Focal Reducer usage and SCT Thread male-to-male adaptor

[RichieJarvis]

Hi All,

As some of you know, I've been using the Celestron SCT 0.63 FR with my AP102ED - I've also just purchased a Meade 0.33 FR as well (first light tonight, and it works!) - there is a slight problem though - I get what seem to be reflection halos around bright stars (see attached for an example)

Now, here are some thoughts:

1. I use Astronomik filters - which have a reflection coating

2. The only place I can put the FR's at the moment is nearest the focuser. i.e. the imaging train goes Focuser -> 2" to SCT adaptor -> FR -> Truetech Filter Wheel -> SCT to T-Adaptor -> SXV-H9

3. Using Astrometic.net, I determined that the FoV on the 0.63 FR in this configuration gives approx the same FoV as I would expect from a 0.8 FR

4. I think that the halos are caused by the discarded light from the filters being reflected back on the FR, as the FR is before the filter in the light path.

5. I think that moving the FR in the imaging train closer to the camera might:

a. Reduce/remove the halos

b. Increase the amount of focal reduction

6. My SCT to T-Adaptor thread is too short to allow me to screw the FR in the other side of the filter wheel

So - am I talking sense?

If so, I need to find an SCT threaded piece of tube about 2cm long - that way, I can use the Meade T-Adaptor to SCT thread to go into the camera, and the piece of SCT threaded tube to go into the truetech - some sort of locking ring would also be a good idea!

Anyone know where I can get such a beastie - or advise what information I need to take to a workshop to get one made?

Thanks,

Richie

[GazOC]

Have you got the spacing from reducer to the chip correct Richie? I believe the 6.3 is quite fussy over this and the 3.3 is VERY fussy.

[RichieJarvis]

Hi Gaz,

To be honest, I don't care about the spacing too much - its more the results - I can only put these FR's in one place at the moment. If I can only extract 0.8 FR from the 0.63 reducer, then thats fine - apart from the halos, it gives a lovely FoV. The 0.33 is definitely giving me a much wider FoV in the same position, so thats cool as well - not sure whether the halos are in evidence yet (time will tell on that one!) - very bad vignetting though (which I expected tbh.)

The main reason for wanting to put them the other side is to remove the halos - which I just realise I forgot to attach, d'oh!

Saying all that - what is the correct spacing?? If I can get the SCT tube sorted, then I have various adaptors which will allow me to get it right....

Thanks,

Richie

[GazOC]

Hi Richie,

I don't image but I'm sure I've read distance from reducer to chip does affect image quality, not just the reduction factor.

Its might be something to consider...

Cheers

Gaz

[RichieJarvis]

Yup - definitely something to consider - once I find out what the correct spacing should be!!

Interestingly, Astrometric.net has just come back with the FoV for my test shots tonight on the Leo Triplet:

Hello, this is the blind astrometry solver. Your results are:

(RA, Dec) center:(169.922984101, 13.2883137745) degrees

(RA, Dec) center (H:M:S, D:M:S):(11:19:41.516, +13:17:17.930)

Orientation:85.25 deg E of N

Pixel scale:5.37 arcsec/pixel

Parity:Normal ("Right-handed")

Field size :1.53 x 1.14 degrees

Now, considering that 1.53 = 91.8 arcmins and 1.14 = 68.4, that means that the 0.33 FR is working at approx 0.47 in its current position, not 0.33 - which is what I expected Incidentally, this speeds up the Focal Ratio of my F/7 scope to about F/3.3 by my reckoning - not bad really!

Having a read around to find the 'correct' spacing, I can't seem to pin it down - this thread on CN seems to be saying pretty much what I thought though - doesn't matter too much, but different spacings have different effects/reductions.

I really just want to get it nearer to the sensor though, hence the query about where/how I can get an SCT threaded tube about 2cm long made up!

Thanks,

Richie

[Merlin66]

Two things:

The back focus distance of 60mm for the X0.33 reducer is pretty critical for success! This reducer comes with a series of T spacers for the purpose of achieving this distance. The instruction sheet for the reducer explains this. ( I have a copy I can scan and send, drop me a PM)

The halos: I've got a reference somewhere, (AstroDon?) from the Web, which explains how to calculate which element in the optical train is causing the problem. By measuring the diameter of the halo you can work out the distance between the CCD chip and the reflecting surface... this can give some guidance on what needs to be changed. I'll see if I can find the link and post it for you.

[roundycat]

you will certainly find some interesting info on Don Goldman's site, just dial in 'astrodon', that should do it. The reflections are a fact of life and changing the optical line up won't get rid of them. There is a recent post over on UKAI under deep sky imaging that shows a view of NGC7000 through the new Astronomik filters. Very little in the way of halos in evidence. A lot less than mine anyway and I dont use a reducer.

Dennis

[RichieJarvis]

Thanks Dennis and Merlin

Merlin - yes please! Mine was second hand and came without the info sheet, so that would be very useful!

Do you mean that it is a fact of life using any reducer?? The reason I ask is that those halos do not appear on my images shot with an FR, so just wondering really!

The shape of them as well seems to suggest that they are a reflection from a curved surface rather than anything else - or that was my thought really!

Anyway - the real question (and the one that no-one has actually answered yet - although I do appreciate the posts and info!) is does anyone know the correct thread information that I need to take to a metalworking shop to get a 2cm SCT threaded tube made up.

The reason that this is important (and is the thread title) is because I cannot change the spacing between the FR and chip whilst the FR is on the other side of the focuser - but I can when it is next in line with the chip. So whether the halos go or not, I still want to put the FR nearest the chip

Thanks,

Richie

[Psychobilly]

SCT thrtead is 2" (50.62mm slightly truncated thread) 24TPI

[RichieJarvis]

Ahhhh - cool - thanks Peter.

I've emailed the info to a chap, so hopefully I can get a suitable piece made up....

I'll report back with what happens!

Cheers,

Richie

[Psychobilly]

Google is a marvellous tool....

If the guys local take the FR to him... he'll measure it up...

The f3.3 reducer is a PITA OK for Small chips but not that useful for anything else... its probably the least used optic in my kit although the variable T adaptors that come with it are among st the most used things...

That might change with the arrival of the DMK though...

If you change the spacing dramatically you'll probably end up sacrificing field flatness and have dodgy stars around the edges of the image.... so although you may have a larger FOV you'll end up chucking a load of it away...

Peter...