option but not a cheap one...

Louise

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SnakeyJ

SnakeyJ

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Hi Jake

I seem to recall reading bad things about Rowe Correctors a while ago so I'd check on that. I'm sure I've come across another cc which could do the job - I'll have to look through the archives, lol. Of course changing to a ccd is an option but not a cheap one...

Louise

Thanks Louise - I'll check out the RCCI reviews and look on the TS site, perhaps GSO do one.   I keep trying to save up for that CCD, but it always seems there's something else I have to get first ;)

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Thalestris24

Thalestris24

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Hi Jake

I had this reference in my archives:

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

It's just one account of someone having some difficulties with the Rowe. I guess you could have a word with FLO. Maybe someone else here uses one?

There was another member who was also thinking about using a Rowe(http://stargazerslounge.com/user/31327-sidelight/) Might be worth asking him?

I had the same difficulties when I was wanting to add a filter wheel to my 150pds/cc/1100d.

Old thread: http://stargazerslounge.com/topic/215058-adding-a-lp-filterfilterwheel-help-and-advice-needed/

I can't help thinking there is an affordable answer out there somewhere!

Hth

Louise

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Thalestris24

Thalestris24

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ps Yes there is a GSO one but it's suspiciously too cheap! Probably best avoided...

One possible way round might be to ditch the coma corrector all together and correct the distortions in software. Quite easy to do with stars. With that and a little cropping I reckon it might be ok for most targets.

Louise

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SnakeyJ

SnakeyJ

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Hi Jake

I had this reference in my archives:

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

It's just one account of someone having some difficulties with the Rowe. I guess you could have a word with FLO. Maybe someone else here uses one?

There was another member who was also thinking about using a Rowe(http://stargazerslounge.com/user/31327-sidelight/) Might be worth asking him?

I had the same difficulties when I was wanting to add a filter wheel to my 150pds/cc/1100d.

Old thread: http://stargazerslounge.com/topic/215058-adding-a-lp-filterfilterwheel-help-and-advice-needed/

I can't help thinking there is an affordable answer out there somewhere!

Louise - thanks for taking the time and providing such a useful response, the articles provided make very interesting reading!

I'll have a look through my box of adaptors to see what I can make with existing bits and how far this will push the 55-57mm limit - I'll also do some test images with and without the coma corrector to see how this fares.

I also see that MA are selling an Ultra Slim EOS to T ring adaptor with a depth of 1mm - this coupled directly to the OAG should give exactly the 55mm required for correct focus!   Looks like this may be a go after all :)

Thanks again - Jake

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SnakeyJ

SnakeyJ

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Hi again

Just as a matter of interest, how much did you get the oag for? I'm always a bit suspicious of 'bargains'!

Louise

Hi Louise - I paid £ 55, though I have seen the same kit go below £ 35.   This seller seems to have got hold of a large supply and keeps relisting them.

Quality feels pretty good and solid, with a nice big prism.    There was a little forward and aft play on the prism stalk even when tightened, though this was easily eliminated with a thin shim.    I have just fitted a 35mm T2 to 1.25 compression fitting for the guide camera and this supports the QHY5L-IIm nicely, fully inserted in to the tube.    The sensor distance from mid point on the prism is approx 44-45mm fully inserted, which should be comfortable and using a compression fitting I have some latitude for sliding this in and out to tune the extension - though a helical focuser would be a more elegant long term solution!

The threads and anodising on the extension tubes look pretty good and a set of four almost justify the price in themselves.

First impressions are good, but I'll reserve judgement until it's up and working - though no doubt this will work better than the poorly mounted Travelmate 70 I have been using so far.

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  • 2 weeks later...
SnakeyJ

SnakeyJ

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Well it sort of works, without too much coma evident on the DSLR, but I am seeing diagonal streaks instead of stars on the guidecam.   Not the best night to test as I was mounting, collimating and commissioning my OO Europa and it was quite windy.

Hope to have another play tomorrow and will test a couple with the OAG, but with and without the CC for comparison to check that the CC is giving some correction/benefit.

The new rig will work at F4.3 with the 0.9x focal reduction of the SW CC. 

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SnakeyJ

SnakeyJ

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OK - made it quite a big improvement in the guide cam star shape, by moving the stalk in a little - though no idea if this will be encroaching over the sensor and causing vignetting.   ideally I will need to mod one of the three thumbscrews on the OAG ring, so that the guide cam can be positioned perpendicular to the bottom of the camera sensor.    This should allow max inwards travel without obscuring/encroaching on the sensor.

No problem with finding a star on the QHY5L-IIM @ 15% gain and 1.5 sec exposure in PHD2 - though will need to reduce pulse size to reflect increased focal length of shooting through scope.

Sufficient for now and just doing a quick test on M27 to see how the CC is working with this spacing; 10 x 120sec, ISO800 with the CC and another 10 subs without CC.   Results to post up later tonight.

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SnakeyJ

SnakeyJ

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Not my prettiest, but here's a quick process with identical processing settings.   Note slightly wider field in image with coma corrector due to focal reduction of 0.9x with SW CC.

With Coma Corrector and OAG (f/l 1080mm):

post-26731-0-09632000-1411340266_thumb.p

OAG Only (f/l 1200mm):

post-26731-0-99583000-1411340288_thumb.p

Results look pretty conclusive, with better quality without the coma corrector due to increased spacing between CC and sensor.   Interestingly the DSS scores were about 30% higher for the non corrected images.

Now can I do anything else to reduce the spacing?

I will measure the final spacing with my digital calipers, once I find out how to replace the batteries that is ;)

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