Field flatness: Chip size vs focal length

[mytola]

Hi,

I've been pondering on this thing for a while. Let's say a scope has a certain field curvature on a certain chip size with corresponding FOV. Now, if a smaller chip is used, and the focal range reduced with a perfect focal reducer such that the FOV would be the same as with the bigger chip, will the aberrations be the same on the two sensors? Or is the second case more demanding of the optics?

Reason I'm pondering this is that my scope, the WO GTF81 is supposed to be flat on a full frame chip. At least I know it's reasonably flat on my APS-C. Now I've got an Atik 314L+ with a substantially smaller chip. The obvious thought is if it's possible to place a reducer in front to obtain a lower focal ratio. I know they do it with the TAK flat field scopes, but they are a different species alltogether...

Thanks,

Andreas

[Sp@ce_d]

I do this with my ZS66 & 314L+ with the 0.8x Flattener II.. works fine. The chip is small enough not to need a flatterer anyway but as you say I also wanted the benefit of the .8x reduction.

Try it and see..

[mytola]

Thanks for the reply! But you are using a reducer with field correction right? My scope has a integrated reducer/flattener. What I'm thinking of is to add a second reducer with no correction - obtaining a similar field of view as the bigger chip. So the question is essentially whether it's the field of view alone that determins the flatness or if there are more to it.

[Sp@ce_d]

Ah.. sorry, I misunderstood.. forgot the GT81 had a MKII variant. I also have a GT81 flattener only that I used with my ZS66 so thought you were doing the same as me. I'm not sure if you can stack reducers. I don't know enough (yet) But I'm guessing that it may not work very well.

[mytola]

That's what I'm thinking too, but then I'm also thinking that if the field is flat at a given fov, why shouldn't it be flat at the same fov, but with a smaller chip and a focal reducer with no correction. Maybe I just have to try with i.e. this one:

http://www.teleskop-express.de/shop/product_info.php/info/p676_Focal-reducer-0-5x---1-25--filter-thread.html

or this one

http://www.teleskop-express.de/shop/product_info.php/info/p4614_GSO-2--0-75x-reducer-without-correcture---Price-Offer.html

[Sp@ce_d]

I see there's a thread over on CN about doing this on video for planetary/lunar http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4908311/Main/4898738

Something to also consider is that reducers tend to bring the point of focus closer in so you may not have enough inward travel.

[Sp@ce_d]

The more I think about this the more interesting it becomes, as I've been thinking about adding a RC scope to the collection which would require reducing a fair bit to get a decent speed.

So some further thoughts on this. The more you reduce the more your FOV increases. Therefore wouldn't the flatness of the edges become more critical? Also, adding more glass must degrade the image to some extent & reduce sensitivity?

[mytola]

For small enough sensors it should be ok I think, but I don't know what is small enough, i.e. if my atik has a small enough sensor combined with my scope. If field of view is the only thing that matters jit should be...

I guess cheap reducers may introduce some additional aberrations also, as you say.

[Gina]

Very interesting I would like to increase my FOV but not at the expense of abberrations. I'm using camera lenses to do this ATM but get CA as expected. For NB there's no problem - I just refocus between filters but for RGB there is a certain amount of CA within each bandwidth, especially the blue it seems. A 0.5 FR for my ED80 and 314L+ would give me a very useful focal length. So... question is, who's going to fork out about 40 euro to see how well it works?

[mytola]

Hmm, I'm looking at this reducer:

http://www.astroshop.eu/field-flatteners---other-photo-accessories/omegon-0-5x-reducer-for-photography-and-observation/p,1043

It's listed with an aperture of 22.5 mm, and focal length of 85 mm. Thats f3.8, and it's a doublet configuration. If the TS one is similar, I'm getting somewhat skeptical regarding CA. But then it will be close to the sensor, so maybe that will help...don't know. I'll probably buy one, but I'll have to wait until I buy some other stuff also since there's a ~20 Euro constant customs fee when importing it...

[mytola]

Just had an answer from TS regarding the reducer, and they didn't recommend the cheap one due to the quality (not very sharp). They recommended this one istead:

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p2181_OPTEC-Brennweitenreduzierung-0-7x-fuer-Geraete-mit-Feldkorrektur.html

Quite a different price range...

[Sp@ce_d]

Hmm I did wonder about the cheaper ones.. otherwise everyone would be doing it! They're very precise on the 63.4mm backfocus that sounds like fun. So if my calculations are correct that would give you f/4.1

[Gina]

Yes, it definitely seemed too good to be true