SXVR-H18 Flat Field with 1.25" Filters...

[RichieJarvis]

Hi All,

I know we were all wondering about how much Vignetting would be caused by using 1.25" filters with an SXVR-H18 - I certainly was!

So here is the answer - this was a 20 second flat through my Ha filter mounted in my TrueTech filter wheel...

Its not as bad as I thought it was going to be!

Cheers,

Richie

[Euan]

Thanks for posting this up, is this with the VC200L?

All I need now is for someone to post up a 383L+ version @ F5

I'm still waiting on Vince from Atik doing the calculations for the new EFW2

[adamsp123]

Cheers Richie, looks good so a bit of money saved then!

I will be very interested Euan, to see what the result is, I am looking to do some F4 imaging so I think the 2" will be needed - pooh!, unless I get a scope that is ok for the 1.25" filters, now theres a thought!

The Baader LRGB are £153 more in the 2" version

[Euan]

Same here, I have a F5 scope but I have been eyeing up an F4 astrograph.

The price of the full Astronomik narrowband & LRGB set at 2" as well as a filter wheel is almost as much as the camera

I'm just sticking with one 1.25" right up to the CCD to get me up and running, I've ordered this adaptor from Scopestuff

[adamsp123]

Oh yes thats a point, narrow band filters are going to be a whole lot dearer.

[Euan]

I've taken a guess at the EFW2 dimensions based on what I know so far

There is a really good discussion on how to calculate the minimum filter size on the Atik user group so using this here is an estimate:

CCD to front edge distance = 17mm

EFW2 edge to filter = 8mm? (I hope)

Total CCD to filter = 25mm

Astronomik 1.25" internal diameter = 24.5mm

@ F5, 400mm focal length, minimum filter size = 25.81mm

@ F4, 800mm focal length, minimum filter size = 27.75mm

As you can see it's tight!

...sorry for the hijack Richie I will PM you the calculations it may be useful for the H18

[RobH]

Interesting stuff.

I will probably be getting an 8300 based camera in the future, buy have a lot invested in 1 1/4 inch filters at the moment....full NB set incl. H-beta, Near IR filter, LRGB, plus another set of Ha,OIII and LRGB for my second imaging rig. Consequently, to replace even half of those with 2 inch filters will cost a fortune, and it also means that you can't have the bulk of them in an 8 position wheel ready to go.

I suspect that unless I find vignetting really bad at aroud F4-5, I'll just deal with it and crop a bit.

I would be very interested to see some F4 shots though.

As most of my imaging is done at F6.9 (552mm), F8(1200mm), and in the future when the new scope arrives, at F10 (2800mm) and F6.3 (around 1800mm), I reckon I'll be OK, but would like to see the hard facts in the form of flats.

I believe the SX camera's chip is further from the front of the camera than the Atiks....is this correct?.

If so, this will affect the results.

Cheers

Rob

[RichieJarvis]

Yes Rob - its 18.5mm on the H18 - I'll take some flats with the Genesis and post em....

[RichieJarvis]

Hi All,

Here is the flat from the Televue Genesis @ F/5.

This is 20 subs stacked to create a master flat.

A bit worse, but certainly not a catastrophe!

Cheers,

Richie

[RobH]

Not a catastophe at all....and a remarkably clean optical train

Cheers

Rob

[RichieJarvis]

Yeah - dunno how that happened!

[Psychobilly]

Thanks for these Richie very imformative...

looks like worst case your going to loose a bit off the edge's and end up with a "squarer" image .. perhaps thats what confused Olley

Peter...

[johnh]

I have been waiting for this test for months - Ritchie:icon_salut:

I use a Newt SPX350 at F4.53 and at F5 looks not to bad, corner crops may add to the frames look rather than side crops.

Thanks again Ritchie.

John.

[MartinB]

The first flat looks great and looks like the filters aren't interfering at all. The second one looks much more worrying. Difficult to judge how much light is lost in the corners without knowing the stretch applied. What are the ADU values Richie? Even if it can be corrected with flats there still looks like a big loss of signal in the corners. OK, you can crop but that's not an insignificant loss of chip real estate and would drive me nuts.

[RichieJarvis]

I'll pull the ADU values later Martin, and post the master FITS file where it can be grabbed.

As far as the effects on images, I used the flat for calibration on my images last night - it did work, however, it had a huge impact on the signal levels as well.

The upshot is that I am probably going to have to sell my H9 and buy some 2" filters

Or just live with it for now....

decisions decisions!

Cheers,

Richie

[Euan]

I hate to be the pessimist but that looks pretty drastic to me even if it's stretched quite far. I wouldn't fancy cropping one of these that defeats the purpose of having it in the first place

I can see me being stuck with Ha only for the foreseeable future

[Astronome]

Richie,

Not been on Twitter much lately - been at Pic du Midi. Well when is the report on the SXVR H18 going to materialise.

What is the score on RBI and having fixed cooler? Does the shutter have any impact on images?

Great profile compared with the FLI 8300, however, they do have RBI solved. I will check on delivery but it seems the one for me over ATIK (not delivering) and FLI (shipping and support by UK dealer?)

Regards,

John (@astrojohn)

[Euan]

Hi Richie,

I noticed you are using the True Tech wheel. The standard adaptor for the H18 that come with it is 2mm deep with quite a deep thread, my 383 has quite a shallow thread so it ended up about 6 or 8mm from the filter wheel

I've bought this adapter:

Adapter SC male to T2 male - short design - Teleskop-Express: Astro-Shop + Fotografie + Naturbeobachtung

It get's the distance between the two down to 0.5mm

Every millimeter comes in handy with these small filters

[Euan]

Here is a bonus prize, I've ran the fits from your website through CCD Inspector

The first is the Vixen, second the Televue

You go from about 45% drop in one of the corners to 65%