Am i out of luck with an OAG

[Ewan]

After having sporadic results with guiding & phd, some good some bad, i started looking at going with an OAG.

My AA 80mm APO i'm guessing should be ok with just a WO extension, OAG then Atik but when i use the flattener i have a spacing distance of just 54.7mm, my imaging train is sensor - FW = 18.6mm, FW = 22mm & FW - Flattener = 8mm total = 48.6mm, now i know it's tight, i have read that you can allow 5% either way which is 2.73mm + the 6.1mm i have left = 8.83.

As i have a fairly small sensor the margin may be bigger than 5%, i see FLO do one from OVl but it's still 15mm overall , can anyone see any way around this problem ?

Could do away with the fw IF one of the adapters takes the 1.25" filters directly, not ideal but a solution, or don't use the flattener then it was pointless buying that as well then.

With the Atik 314L+ is spacing not as critical & has anyone here got away with larger spacing than they should have done ( i appreciate the scope probably comes into play as well).

I have my finder guider mounted on the side side of the APO will it make any difference to guiding ie flexure, having it on top of the scope attached to the rings ?

This is probably not the first time this has been asked but answers i have found so far are few & far between.

[steppenwolf]

Where does the 18.6mm figure come from? The Atik 314L+ has a backfocus of 13.5mm and it has a female 'T' thread - is the extra 5.1mm a projection on the rear of the 22.0mm of the filter wheel? Likewise, the 8.0mm of the filter wheel to focal reducer, how is this made up in addition to the 22.0mm?

The sensor size only masks the mismatch, of course but the non-corrected errors will still be there, just not so prominent. 2.73mm is probably pushing things a bit although you will gain an allowance of 0.7mm through the use of your Baader filters.

Getting this spacing right can be an absolute nightmare - I ended up making a hybrid filter wheel/OAG using both Atik and Starlight Xpress components!

[Ewan]

Hi Steve,

The 18.6mm is 13.1mm back focus to sensor (from what i found on the net not 13.5 this i will check again i think) + a male to male thread of 5.5mm & the 8.0mm is from the flattener top flange (as shown by Altair) to the FW.

I am thinking it's going to be almost if not possible at all to use an OAG with the flattener + the APO, i will still think about getting one though to use on my C8 instead though.

Thanks for the reply Steve it's given me more to think about.

[Gina]

I know I'm a bit tired from a disturbed night but I can't quite make out those figures   I've used the Evostar ED80 with reducer and 55mm back focus required like yours (near enough) with OAG from FLO 16mm,  EFW2 22mm, 314L+ 13.5 according to Atik.  What have I missed?

[Ewan]

I know I'm a bit tired from a disturbed night but I can't quite make out those figures   I've used the Evostar ED80 with reducer and 55mm back focus required like yours (near enough) with OAG from FLO 16mm,  EFW2 22mm, 314L+ 13.5 according to Atik.  What have I missed?

Hi Gina, maybe i have read the drawing wrong but i am assuming on the picture above where it sais 'Otimum image plane from the flat shoulderetc' means to the atik sensor face ?

When my image train is attached together, flattener - M48 to T2 adapter - filter wheel - variable T adapter , this all measures about 36mm to the very edge of the atik, so then i added 13.1mm for the distance to the sensor. Have a made a boob on this then ? I hope not.

Does the OAG replace one of the adapters with its own connection ?

I will take a snap & post it on here, may be easier to see what i have added together.

[ollypenrice]

My concern would be that all this effort might not fix your guiding issue. With an 80mm refractor a parallel guidescope should work perfectly. I use nothing else, only using an OAG on the 14 inch reflector. Forgive the rather 'off topic' approach but I think it's worth considering.

Olly

[Ewan]

My concern would be that all this effort might not fix your guiding issue. With an 80mm refractor a parallel guidescope should work perfectly. I use nothing else, only using an OAG on the 14 inch reflector. Forgive the rather 'off topic' approach but I think it's worth considering.

Olly

Something i have also contemplated Olly, i hope your refering to the mount & not the operator :-)

My PA is bang on & i have had 10 minue guided sub sucess, i am going to do a strip down & reset of the mount then start afresh.

Olly i appreciate you dont use this mount but i havea question, i didn't get 1 answer when i posted this in another topic, i have a Guide Rate setting under the Autoguide rate handset menu, i can select from 10-90% of the sidreal rate in both RA or DEC, it was originally set to 50% in Ra & Dec, my RA graph in phd is pretty level but my Dec either starts travelling upwards or downwards & will carry on going, i have looked at guide star size/focus, cabling, balance etc. Sorry if its off topic but its the main reason i was looking at an OAG.

[Ewan]

Btw Olly very nice shot of yours in Astronomy Now magazine for November of Simeis 147, really very subtle detail.

[Ewan]

Just to add i have just read a review on the new Starlight Express Mini Filter wheel with built in OAG, it is also electronic for the small(ish) sum of £295.00.

Anyway here is a photo of my imaging train, have i made a massive error ?

My 'guidescope' is an Altair 10 x 60mm finder, f3.75 & 225mm FL, i have seen others using similar so it should 'just' work.

[steppenwolf]

but my Dec either starts travelling upwards or downwards & will carry on going

I hate to ask this but have you definitely got Dec guiding enabled?

[RogerTheDodger]

Going back to the OP, don't you just need to add 6mm of spacers to get the image plane up to 54.7?

[Ewan]

I hate to ask this but have you definitely got Dec guiding enabled?

That's ok Steve no worries, yes i leave it on Auto, i have tried the other options but Starks tutorial sais it can stay on Auto so i left it there, still worth asking though.

My frac is light @ 2.85kg, Atik - 400g, fw approx 300g (dont know the exact weight), flattener - 150g, AA finder - 795g so i should not be overweighted mount wise either.

I am going to take the mount head off tonight & check for any loose bolts etc anyway.

Going back to the OP, don't you just need to add 6mm of spacers to get the image plane up to 54.7?

That's correct for 'normal' guiding but i am thinking of getting an OAG, this i could also use with my SCT when it's mounted.

It would appear that unless i have made a huge error somewhere or the Altair Astro picture of the flattener is wrong then i don't have the gap to fit an OAG in the image train, the only thing i can think of is if the OAG has a built in adapter that would replace one of mine i use at the moment.

[Davey-T]

Are you using the ST4 cable or ASCOM ? I had problems with the mount ignoring guiding corrections sent by PHD via ST4 and just carrying on Southwards 'till it lost the guide star. No problem guiding with ASCOM.

Dave

[steppenwolf]

Ewan, I have just checked out my own solution to this issue using my EFW2/SX OAG hybrid and the combination is 39.0mm deep soooooo:-

Atik sensor the mounting face = 13.5

EFW2/SX OAG hybrid             = 39.0

Altair Astro M48(F) to 'T' (M)   =  2.0 (estimated)

Total = 13.5 + 39.0 + 2.0         = 54.5

Here's a possible solution then?

[dmahon]

Every bit of glass increases the backfocus a bit too. There is glass in a filter and glass at the front of the CCD. You might just manage with that slim 9mm OAG. Some OAGs also remove a spacer off the filterwheel too if they can be bolted directly to it (like the SX OAG and SX filterwheel.

[Ewan]

Are you using the ST4 cable or ASCOM ? I had problems with the mount ignoring guiding corrections sent by PHD via ST4 and just carrying on Southwards 'till it lost the guide star. No problem guiding with ASCOM.

Dave

Sorry about all this guys,

I do guiding through the autoguide port on the cg5-gt via a Hitec USB Guider Interface & the DMK attached to the laptop via USB, i select on camera guidescope, Camera - Windows WDM-style webcam camera.

I have just seen that the Ascom Platform driver is now at 6SP3 so i am updating that, then i will reinstall the Hitec Ascom driver as well.

Ewan, I have just checked out my own solution to this issue using my EFW2/SX OAG hybrid and the combination is 39.0mm deep soooooo:-

Atik sensor the mounting face = 13.5

EFW2/SX OAG hybrid = 39.0

Altair Astro M48(F) to 'T' (M) = 2.0 (estimated)

Total = 13.5 + 39.0 + 2.0 = 54.5

Here's a possible solution then?

Could be Steve thanks mate for that, i am interested in the new OAG / FW from Starlight Express it has a backfocus of 37mm for £295.00, i thought i posted this earlier but maybe i didn't actually click Post, very sorry.

[dmahon]

If your flattener offers a screw thread, SX do a huge range of attachments for their filterwheel so that it can attach direct rather than need an additional adapter.

[Ewan]

Dmahon i will look into that option, i feel a bit bad 'dumping' my problem on here tbh.

I noticed last night while taking a look at my mount that when the left hand button on the handset is pressed the mount responds almost straight away, after taking my finger off the same button the scope still travels in that direction  does anyone know what maybe causing this ? backlash setting on the mount maybe ?

The up button takes about 2 seconds to respond as well so i obviously have a mount problem somewhere.

One last thing the PA routine sais finish aligning the stars with an up & right motion NOT down & left now does this mean up & right on the HANDSET or while looking through the ep ? this may sound like i stupid question.

[Ewan]

Dont worry i found the inf needed about the final approach, out in 20 to setup as it's clear all night for a change

[badgers]

Hi Ewan,

I'm working with the previous version of the Flattener on the same scope and the old planostar required 51.5mm backfocus which was even less space to play with.

I use a Quantum Filter 7x electric filter wheel (21mm) and the TS OAG 9mm + Atik Backfocus (13mm). If memory serves I come in at about 52mm when everything is connected up due to mms lost at thread interconnects.

I have the Planostar connecting directly into the TS OAG and there is very little wasted space with that setup with the connections. 

You shouldn't have any problem at all getting the spacing right, but it may take you a few nights of experimentation.

Here is my current setup -> Around 53mm flange to CCD spacing. You could do the same but you'd probably need a very short extension.

Get a good vernier calipers and don't underestimate how many mm's you can gain or loose by swapping out different adapters.

I changed the T-Adapters on the filter-wheel with new ones from astroboot which were shorter and cost me less space at the interconnect.

If the SXV filterwheel + OAG had been available back then, I'd have bought it in a heartbeat.

Anyway, all I can say is that my current setup is consistently delivering flat stars and 30 minute guiding, so I'm not going to tinker with it again : )

But it took 3-4 nights to get the spacing right.

Here is my old setup before moving to the new filter wheel, I gained a mm or so 

Hope this is helpful in some way!

Anton

[Ewan]

Sure is Anton, it was because of guiding i was looking at this avenue & still may go down it for my SCT.

Thank you for getting back to me much appreciated.

[badgers]

No worries Ewan.

The OAG did work wonders for me. My home-made guidescope rings and finder-guider seemed to cause quite a bit of differential flexure that crept in above 10-15minute exposures.