I am after a focal reducer for a North Group ED127 F7.5. Has anyone used one with this scope?. It will be used with a Meade DSI II on DSO's. Would I need a field flatener too.
I am after a focal reducer for a North Group ED127 F7.5. Has anyone used one with this scope?. It will be used with a Meade DSI II on DSO's. Would I need a field flatener too.
John,
the Ed127 has a fairly flat field as is. That is not to say it can't be improved, but if you're just getting your feet under the astroimaging table, I woudn't spend mooney on one just yet. Similarly with a reducer. They have a place but generally, you need to get your head around other stuff first.
I have an ED127 and use a Williams Optics IV reducer/flattener and am happy with the results but it wasn't cheap. Hotech also make a good flattener but my suggestion is to get your imaging skills in place first and worry about that last 5% afterwards.
Peter
I've had a look at John's scope and we both had the same question on the reducer/flattener.
I believe the current F ratio is F7.5, we've tried a DSLR as well as John's DSI.
Wouldn't a 0.5x reducer or similar be required to widen the FOV and image DSO's like eta carina or the rosette? Else is it dependent on the imaging chip?
and once he uses a 0.5x reducer would that then need a flattener?
The current field is flat but what would it be like with a reducer?
Also, wouldn't the reducer lower imaging time to achieve the same apparent brightness?
I've had a look at John's scope and we both had the same question on the reducer/flattener.
I believe the current F ratio is F7.5, we've tried a DSLR as well as John's DSI.
Wouldn't a 0.5x reducer or similar be required to widen the FOV and image DSO's like eta carina or the rosette? Else is it dependent on the imaging chip?
and once he uses a 0.5x reducer would that then need a flattener?
The current field is flat but what would it be like with a reducer?
Also, wouldn't the reducer lower imaging time to achieve the same apparent brightness?
Yes, a reducer will widen the field - and reduce exposure times. WHether it produces some optical side effects will depend a bit on the reducer. I can't speak for the ones mentioned in the original post I'm afraid.
A combination reducer/flattener would answer both needs.
Peter
I am after a focal reducer for a North Group ED127 F7.5. Has anyone used one with this scope?. It will be used with a Meade DSI II on DSO's. Would I need a field flatener too.
John...if you go to the ED127 yahoo forum in my sig then there are posts all about this.
But here is the guff for others....Explore Scientific are just about to beta test a new dedicated 2" FR/FF and a 3" FF.
They tried this around 2 years ago but those units failed testing so this is the "back to the lab result from them".
I spoke to one of the guys who is going to do testing (and to ES). He Knows of it but doesn't know when he'll get one to test. I believe (I could be wrong) ES has them built already and are awaiting this testing to release them. ES told me of the 3" FF unit.
Ok a 2" FR/FF will be restrictive but better than nothing. A 3" FF...hmmm I'll have one.
Hope this answers the question...oh and other FR/FF don't work 100% well with this scope (IMHO) but there are quite a few FF that work ok.
The common mass produced focal reducers are meant to be used on SCT's with a secondary F ratio of 10. The common size is refered to as F6.3. and reduces the effective focal length by about a third. However these devices do more than reduce the F ratio as they are designed to match the optical path of a SCT and perform field flattening as well.
A second common type is refered to as F3.3 will reduce the focal length by two thirds. However these also are designed for use with a F10 SCT for older digital imagers using small physical size chips to get an apparent wider field. These type are not suitable for use with the large CCD's that are now used extensively as distortion outside the designed area can be quite severe.
This means that they are not suitable for use on a refractor. Admittedly they will reduce the effective focal length by a third or two thirds but the other corrections will not match the optical path.
For a refractor you will need a focal reducer designed for the actual telescope and it will probably cost more than replacing the OTA with one of a shorter focal length.
The calculators can be used to give the approximate photo dimensions if you substitute the amended reduction in focal length.
Here is a FR that works perfectly with the 127mm scope http://www.iceinspace.com.au/forum/s...ad.php?t=87167
Tried and tested and an image to prove it.
Works exceptionally well with any flat fielded scope, here it was tested with a GSO RC but it also works with the 127mm that I have.
Last edited by allan gould; 28-02-2012 at 02:50 PM.
Thanks everyone for the replies.
Hi Peter the price differance between the 2 I mentioned and the WO unit or similar is my main concern. As they're a lot cheaper I was concerned about how the result would compare.
I might wait and see if I can try one or two at snake valley camp or other viewing nights.
Hi Brendan, I wont be getting a 3" as I only have a 2.5" DT but sounds like your saying that bigger is better. i.e. a 2" would be better than a 1.25" or are there other factors involved that would negate the bigger is better.
I have joined the yahoo group and will have a read thru that. Is it just me or are the yahoo groups a pain to browse.
Hi Barry Thanks for the explaination. I have been staying away from the sct ones. I have been using ccdcalc to check my FOV and realised how small the meade dsi chip is, so thought if I had a reducer I would be able to get a bit wider FOV.
Hi Allan, I didnt see your reply before I posed the above reply.
I did read your post but as it was tested on an RC I didn't know if it would be usable/suitable on a refractor. I had a look at the lumicon web site but it has been discontinued.
Boy, you guys don't read other threads do you.
Here a FR that works perfectly with the 127mm scope
.....
Hi Allan, thanks....problem is I've purchased an expensive unit (and associated adaptors, RFL-4087) that didn't fully work (corner stars elongated). I have spoken with quite a few other users on CN & other places WRT other FR/FF's tried and so far we found none perfectly suitable.
I'm a bit gun shy on $ out-laying on another unit on a maybe. I really need image proof before I'll buy another FR/FF.
Brendan
Last edited by wasyoungonce; 28-02-2012 at 05:45 PM.
I'm going to test another fr/ff on the GSO RC and the 127mm scope. I believe it will work equally well on both of them. The fr is the Astrophysics ccd67 which has been shown to take the GSO RC from f8 to f4.8 and maintain a flat field over a 22mm diam chip. This fr should also work well with the 127mm scope.
As soon as I get some cloud free skies I'll post the results here.
I have been using ccdcalc to check my FOV and realised how small the meade dsi chip is, so thought if I had a reducer I would be able to get a bit wider FOV.
Regards
Hi John,
Not sure if this is required or relevant, but you might want to think of how it would work with bigger chip sizes as well in case you get a dslr or ccd, not just the dsi.
Hi Allan, thanks....problem is I've purchased an expensive unit (and associated adaptors, RFL-4087) that didn't fully work (corner stars elongated). I have spoken with quite a few other users on CN & other places WRT other FR/FF's tried and so far we found none perfectly suitable.
I'm a bit gun on $ out-laying on another unit on a maybe. I really need image proof before I'll buy another FR/FF.
Brendan
Fully understand that Brendan as it can be a minefield and you have to see the field with your own eyes. The only reason I got the ccd67 was that I have seen several user with this reducer on the GSO RC and they have very flat fields and I suspect it will also work with the 127mm but that is yet to be tested. I might be able to get out tonight clouds permitting and so I will see.
OK I managed t get a few shots with my qhy10 and 127mm scope.
the first image is the full frame of M42 with the CCD67FR at a total of 101mm from the chip. The next is 127 mm frame and the last is a large magnification of each of the four corners of the CCD67 image.
Hope this helps but so far this looks about the best Ive used on this scope, the Lumicon 2" FR comes a close second while the others are hopeless.
I may have to do a little juggling with the chip distance and to get the stars a little tighter in the corners but its very close.
Hope this helps, Allan
