Time for some barlow shopping, which one? - Planetary/Lunar

[ONIKKINEN]

Currently i am imaging with an f/4.4 newtonian, TS 2.5x APO barlow + paracorr and ZWO 678MC (2 micron pixels).

This barlow is of good quality, but it cant easily be shortened to be of lower magnification due to some weird proprietary thread that only connects to the body it comes with.

For Lunar, i want to have some coma correction because the usable field of view is abysmally small otherwise. For planetary, dont care since Jupiter will be just 200 or so pixels anyway.

So maybe the coma corrected 2.7x APM one? That works out to f/11.8 which ia too much if i were to aim for critical sampling at pixelsize x 4-5. Could i use it with a shorter nosepiece and get a little bit of coma correction and a more appropriate barlow factor? Let me know if you have tried this.

Or a 2" barlow that i put the paracorr in. Also dont know if that would work well.

For 1.25" general use barlows there seem to be a dozen different options to choose from. Which ones are worth the money?

Not really made of money at the moment so cant afford to buy the wrong one, any ideas appreciated!

[vlaiv]

51 minutes ago, ONIKKINEN said:

So maybe the coma corrected 2.7x APM one? That works out to f/11.8 which ia too much if i were to aim for critical sampling at pixelsize x 4-5. Could i use it with a shorter nosepiece and get a little bit of coma correction and a more appropriate barlow factor? Let me know if you have tried this.

I think that you should probably consider different barlow if you'll shoot for F/10.

According to Bill Paolini - it works good down to x2.2

https://www.cloudynights.com/topic/542326-apm-coma-correcting-barlow/#entry7310653

That is magnification you'd need to get x5 pixel size.

Btw, if you go for x2.2 barlow and F/10 - then you'll have about 2mm radius of diffraction limited field (4mm diagonal).

That is 1600px by whatever is offered as height for that width as ROI (since diagonal is ~8.8mm - you want x2.2 smaller ROI to get diagonal to be ~4mm).

[vlaiv]

Alternative is to get 2.9um camera instead of 678 and use APM barlow at ~x2.7

[ONIKKINEN]

3 hours ago, vlaiv said:

I think that you should probably consider different barlow if you'll shoot for F/10.

According to Bill Paolini - it works good down to x2.2

https://www.cloudynights.com/topic/542326-apm-coma-correcting-barlow/#entry7310653

That is magnification you'd need to get x5 pixel size.

Btw, if you go for x2.2 barlow and F/10 - then you'll have about 2mm radius of diffraction limited field (4mm diagonal).

That is 1600px by whatever is offered as height for that width as ROI (since diagonal is ~8.8mm - you want x2.2 smaller ROI to get diagonal to be ~4mm).

Read through the thread, it wasn't super helpful to be honest. People talking about the thing with refractors and entirely for visual, so the question remains unanswered as far as im concerned.

1600px is acceptable for sure with Lunar closeup feature imaging, but the problem is that everything in the optics must be exactly aligned, or that 1600px area will not coincide with the center of the sensor. At f/4.4 its very easy to be half a millimeter or a millimeter off center, and that would ruin half or most of the frame. Have tried that and had that exact problem (without the paracorr). Im guessing this is why almost no-one is doing lunar/planetary with f/4-f/4.5 type newtonians.

I have sent APM a message, if anyone knows the answer to using the barlow with lesser backfocus, its them.

3 hours ago, vlaiv said:

Alternative is to get 2.9um camera instead of 678 and use APM barlow at ~x2.7

Or another scope at that point to be honest... Rather not go that route

[ONIKKINEN]

Just received an email from Markus Ludes, who did say that down to 2x is the limit for still good coma correction for the APM 2.7x one.

Pretty sure this is what i will end up going with. Ideally i would be imaging with the entire chip for lunar, but that might be a tall order. 2560x1440 capture size would be very nice though, conveniently my desktop resolution and wide enough for most interesting features on the Moon. Seems likely to work, only means a 50% increase of the coma free field compared to just the barlow effect.

[CraigT82]

I used the APM 2.7x barlow extensively and loved it, performed well in my variety of newts but I admit I never used it at anything less than 2.7x, usually a touch more. You can buy just the lens cell rather than the whole thing if you don’t need the nosepiece and body (which are well made but heavy). I think owes it’s coma correcting abilities to it’s long focal length of 62.9mm (I read somewhere than any long FL doublet barlow will correct coma?)

Alternative if you want wide field lunar is to capture mosaics, use the small centre portion where image is sharpest and capture a few panes to stitch. The faster frame rates you get with the smaller RoI offset the extra time of shooting multiple panes (to some extent)

 

[ONIKKINEN]

Mosaicing would be plan b if the wider shots dont end up working from the coma perspective. I find it much easier to babysit a single big panel for a while longer though so that is preferable.

Still banking on seeing being better which might not happen, so a bit of a gamble anyway.